JP4072400B2 - Capacitance detection circuit, capacitance detection device, and microphone device - Google Patents

Description

となる。
【００２３】
上記式１と式２とから、Ｖ2を消去すると、
Ｖ1＝−（Ｒ2／Ｒ1）・（Ｖin／Ａ） （式４）
が得られ、このＶ1を上記式３に代入すると、
Ｖout＝−（１＋Ｃs／Ｃf）・（Ｒ2／Ｒ1）・（Ｖin／Ａ） （式５）
が得られる。
【００２４】
この式５から分かるように、静電容量検出回路１０の信号出力端子２０から出力される検出信号の電圧Ｖoutは、被検出コンデンサ１７の容量Ｃsに依存した値となる。従って、この電圧Ｖoutに対して種々の信号処理を施すことによって、容量Ｃsを特定することができる。また、この式５には角周波数ωが含まれていないことから分かるように、この検出信号の電圧Ｖoutは、交流電圧発生器１１からの交流信号Ｖinの周波数及び被検出コンデンサの周波数の変化に依存しない。これによって、被検出コンデンサ１７に印加される交流電圧の周波数に依存することなく、被検出コンデンサ１７の容量を検出することができる（回路での周波数依存特性を有しない）静電容量検出回路が実現される。したがって、コンデンサマイクロホン等、容量値がある周波数（音声帯域）で変化するような被検出コンデンサ１７に対して、検出された信号を周波数補正することなく、その電圧値から直接、容量値を特定することが可能となる。
【００２５】
また、本実施の形態例の静電容量検出回路１０では、コンデンサ１５及び被検出コンデンサ１７に電流を供給している演算増幅器１４は、その非反転入力端子が所定の電位に接続され、固定化されている。したがって、図９に示される従来の回路における演算増幅器９５と異なり、演算増幅器１４は、入力される交流信号の周波数等に依存することなく、ノイズの少ない安定した電流をコンデンサ１５及び被検出コンデンサ１７に供給するので、被検出コンデンサ１７の微小な容量の検出が可能となる。
【００２６】
なお、本発明に関する実験によれば、図１の静電容量検出回路において、例えば、Ｃｓ（被検出コンデンサ：本実施の形態ではマイクロホン）の元々の静電容量が２０ｐＦのときに、信号線の浮遊容量が２００ｐＦを越すと、かなり検出感度が悪化した。また、前記Ｃｓについて、いくつかの別の静電容量値で確認したところ、同じ傾向の結果を得た。
また、第１インピーダンス素子である容量Ｃｆと被検出コンデンサＣｓとは、この回路中ではともに信号線に接続された容量素子であり、どちらの素子についてみても計算上は前記と同じ結果をもたらすものと考えられる。
これらの実験結果及び経験から、信号線の浮遊容量が、当該Ｃｓ又はＣｆの容量値の一桁上の値を越えないように、被検出コンデンサと第１インピーダンス素子とインピーダンス変換器とを近接させると良好な検出感度が得られることが分かった。
【００２７】
図２は、図１に示された静電容量検出回路１０におけるインピーダンス変換器１６の具体的な回路例を示す。図２（ａ）は、演算増幅器１００を用いたボルテージフォロワを示している。演算増幅器１００の反転入力端子と出力端子とが短絡されている。この演算増幅器１００の非反転入力端子をインピーダンス変換器１６の入力とし、演算増幅器１００の出力端子をインピーダンス変換器１６の出力とすることで、入力インピーダンスが極めて高く、電圧ゲインＡが１となるインピーダンス変換器１６が得られる。
【００２８】
図２（ｂ）は、演算増幅器１０１を用いた非反転増幅回路を示している。演算増幅器１０１の反転入力端子とグランド間に抵抗（Ｒ10）１１０が接続され、演算増幅器１０１の反転入力端子と出力端子間にフィードバック抵抗（抵抗（Ｒ11）３３）が接続されている。この演算増幅器１０１の非反転入力端子をインピーダンス変換器１６の入力とし、演算増幅器１０１の出力端子をインピーダンス変換器１６の出力とすることで、入力インピーダンスが極めて高く、電圧ゲインＡが（Ｒ10＋Ｒ11）／Ｒ10となるインピーダンス変換器１６が得られる。
【００２９】
図２（ｃ）は、図２（ａ）や図２（ｂ）に示されるような演算増幅器の入力段にＣＭＯＳ構造のバッファを付加した回路を示している。図示されるように、正負電源間にＮ型ＭＯＳＦＥＴ３４とＰ型ＭＯＳＦＥＴ３５とが抵抗１１２、１１３を介して直列に接続され、バッファの出力が演算増幅器１００（又は１０１）の入力に接続されている。このバッファの入力をインピーダンス変換器１６の入力とし、演算増幅器の出力端子をインピーダンス変換器１６の出力とすることで、入力インピーダンスが極めて高いインピーダンス変換器１６が得られる。
【００３０】
図２（ｄ）は、図２（ｃ）の入力段のバッファのような回路を示している。図示されるように、正負電源間に、Ｎ型ＭＯＳＦＥＴ３４とＰ型ＭＯＳＦＥＴ３５とが直列に接続され、両ＭＯＳＦＥＴの接続部から出力がなされる。
【００３１】
図２（ｅ）は、演算増幅器１０２の非反転入力をインピーダンス変換器の入力とし、演算増幅器１０２の反転入力端子に抵抗１１４の一端を接続し、演算増幅器１０２の出力と反転入力間を抵抗１１５を介して接続したものとなっている。図２（ｄ）及び図２（ｅ）に示されるように、こうした構成をとることで入力インピーダンスが極めて高いインピーダンス変換器１６が得られる。
【００３２】
（第２の実施の形態）
次に、本発明の第２の実施の形態における静電容量検出回路について説明する。
【００３３】
図３は、第２の実施の形態例における静電容量検出回路３０の回路図である。この静電容量検出回路３０は、大きく分けて、図１に示された静電容量検出回路１０に相当するコア部３１、そのコア部３１の信号出力端子２０での信号電圧Ｖ01を入力として反転する反転部３２、及び、その反転部３２の出力端子２３での信号電圧Ｖ03とコア部３１の交流出力端子２２での信号電圧Ｖ02とを加算し、出力端子２４に電圧Ｖ04の検出信号を出力する加算部３３から構成される。
【００３４】
コア部３１は、図１に示された静電容量検出回路１０と同一の回路である。したがって、コア部３１の信号出力端子２０の電圧Ｖ01は、上記式５より、
Ｖ01＝−（１＋Ｃs／Ｃf）・（Ｒ2／Ｒ1）・（Ｖin／Ａ） （式６）
となり、コア部３１の交流出力端子２２の電圧Ｖ02は、上記式１より、
Ｖ02＝−（Ｒ2／Ｒ1）・（Ｖin／Ａ） （式７）
となる。
【００３５】
反転部３２は、可変抵抗（Ｒ4）４０、抵抗（Ｒ5）４１、可変抵抗（Ｒ6）４２、コンデンサ４３及び演算増幅器４４を備えた反転増幅回路であり、電圧利得が−１で、かつ、その出力端子２３での信号Ｖ03の位相がコア部３１の交流出力端子２２での信号Ｖ02と同一になるように、可変抵抗（Ｒ4）４０及び可変抵抗（Ｒ6）４２の抵抗値が調整されている。したがって、この反転部３２の入力電圧Ｖ01と出力電圧Ｖ03とは、理想的には以下の関係が成り立っている。
Ｖ03＝−Ｖ01 （式８）
【００３６】
加算部３３は、抵抗値の等しい３つの抵抗（Ｒ7）４５、抵抗（Ｒ8）４６及び抵抗（Ｒ9）４７が演算増幅器４８に接続された加算器である。つまり、２つの入力信号の電圧Ｖ02及びＶ03と、出力電圧Ｖ04とは、以下の関係が成り立つ。
Ｖ04＝−（Ｖ02＋Ｖ03） （式９）
この式９に、上記式８を代入してＶ03を消去した後に、上記式６及び式７を代入すると、

が成り立つ。つまり、この静電容量検出回路３０の出力端子２４から出力される検出信号の電圧Ｖ04は、容量値Ｃsに比例することが分かる。よって、この電圧Ｖ04に基づいて、種々の信号処理を施すことで、未知の容量値Ｃｓ又は容量変化を容易に特定することができる。
【００３７】
この式１０と第１の実施の形態例における検出信号の電圧Ｖoutを示す式５とを比較して分かるように、第２の実施の形態例における静電容量検出回路３０で得られる検出信号は、第１の実施の形態例と異なり、被検出コンデンサ１７の容量に比例する成分だけを含み、不要なオフセット分（被検出コンデンサ１７に依存しない電圧）を含んでいない。したがって、第２の実施の形態例における検出信号から被検出コンデンサ１７の容量を特定する信号処理は、簡易なもので済む。
なお、本例では、Ｖ03＝−Ｖ01となる例で説明したが、本発明は、これに限定されるものではない。容量センサの種類により、Ｖ03＝ｋ・Ｖ01（ｋは反転増幅部の増幅率）として、出力電圧Ｖ04が、
Ｖ04＝｛ｋ・（Ｃs／Ｃf）＋（ｋ＋１）｝・（Ｒ2／Ｒ1）・Ｖin
となるように設定してもよい。
【００３８】
図４は、上記第１及び第２の実施の形態例における静電容量検出回路の電子機器への応用例を示す図である。ここでは、コンデンサマイクロホンと静電容量検出回路とが一体化された、携帯電話機等に用いられるマイクロホン５０の断面図が示されている。このマイクロホン５０は、音孔５２を有する蓋体５１と、音によって振動する振動膜５３と、振動膜５３を固定しているリング５４と、スペーサ５５ａと、スペーサ５５ａを介して振動膜５３と対抗して設けられた固定電極５６と、固定電極５６を支持する絶縁板５５ｂと、絶縁板５５ｂの裏面に固定された上記施の形態の静電容量検出回路が形成されたＩＣチップ５８と、ＩＣチップ５８をモールドしているＩＣパッケージ５９と、ＩＣチップ５８とワイヤボンディング等で接続された外部電極６１ａ、６１ｂ等とから構成される。
【００３９】
コンデンサを形成している一方の電極である振動膜５３は、所定の電位（本例では、接地）に接続され、他方の電極である固定電極５６は、アルミニウム板やワイヤボンディング、コンタクトホール等の導電体を介してＩＣチップ５８の回路に接続されている。振動膜５３と固定電極５６とからなるコンデンサの容量又はその変化は、絶縁板５５ｂを介して隣接するＩＣチップ５８内の静電容量検出回路によって検出され、電気信号に変換されて、外部電極６１ａ、６１ｂ等から出力される。なお、蓋体５１は、アルミニウム等の金属からなり、絶縁基板６０の上面に形成された導電膜（図示せず）とともに、内部のコンデンサ５３、５６やＩＣチップ５８への外乱ノイズの侵入を遮蔽するシールドボックスとしての役割を果たしている。また本例では、固定電極５６と回路とを接続し、振動膜５３を所定の電位に接続しているが、振動膜５３と回路とを接続し、固定電極５６を所定の電位に接続してもよい。ただし、経験的には前者の方が好ましい。
【００４０】
図５は、図４に示されたマイクロホン５０の概略的な外観図である。図５（ａ）は平面図、図５（ｂ）は正面図、図５（ｃ）は底面図である。図５（ａ）、（ｂ）に示された蓋体５１の大きさは、例えば、およそφ縦５ｍｍ×高さ２ｍｍである。図５（ｃ）に示された４つの外部電極６１ａ〜６１ｄは、例えば、静電容量検出回路の電源用の２つの端子と、出力信号用の２つの端子である。
【００４１】
このような応用例においては、被検出コンデンサ（ここでは、コンデンサマイクロホン）と静電容量検出回路（ここでは、ＩＣチップ）とは隣接して設けられ、信号線は極めて短く、その浮遊容量がコンデンサマイクロホンか回路内の第１インピーダンス素子のいずれか大きい方の容量値の１０倍を超えないような長さの導電体によって接続されている。そして、それらの部品は、金属製の蓋体等のシールド部材で覆われている。したがって、このような応用例においては、被検出コンデンサと静電容量検出回路とを接続する信号線（導電体）に混入する外乱ノイズ等の悪影響については無視することができると考えられる。
【００４２】
つまり、このような小型のマイクロホンにおいては、被検出コンデンサと静電容量検出回路とは極めて短い導電体で接続されるので、その間をシールド付きケーブルで接続したり、そのシールドにガード電圧を印加するための特殊な回路を設けることは、却って、回路規模を大きくし、回路のコンパクト化を妨げる。したがって、被検出コンデンサと静電容量検出回路とは、非シールドの（シールドされていない）導電板、配線パターン、ワイヤボンディング、リード線等により、最短経路を接続するのが好ましい。他のマイクロホンの例として、図６及び図７に、回路を基板にのせたものを示す。上記実施の形態例の静電容量検出回路が基板６２に搭載された以外は基本的に同じである。
【００４３】
以上、本発明に係る静電容量検出回路について、２つの実施の形態例及び製品への応用例に基づいて説明したが、本発明は、これらの実施の形態例及び応用例に限定されるものではない。
【００４４】
例えば、静電容量検出回路１０及び３０において、被検出コンデンサ１７に流れる電流を検出するために、演算増幅器１４とインピーダンス変換器１６との間に、コンデンサ１５が接続されたが、抵抗やインダクタンス等のインピーダンス素子を接続することも考慮可能である。
【００４５】
また、図８に示されるように、上記実施の形態における静電容量検出回路１０及び３０におけるコンデンサ１５と並列に抵抗１８を付加して接続してもよい。これによって、コンデンサ１５と被検出コンデンサ１７との接続点は、抵抗１８を介して第１演算増幅器１４の出力端子と接続されることになり、直流的にフローティング状態となることが解消され、電位が固定される。
【００４６】
また、被検出コンデンサ１７として接続される容量型センサは、コンデンサマイクロホンだけに限られず、加速度センサ、地震計、圧力センサ、変位センサ、変位計、近接センサ、タッチセンサ、イオンセンサ、湿度センサ、雨滴センサ、雪センサ、雷センサ、位置合わせセンサ、接触不良センサ、形状センサ、終点検出センサ、振動センサ、超音波センサ、角速度センサ、液量センサ、ガスセンサ、赤外線センサ、放射線センサ、水位計、凍結センサ、水分計、振動計、帯電センサ、プリント基板検査機等の公知の容量型センサなど、静電容量の変化を利用して各種物理量を検出する全てのトランスデューサ（デバイス）が含まれる。
【００４７】
【発明の効果】
以上の説明から明らかなように、本発明に係る静電容量検出回路、静電容量検出装置及びマイクロホン装置は、抵抗を介して演算増幅器に交流電圧を印加し、信号線に被検出コンデンサを接続することで、被検出コンデンサの容量を検出している。つまり、非反転入力端子を所定の電位に接続した演算増幅器の出力端子とインピーダンス変換器の入力端子間にコンデンサを接続するとともに、インピーダンス変換器の入力端子と所定の電位間に被検出コンデンサを接続している。
【００４８】
これによって、被検出コンデンサに流れる電流の全てがコンデンサに流れ、演算増幅器の出力端子には被検出コンデンサの容量に対応する正確な信号が出力されることとなり、数ｐＦあるいはｆＦオーダー以下の微小な容量の検出が可能となる。
【００４９】
そして、演算増幅器の非反転入力端子は所定の電位に接続され、入力端子の一方の電位が固定されるので、演算増幅器は安定して動作し、演算誤差が低減し、検出信号に含まれるノイズが抑制される。
【００５０】
また、演算増幅器とインピーダンス変換器との間にコンデンサが接続されているので、演算増幅器に印加される交流電圧の周波数に依存せず、被検出コンデンサの容量変化の周波数にも依存しない検出感度が確保される。さらに、演算増幅器とインピーダンス変換器との間に抵抗を接続した場合におけるその抵抗からの熱雑音によるＳ／Ｎ比の劣化という問題も生じない。
【００５１】
なお、この静電容量検出回路と被検出コンデンサとを隣接した位置に設けておくか、又は、信号線に接続される回路素子を近接して設けることで、この間を接続するシールドケーブルや、そのケーブルで発生する浮遊容量をキャンセルする特殊な回路等は不要となる。
【００５２】
ここで、前記静電容量検出回路に、信号出力端子での信号を反転する反転増幅回路と、インピーダンス変換器の出力信号と反転増幅回路の出力信号とを加算する加算回路とを付加してもよい。これによって、静電容量検出回路の出力信号に含まれる不要なオフセット成分が除去され、被検出コンデンサの容量に対応する正味の信号を大きく増幅することができる。
【００５３】
また、被検出コンデンサをコンデンサマイクロホンとし、静電容量検出回路についてはＩＣで実現し、それらコンデンサマイクホンとＩＣとを一体化し、携帯電話機等に使用されるマイクロホンとして１つの筐体（シールドボックス）に収めることで、コンデンサマイクロホンと静電容量検出回路とは極めて隣接した位置に配置されるので、被検出コンデンサと静電容量検出回路とを接続するための径の大きなシールドケーブルやガード電圧を印加するための特殊な回路等が不要となる。
【００５４】
さらに、本発明に係る静電容量検出回路は、被検出コンデンサに電流を流すことによって容量を検出しているので、エレクトレットコンデンサマイクロホン等のように、被検出コンデンサの電極に高分子フィルム等を貼り付けてエレクトレット化する必要がなく、通常の静電容量型センサに適用することができる。
【００５５】
以上のように、本発明により、使用環境の限定も少なくなり、微小な容量を正確に検出することができ、かつ、小型化に適した静電容量検出回路等が実現され、特に、携帯電話機等の軽量・小型の音声通信機器の音声性能が飛躍的に向上され、その実用的価値は極めて高い。
【図面の簡単な説明】
【図１】本発明の第１の実施の形態における静電容量検出回路の回路図である。
【図２】（ａ）〜（ｅ）は、本発明で使用可能なインピーダンス変換器の例を示す図である
【図３】本発明の第２の実施の形態における静電容量検出回路の回路図である。
【図４】本発明の静電容量検出回路の電子機器への応用例を示す図（マイクロホンの断面図）である。
【図５】図３に示されたマイクロホンの概略的な外観図であり、（ａ）は平面図、（ｂ）は正面図、（ｃ）は底面図である。
【図６】マイクロホンの他の一例の断面図である。
【図７】図５に示されたマイクロホンの概略的な外観図であり、（ａ）は平面図、（ｂ）は正面図である。
【図８】本発明の他の実施の形態における静電容量検出回路の回路図である。
【図９】従来の静電容量検出回路の回路図である。
【符号の説明】
１０、３０ 静電容量検出回路
１１ 交流電圧発生器
１２、１３、１８、４１、４５〜４７、１１０〜１１５ 抵抗
１４、４４、４８、１００〜１０２ 演算増幅器
１５ コンデンサ（インピーダンス素子）
１６ インピーダンス変換器
１７ 被検出コンデンサ
２０ 信号出力端子
２１ インピーダンス変換器の入力端子
２２ 交流出力端子
２３ 反転部の出力端子
２４ 静電容量検出回路の出力端子
３１ コア部
３２ 反転部
３３ 加算部
３４、３５ ＭＯＳＥＴ
４０、４２ 可変抵抗
４３ コンデンサ
５０ マイクロホン
５１ 蓋体
５２ 音孔
５３ 振動膜
５４ リング
５５ａ スペーサ
５６ 固定電極
５５ｂ 絶縁板
５８ ＩＣチップ
５９ ＩＣパッケージ
６０ 絶縁基板
６１ａ〜６１ｄ 外部電極
６２ 回路基板[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a circuit and device for detecting capacitance, and more particularly to a circuit, device and microphone device for detecting a minute capacitance with high accuracy.
[0002]
[Prior art]
As a conventional example of the capacitance detection circuit, a capacitance displacement meter can be cited (for example, see Patent Document 1). FIG. 9 is a circuit diagram showing this capacitance detection circuit. In this detection circuit, a capacitance sensor 92 formed of electrodes 90 and 91 is connected to an inverting input terminal of an operational amplifier 95 via a signal line 93. A capacitor 96 is connected between the output terminal of the operational amplifier 95 and the inverting input terminal, and an AC voltage Vac is applied to the non-inverting input terminal. The signal line 93 is covered with a shield line 94 and is electrically shielded against disturbance noise. The shield line 94 is connected to the non-inverting input terminal of the operational amplifier 95. The output voltage Vd is taken out from the output terminal of the operational amplifier 95 through the transformer 97.
[0003]
In this detection circuit, the inverting input terminal and the non-inverting input terminal of the operational amplifier 95 are in an immediate short state, and the signal line 93 connected to the inverting input terminal and the shield line 94 connected to the non-inverting input terminal are defined. The potentials are substantially the same. As a result, the signal line 93 is guarded by the shield line 94, that is, the stray capacitance between the two 93 and 94 is canceled, and an output voltage Vd that is hardly affected by the stray capacitance is obtained.
[0004]
[Patent Document 1]
JP-A-9-280806 (FIG. 2)
[0005]
[Problems to be solved by the invention]
However, according to such a conventional technique, when the capacitance of the capacitance sensor 92 is certainly large to some extent, an accurate output voltage Vd that is not affected by the stray capacitance between the signal line 93 and the shield line 94 can be obtained. However, there is a problem that an error becomes large in detecting a minute capacitance of several pF or fF (femtofarad) order or less.
[0006]
Further, depending on the frequency of the AC voltage Vac to be applied, due to a tracking error in the operational amplifier 95 or the like, there is a slight phase difference between the voltage of the inverting input terminal and the non-inverting input terminal that is in the short circuit state. There is also a problem that an amplitude shift occurs and a detection error increases.
[0007]
On the other hand, in light and small audio communication devices represented by cellular phones and the like, there is a demand for a compact amplifier circuit that converts sound detected by a capacitance sensor such as a condenser microphone into an electric signal with high sensitivity and high fidelity. . If it is possible to accurately detect a minute capacitance of several pF or fF order or a change thereof, a high-performance microphone capable of detecting voice with high sensitivity and high fidelity can be realized and carried. The performance of voice pickup in voice communication devices such as telephones is greatly improved.
[0008]
Accordingly, the present invention has been made in view of such a situation, and can detect a minute capacitance accurately and can be used for a capacitive microphone or the like used in a lightweight and small audio communication device. It is an object of the present invention to provide a capacitance detection circuit and the like suitable for the detection of capacitance.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, a capacitance detection circuit according to the present invention is a capacitance detection circuit that outputs a detection signal corresponding to the capacitance of a capacitor to be detected, and has a high input impedance and a high output impedance. A low impedance converter, a capacitive first impedance element, an operational amplifier, an AC voltage generator for applying an AC voltage to the operational amplifier, and a signal output terminal connected to the output of the operational amplifier, One terminal of the capacitor to be detected and one terminal of the first impedance element are connected to the input terminal of the impedance converter, and the first impedance element and the impedance converter are included in the negative feedback path of the operational amplifier, The detected capacitor and the capacitance detection circuit are provided adjacent to each other.
[0010]
The capacitance detection circuit according to the present invention is a capacitance detection circuit that outputs a detection signal corresponding to the capacitance of the capacitor to be detected, and an impedance converter having a high input impedance and a low output impedance; A capacitive first impedance element; an operational amplifier; an AC voltage generator for applying an AC voltage to the operational amplifier; and a signal output terminal connected to an output of the operational amplifier, the input of the impedance converter One end of the detected capacitor and one end of the first impedance element are connected to the terminal, and the first impedance element and the impedance converter are included in the negative feedback path of the operational amplifier. The first impedance element and the impedance converter are provided close to each other.
[0011]
As a specific example, an AC voltage generator, an operational amplifier whose non-inverting input terminal is connected to a predetermined potential, an impedance converter, and an inverting input terminal of the operational amplifier and an output terminal of the impedance converter are connected. A capacitance detection circuit comprising a resistor and a capacitor (first impedance element) connected between the output terminal of the operational amplifier and the input terminal of the impedance converter is configured, and the detected capacitor is connected to the input terminal of the impedance converter. Connected between predetermined potentials, the capacitance detection circuit and the capacitor to be detected are adjacent to each other, or short and close so that the stray capacitance of the signal line does not exceed 10 times the maximum value of the capacitance of the connected device. Keep it in place. Here, the predetermined potential refers to any one of a certain reference potential, a predetermined DC potential, a ground potential, or a floating state, and an optimum one is selected according to the embodiment. Further, a resistor as a second impedance connected between the AC voltage generator and the inverting input terminal of the operational amplifier may be provided.
[0012]
With this configuration, a constant voltage is applied to the detected capacitor, and almost all of the current flowing through the detected capacitor flows to the capacitor (first impedance element). From the signal output terminal, the detected capacitor A signal corresponding to the capacitance is output.
[0013]
Note that the capacitor to be detected and the capacitance detection circuit can be used to reduce noise from entering the signal line connecting the capacitance detection circuit and the capacitor to be detected, and to reduce the generation of stray capacitance on the signal line. Provide as close as possible. Alternatively, the capacitor to be detected, the first impedance element, and the impedance converter are provided as close as possible.
Here, in the present specification, “adjacent” means that the stray capacitance of the signal line is 10 times as large as the capacitance value of the capacitance value of the capacitor to be detected or the capacitance value of the capacitive first impedance element. It means that it is in a state not exceeding. This is because when the stray capacitance of the signal line is set to a capacitance value that does not exceed the numerical value of the connected element by one digit, the capacitance detection circuit of the present invention has a significant deterioration in detection sensitivity. It has been found that this can be prevented, and this has been obtained empirically. The stray capacitance of the signal line can be measured by measuring the capacitance without connecting the capacitor to be detected, the first impedance element, and the impedance converter to the signal line. And in this-application specification, the state which adjoins and adjoins on said close conditions is called "adjacent."
[0014]
Here, in addition to the capacitance detection circuit, an inverting amplifier circuit that inverts the signal at the signal output terminal and an adder circuit that adds the output signal of the impedance converter and the output signal of the inverting amplifier circuit are added. May be. A resistor may be connected in parallel with the capacitor (first impedance element).
[0015]
As an application of the present invention, the capacitor to be detected is a capacitive sensor that detects a physical quantity in accordance with a change in capacitance, and the capacitance detection circuit is formed on a printed circuit board or a silicon substrate. It is preferable that the substrate is fixed or integrally formed. As a specific example, more preferably, a condenser microphone is used as the capacitor to be detected, and the capacitance detection circuit is realized by an IC. The condenser microphone and the IC are integrated, and the microphone used for a mobile phone or the like. As well as a single housing (shield box). At this time, the condenser microphone and the IC are fixed at adjacent positions and connected by a conductive plate, a wiring pattern, wire bonding, or the like.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIG. 1 is a circuit diagram of a capacitance detection circuit 10 according to the first embodiment of the present invention. In this figure, the capacitance detection circuit 10 includes a detected capacitor 17 to be detected (capacitance sensor that detects various physical quantities using a change in capacitance Cs such as a capacitor microphone). Is connected.
[0017]
The capacitance detection circuit 10 includes an AC voltage generator 11 that generates an AC voltage, a resistor (R1) 12, a resistor (R2) 13, an operational amplifier 14, an impedance element (here, a capacitor having a capacitance Cf) 15, and an impedance. The detection signal (voltage Vout) corresponding to the capacitance of the capacitor 17 to be detected is output from the signal output terminal 20.
[0018]
One end of the AC voltage generator 11 is connected to a predetermined potential (in this example, ground), and a constant AC voltage (voltage Vin, angular frequency ω) is generated from the other end (output terminal). A resistor (R 1) 12 is connected between the output terminal of the AC voltage generator 11 and the inverting input terminal of the operational amplifier 14.
[0019]
The operational amplifier 14 is a voltage amplifier having extremely high input impedance and open loop gain. Here, the non-inverting input terminal is connected to a predetermined potential (ground in this example), and the non-inverting input terminal and the inverting input terminal are connected to each other. I'm in an emergency short state. A capacitor 15, an impedance converter 16 and a resistor (R2) 13 are connected in series in this order between the negative feedback path of the operational amplifier 14, that is, from the output terminal of the operational amplifier 14 to the inverting input terminal.
[0020]
The impedance converter 16 is a voltage amplifier having an extremely high input impedance, an extremely low output impedance, and a voltage gain of A times. One end of the detected capacitor 17 is connected to the input terminal 21 of the impedance converter 16 via a conductor such as a signal line or a wiring pattern on a printed circuit board, while the other end of the detected capacitor 17 is It is connected to a predetermined potential (ground in this example). The output terminal of the operational amplifier 14 is connected to a signal output terminal 20 for outputting an output signal of the capacitance detection circuit 10, that is, a detection signal corresponding to the capacitance of the capacitor 17 to be detected. In the present application, any variable A indicated by A times or the like represents a real number other than zero (0).
[0021]
In order to avoid unnecessary stray capacitance being added as a detection error or mixing of disturbance noise in connection between the capacitor 17 to be detected and the capacitance detection circuit 10, the shortest possible conductor ( It is preferable to use a cable, a copper foil wiring pattern, a connection terminal, or the like. Further, if possible, it is preferable that the detected capacitor 17 and the entire capacitance detection circuit 10 are covered with a grounded shield member or housed in a shield box in order to enhance shielding against disturbance noise.
[0022]
The operation of the capacitance detection circuit 10 configured as described above is as follows.
When attention is paid to an inverting amplifier circuit composed of a resistor (R1) 12, a resistor (R2) 13, an operational amplifier 14, and the like, both input terminals of the operational amplifier 14 are in the state of an equal short circuit and have the same potential (eg, 0 V). Since the input impedance is extremely high and no current flows, the current flowing through the resistor (R1) 12 becomes Vin / R1, and all of it flows through the resistor (R2) 13. Therefore, the impedance converter 16 If the output voltage of V2 is V2,
Vin / R1 = -V2 / R2
Holds. By arranging this, the output voltage V2 of the impedance converter 16 is
V2 =-(R2 / R1) · Vin (Formula 1)
It becomes. Since the voltage gain of the impedance converter 16 is A, the input voltage V1 is given by the relationship between the input voltage (voltage at the input terminal 21) V1 and the output voltage (voltage at the output terminal 22) V2.
V1 = (1 / A) · V2 (Formula 2)
Holds. Further, if the current flowing through the capacitor 15 toward the detected capacitor 17 is i, the input impedance of the impedance converter 16 is extremely high. Therefore, since all of the current i flows to the detected capacitor 17, the current i is jωCs · V1, and the voltage Vout of the detection signal output from the signal output terminal 20 is

It becomes.
[0023]
From equation 1 and equation 2, if V2 is eliminated,
V1 =-(R2 / R1). (Vin / A) (Formula 4)
And substituting this V1 into Equation 3 above,
Vout =-(1 + Cs / Cf). (R2 / R1). (Vin / A) (Formula 5)
Is obtained.
[0024]
As can be seen from Equation 5, the voltage Vout of the detection signal output from the signal output terminal 20 of the capacitance detection circuit 10 is a value depending on the capacitance Cs of the capacitor 17 to be detected. Therefore, the capacitance Cs can be specified by performing various signal processing on the voltage Vout. Further, as can be seen from the fact that the angular frequency ω is not included in the expression 5, the voltage Vout of the detection signal is a change in the frequency of the AC signal Vin from the AC voltage generator 11 and the frequency of the detected capacitor. Do not depend. As a result, an electrostatic capacitance detection circuit that can detect the capacitance of the detected capacitor 17 (without frequency dependency characteristics in the circuit) without depending on the frequency of the AC voltage applied to the detected capacitor 17. Realized. Therefore, for a capacitor to be detected 17 whose capacitance value changes at a certain frequency (audio band), such as a capacitor microphone, the capacitance value is directly specified from the voltage value without frequency correction of the detected signal. It becomes possible.
[0025]
Further, in the capacitance detection circuit 10 according to the present embodiment, the operational amplifier 14 that supplies current to the capacitor 15 and the capacitor 17 to be detected has its non-inverting input terminal connected to a predetermined potential and fixed. Has been. Therefore, unlike the operational amplifier 95 in the conventional circuit shown in FIG. 9, the operational amplifier 14 supplies a stable current with less noise and the capacitor 15 and the detected capacitor 17 without depending on the frequency of the input AC signal. Therefore, it is possible to detect a minute capacitance of the capacitor 17 to be detected.
[0026]
According to the experiment relating to the present invention, in the capacitance detection circuit of FIG. 1, for example, when the original capacitance of Cs (detected capacitor: microphone in this embodiment) is 20 pF, the signal line When the stray capacitance exceeded 200 pF, the detection sensitivity deteriorated considerably. Further, when Cs was confirmed with several different capacitance values, the same tendency was obtained.
In addition, the capacitor Cf and the capacitor Cs to be detected, which are the first impedance elements, are both capacitive elements connected to the signal line in this circuit, and both elements yield the same results as described above in terms of calculation. it is conceivable that.
From these experimental results and experience, the capacitor to be detected, the first impedance element, and the impedance converter are brought close to each other so that the stray capacitance of the signal line does not exceed the value of the capacitance value of Cs or Cf by one digit. It was found that good detection sensitivity was obtained.
[0027]
FIG. 2 shows a specific circuit example of the impedance converter 16 in the capacitance detection circuit 10 shown in FIG. FIG. 2A shows a voltage follower using the operational amplifier 100. The inverting input terminal and the output terminal of the operational amplifier 100 are short-circuited. The non-inverting input terminal of the operational amplifier 100 is used as the input of the impedance converter 16 and the output terminal of the operational amplifier 100 is used as the output of the impedance converter 16 so that the input impedance is extremely high and the voltage gain A is 1. A transducer 16 is obtained.
[0028]
FIG. 2B shows a non-inverting amplifier circuit using the operational amplifier 101. A resistor (R10) 110 is connected between the inverting input terminal of the operational amplifier 101 and the ground, and a feedback resistor (resistor (R11) 33) is connected between the inverting input terminal and the output terminal of the operational amplifier 101. By setting the non-inverting input terminal of the operational amplifier 101 as the input of the impedance converter 16 and the output terminal of the operational amplifier 101 as the output of the impedance converter 16, the input impedance is extremely high and the voltage gain A is (R10 + R11) / The impedance converter 16 which becomes R10 is obtained.
[0029]
FIG. 2C shows a circuit in which a CMOS structure buffer is added to the input stage of the operational amplifier as shown in FIGS. 2A and 2B. As shown in the figure, an N-type MOSFET 34 and a P-type MOSFET 35 are connected in series via resistors 112 and 113 between positive and negative power supplies, and the output of the buffer is connected to the input of the operational amplifier 100 (or 101). By using the input of this buffer as the input of the impedance converter 16 and the output terminal of the operational amplifier as the output of the impedance converter 16, the impedance converter 16 having an extremely high input impedance can be obtained.
[0030]
FIG. 2D shows a circuit like the input stage buffer of FIG. As shown in the figure, an N-type MOSFET 34 and a P-type MOSFET 35 are connected in series between positive and negative power supplies, and an output is made from the connection portion of both MOSFETs.
[0031]
2E, the non-inverting input of the operational amplifier 102 is used as the input of the impedance converter, one end of the resistor 114 is connected to the inverting input terminal of the operational amplifier 102, and the resistor 115 is connected between the output of the operational amplifier 102 and the inverting input. It has become connected via. As shown in FIGS. 2D and 2E, an impedance converter 16 having an extremely high input impedance can be obtained by adopting such a configuration.
[0032]
(Second Embodiment)
Next, a capacitance detection circuit according to the second embodiment of the present invention will be described.
[0033]
FIG. 3 is a circuit diagram of the capacitance detection circuit 30 in the second embodiment. The capacitance detection circuit 30 is roughly divided into a core portion 31 corresponding to the capacitance detection circuit 10 shown in FIG. 1 and inverted with the signal voltage V01 at the signal output terminal 20 of the core portion 31 as an input. And the signal voltage V03 at the output terminal 23 of the inverter 32 and the signal voltage V02 at the AC output terminal 22 of the core 31 are added to output a detection signal of the voltage V04 to the output terminal 24. The adding unit 33 is configured.
[0034]
The core unit 31 is the same circuit as the capacitance detection circuit 10 shown in FIG. Therefore, the voltage V01 of the signal output terminal 20 of the core unit 31 is expressed by the above equation 5.
V01 =-(1 + Cs / Cf). (R2 / R1). (Vin / A) (Formula 6)
Thus, the voltage V02 of the AC output terminal 22 of the core part 31 is expressed by the above equation 1.
V02 =-(R2 / R1) (Vin / A) (Formula 7)
It becomes.
[0035]
The inverting unit 32 is an inverting amplifier circuit including a variable resistor (R4) 40, a resistor (R5) 41, a variable resistor (R6) 42, a capacitor 43, and an operational amplifier 44, and has a voltage gain of −1 and The resistance values of the variable resistor (R4) 40 and the variable resistor (R6) 42 are adjusted so that the phase of the signal V03 at the output terminal 23 is the same as that of the signal V02 at the AC output terminal 22 of the core unit 31. . Accordingly, the input voltage V01 and the output voltage V03 of the inverting unit 32 ideally have the following relationship.
V03 = -V01 (Formula 8)
[0036]
The adder 33 is an adder in which three resistors (R7) 45, resistors (R8) 46, and resistors (R9) 47 having the same resistance value are connected to the operational amplifier 48. That is, the following relationship holds between the voltages V02 and V03 of the two input signals and the output voltage V04.
V04 =-(V02 + V03) (Formula 9)
Substituting Equation 6 and Equation 7 after substituting Equation 8 into Equation 9 and erasing V03,

Holds. That is, it can be seen that the voltage V04 of the detection signal output from the output terminal 24 of the capacitance detection circuit 30 is proportional to the capacitance value Cs. Therefore, by performing various signal processing based on the voltage V04, an unknown capacitance value Cs or capacitance change can be easily specified.
[0037]
As can be seen by comparing Equation 10 with Equation 5 representing the voltage Vout of the detection signal in the first embodiment, the detection signal obtained by the capacitance detection circuit 30 in the second embodiment is: Unlike the first embodiment, it includes only a component proportional to the capacitance of the detected capacitor 17 and does not include an unnecessary offset (voltage that does not depend on the detected capacitor 17). Therefore, the signal processing for specifying the capacitance of the detected capacitor 17 from the detection signal in the second embodiment is simple.
In this example, V03 = −V01 has been described as an example, but the present invention is not limited to this. Depending on the type of capacitance sensor, V03 = k · V01 (k is the amplification factor of the inverting amplifier), and the output voltage V04 is
V04 = {k · (Cs / Cf) + (k + 1)} · (R2 / R1) · Vin
You may set so that.
[0038]
FIG. 4 is a diagram showing an application example of the capacitance detection circuit in the first and second embodiments to an electronic device. Here, a cross-sectional view of a microphone 50 used in a mobile phone or the like in which a condenser microphone and a capacitance detection circuit are integrated is shown. The microphone 50 is opposed to the vibrating membrane 53 via the lid 51 having the sound hole 52, the vibrating membrane 53 vibrating by sound, the ring 54 fixing the vibrating membrane 53, the spacer 55a, and the spacer 55a. The fixed electrode 56, the insulating plate 55b that supports the fixed electrode 56, the IC chip 58 on which the capacitance detection circuit of the above-described embodiment fixed to the back surface of the insulating plate 55b is formed, and the IC An IC package 59 in which the chip 58 is molded, and external electrodes 61a and 61b connected to the IC chip 58 by wire bonding or the like are included.
[0039]
The vibrating film 53 that is one electrode forming the capacitor is connected to a predetermined potential (in this example, ground), and the fixed electrode 56 that is the other electrode is an aluminum plate, wire bonding, contact hole, or the like. It is connected to the circuit of the IC chip 58 through a conductor. The capacitance of the capacitor formed of the vibration film 53 and the fixed electrode 56 or its change is detected by a capacitance detection circuit in the adjacent IC chip 58 via the insulating plate 55b, converted into an electric signal, and the external electrode 61a. , 61b, etc. The lid 51 is made of a metal such as aluminum, and shields disturbance noise from entering the internal capacitors 53 and 56 and the IC chip 58 together with a conductive film (not shown) formed on the upper surface of the insulating substrate 60. It plays a role as a shield box. In this example, the fixed electrode 56 and the circuit are connected and the vibrating membrane 53 is connected to a predetermined potential. However, the vibrating membrane 53 and the circuit are connected and the fixed electrode 56 is connected to a predetermined potential. Also good. However, the former is preferable from experience.
[0040]
FIG. 5 is a schematic external view of the microphone 50 shown in FIG. 5A is a plan view, FIG. 5B is a front view, and FIG. 5C is a bottom view. The size of the lid 51 shown in FIGS. 5A and 5B is, for example, approximately φ length 5 mm × height 2 mm. The four external electrodes 61a to 61d shown in FIG. 5C are, for example, two terminals for power supply of the capacitance detection circuit and two terminals for output signal.
[0041]
In such an application example, the capacitor to be detected (here, the condenser microphone) and the capacitance detection circuit (here, the IC chip) are provided adjacent to each other, the signal line is extremely short, and the stray capacitance is the capacitor. They are connected by a conductor having a length that does not exceed 10 times the larger capacitance value of the microphone or the first impedance element in the circuit, whichever is larger. These parts are covered with a shield member such as a metal lid. Therefore, in such an application example, it is considered that adverse effects such as disturbance noise mixed in a signal line (conductor) connecting the capacitor to be detected and the capacitance detection circuit can be ignored.
[0042]
In other words, in such a small microphone, the capacitor to be detected and the capacitance detection circuit are connected by a very short conductor, so that a shielded cable is connected between them or a guard voltage is applied to the shield. However, the provision of a special circuit for increasing the circuit scale prevents the circuit from being made compact. Therefore, it is preferable to connect the shortest path between the capacitor to be detected and the capacitance detection circuit by an unshielded (unshielded) conductive plate, wiring pattern, wire bonding, lead wire or the like. As another example of the microphone, FIGS. 6 and 7 show a circuit placed on a substrate. This is basically the same except that the capacitance detection circuit of the above embodiment is mounted on the substrate 62.
[0043]
As described above, the capacitance detection circuit according to the present invention has been described based on two embodiment examples and application examples to products. However, the present invention is limited to these embodiment examples and application examples. is not.
[0044]
For example, in the capacitance detection circuits 10 and 30, the capacitor 15 is connected between the operational amplifier 14 and the impedance converter 16 in order to detect the current flowing through the detected capacitor 17. It is also possible to consider connecting other impedance elements.
[0045]
Further, as shown in FIG. 8, a resistor 18 may be added and connected in parallel with the capacitor 15 in the capacitance detection circuits 10 and 30 in the above embodiment. As a result, the connection point between the capacitor 15 and the capacitor 17 to be detected is connected to the output terminal of the first operational amplifier 14 via the resistor 18, thereby eliminating the DC floating state. Is fixed.
[0046]
Further, the capacitive sensor connected as the capacitor to be detected 17 is not limited to the condenser microphone, but an acceleration sensor, seismometer, pressure sensor, displacement sensor, displacement meter, proximity sensor, touch sensor, ion sensor, humidity sensor, raindrop Sensor, snow sensor, lightning sensor, alignment sensor, contact failure sensor, shape sensor, end point detection sensor, vibration sensor, ultrasonic sensor, angular velocity sensor, liquid level sensor, gas sensor, infrared sensor, radiation sensor, water level gauge, freezing sensor All transducers (devices) that detect various physical quantities using changes in capacitance, such as known capacitive sensors such as moisture meters, vibrometers, charge sensors, and printed circuit board inspection machines, are included.
[0047]
【The invention's effect】
As is clear from the above description, the capacitance detection circuit, the capacitance detection device, and the microphone device according to the present invention apply an AC voltage to the operational amplifier through the resistor and connect the detected capacitor to the signal line. By doing so, the capacitance of the capacitor to be detected is detected. In other words, a capacitor is connected between the output terminal of the operational amplifier with the non-inverting input terminal connected to a predetermined potential and the input terminal of the impedance converter, and a detected capacitor is connected between the input terminal of the impedance converter and the predetermined potential. is doing.
[0048]
As a result, all of the current flowing through the capacitor to be detected flows through the capacitor, and an accurate signal corresponding to the capacitance of the capacitor to be detected is output to the output terminal of the operational amplifier. The capacity can be detected.
[0049]
Since the non-inverting input terminal of the operational amplifier is connected to a predetermined potential and one potential of the input terminal is fixed, the operational amplifier operates stably, the operational error is reduced, and the noise included in the detection signal Is suppressed.
[0050]
In addition, since a capacitor is connected between the operational amplifier and the impedance converter, the detection sensitivity does not depend on the frequency of the AC voltage applied to the operational amplifier and does not depend on the frequency of the capacitance change of the detected capacitor. Secured. Further, when a resistor is connected between the operational amplifier and the impedance converter, there is no problem of deterioration of the S / N ratio due to thermal noise from the resistor.
[0051]
The electrostatic capacitance detection circuit and the capacitor to be detected are provided at adjacent positions, or by providing a circuit element connected to the signal line close to each other, a shielded cable that connects between them, A special circuit that cancels the stray capacitance generated in the cable is not necessary.
[0052]
Here, an inverting amplifier circuit that inverts the signal at the signal output terminal and an adder circuit that adds the output signal of the impedance converter and the output signal of the inverting amplifier circuit may be added to the capacitance detection circuit. Good. Thereby, an unnecessary offset component included in the output signal of the capacitance detection circuit is removed, and the net signal corresponding to the capacitance of the detected capacitor can be greatly amplified.
[0053]
The capacitor to be detected is a condenser microphone, and the capacitance detection circuit is realized by an IC. The condenser microphone and the IC are integrated to form a single casing (shield box) as a microphone used in a mobile phone or the like. Since the condenser microphone and the capacitance detection circuit are placed very close to each other, a shield cable with a large diameter and a guard voltage are applied to connect the capacitor to be detected and the capacitance detection circuit. A special circuit or the like is not required.
[0054]
Furthermore, since the capacitance detection circuit according to the present invention detects the capacitance by passing a current through the capacitor to be detected, a polymer film or the like is attached to the electrode of the capacitor to be detected, such as an electret condenser microphone. There is no need to attach it to an electret and it can be applied to a normal capacitance type sensor.
[0055]
As described above, according to the present invention, there is less limitation on the use environment, and a capacitance detection circuit and the like that can accurately detect a minute capacitance and are suitable for miniaturization are realized. The voice performance of such lightweight and small voice communication devices is dramatically improved, and its practical value is extremely high.
[Brief description of the drawings]
FIG. 1 is a circuit diagram of a capacitance detection circuit according to a first embodiment of the present invention.
2A to 2E are diagrams showing examples of impedance converters that can be used in the present invention. FIG. 3 is a circuit diagram of a capacitance detection circuit according to a second embodiment of the present invention. FIG.
FIG. 4 is a diagram (cross-sectional view of a microphone) showing an application example of the capacitance detection circuit of the present invention to an electronic device.
5A and 5B are schematic external views of the microphone shown in FIG. 3, wherein FIG. 5A is a plan view, FIG. 5B is a front view, and FIG. 5C is a bottom view.
FIG. 6 is a cross-sectional view of another example of a microphone.
7 is a schematic external view of the microphone shown in FIG. 5, where (a) is a plan view and (b) is a front view. FIG.
FIG. 8 is a circuit diagram of a capacitance detection circuit according to another embodiment of the present invention.
FIG. 9 is a circuit diagram of a conventional capacitance detection circuit.
[Explanation of symbols]
10, 30 Capacitance detection circuit 11 AC voltage generators 12, 13, 18, 41, 45-47, 110-115 Resistors 14, 44, 48, 100-102 Operational amplifier 15 Capacitor (impedance element)
16 Impedance converter 17 Capacitor to be detected 20 Signal output terminal 21 Impedance converter input terminal 22 AC output terminal 23 Inverting unit output terminal 24 Capacitance detection circuit output terminal 31 Core unit 32 Inverting unit 33 Adding units 34 and 35 MOSET
40, 42 Variable resistance 43 Capacitor 50 Microphone 51 Cover body 52 Sound hole 53 Vibration film 54 Ring 55a Spacer 56 Fixed electrode 55b Insulating plate 58 IC chip 59 IC package 60 Insulating substrates 61a to 61d External electrodes 62 Circuit substrate

Claims (9)

A capacitance detection circuit that outputs a detection signal corresponding to the capacitance of the capacitor to be detected,
An impedance converter having a high input impedance and a low output impedance, a capacitive first impedance element, an operational amplifier, an AC voltage generator for applying an AC voltage to the operational amplifier, and an output of the operational amplifier Signal output terminal,
One end of the detected capacitor and one end of the first impedance element are connected to the input terminal of the impedance converter with a signal line ,
The negative feedback path of the operational amplifier includes the first impedance element and the impedance converter;
The detected capacitor, the first impedance element, and the impedance converter are configured such that the stray capacitance of the signal line is larger than a capacitance value of the capacitance value of the detected capacitor or the capacitance value of the first impedance element. The capacitance detection circuit is provided so as not to exceed 10 times . The capacitance detection circuit according to claim 1 , wherein the capacitor to be detected and the capacitance detection circuit are provided adjacent to and in contact with each other . The capacitance detection circuit according to claim 1, wherein the capacitance detection circuit further includes a resistance element connected in parallel with the first impedance element. The capacitance detection circuit according to claim 1, further comprising a second impedance provided between the AC voltage generator and the operational amplifier. The capacitance detection circuit further includes:
An inverting amplifier circuit for inverting the signal at the signal output terminal;
The capacitance detection circuit according to any one of claims 1 to 4, further comprising: an addition circuit that adds an output signal of the impedance converter and an output signal of the inverting amplification circuit. The capacitance detection circuit according to claim 1, wherein one end of the capacitor to be detected and the input terminal of the impedance converter are connected by an unshielded conductor. The capacitance detection circuit according to claim 1, wherein the capacitor to be detected and the capacitance detection circuit are housed in one shield box. A capacitive sensor as the detected capacitor for detecting a physical quantity in accordance with a change in capacitance;
An electrostatic capacitance comprising: the electrostatic capacitance detection circuit according to claim 1, wherein the electrostatic capacitance detection circuit is provided on a printed circuit board or a silicon substrate, and is fixed to the capacitive sensor. Capacity detection device. A condenser microphone as the capacitor to be detected;
A microphone device comprising: the capacitance detection circuit according to claim 1.

Images