JPS62264713A - Switched capacitor filter - Google Patents

Abstract

PURPOSE:To decrease the DC offset without changing the transmission characteristic with the same total capacitance as that of a conventional filter by dividing the capacitor of a bicut type SCF into two groups and changing independently the unit capacitance of each group. CONSTITUTION:Plural switches and capacitors, and two operational amplifiers constitute the bicut switched capacitor filter. The capacitors constituting the filter is divided into capacitors 17, 20, 23, 24, 19 of a group using a capacitance BC1 and capacitors 21, 22, 15, 16, 18 of a group using a capacitance DC0 as a unit. Through the constitution as above, the unit capacitances are selected independently for each group without changing the transmission characteristic. In selecting the unit capacitance C1 as a half of the unit capacitance C0 in this grouping, the DC offset is reduced to 60% of a conventional offset.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a switched capacitor filter,
0 regarding the biquad type switched capacitor filter [promotion → 叉 t] The biquad type switched capacitor filter (hereinafter abbreviated as S Ci=') is gradually introduced as shown in Figure 2. switch], 2.3.4.5.6 and capacitance value G CO, HCo
, AC6's capacitors 15, 16, and 17 are connected in cascade to 3X amplifiers AI and A2.2, and switches 7 to 14 and capacitors D are connected to the long circuits of the amplifiers.
Co, Bco, i+'co.

Consisting of CC6, EC6, IC6, and JCo valley shapes 18 to 24, each capacity is a vertical multiple of the unit capacity Co, and A, B, C, . . . , I, J are crab numbers indicating the magnification, respectively. It is. The valley switch can be driven by the two-phase clock of φ and φ shown in Fig. 3.The transfer function of this SCF is 2πfτ, where the input voltage is “It” and the output voltage is v2.Here, Z=eJ, f is the wave number of the input signal, and τ is one period of sampling.

In Fig. 2, the voltage when converting the offset voltage of the input offset of the 1-line generator into the input offset is . l*v
If Off, the transfer functions of the output voltage v2 to the offset voltage Vold and VOZ when the input signal Vl is grounded are as follows. Here, in order to find the transfer function for the DC offset, if Z = 1, then the direct offset of the biquad type SCF in Fig. 2 is the human power conversion offset of the first stage operational amplifier A1 and the valley common value CC, ,GC,,HC,.

[Problem that the invention seeks to solve]

In the conventional biquad-type SCF described above, the transfer function is determined by the ratio between the peak and the peak. Therefore, the unit capacitance co can be set to an extremely small value within the permissible range of capacitance ratio accuracy, which makes it possible to integrate the SCF into an integrated circuit. However, there was a drawback that by reducing the unit 81tk, the DC offset increased.

Fig. 4 shows a primary value divider, which is the basic configuration of the SCF, and switches SWI and SW3 connect the input and the grounding point via the valley tea, and the grounding point and the operational amplifier A10 via the capacitor Ca. Switch 8Wz, SW4 to connect the inverting input
and J4--consisting of a capacitance cb connected to the inverting input terminal and output terminal of the amplifier A10. Each switch has pffl
The transfer gate is composed of a transfer gate and an n-type transistor connected in parallel. When the switch SW4 turns on and off, the charge leaks to the integral capacitor tCb through the parasitic capacitance *Cos and Cot between the gate, source, and drain of each transistor, and when the switch 8W4 turns off, the channel charge redundant in the integral capacitor cb. , and if the sum of the charges flowing through the integral volume CbK is △Q, then the DC offset △V of the output is △V=△
It becomes Q/Cb. 1 leaking to integral capacitance Cb! The load △Q is
The capacitance value cb depends only on the size of the transistors that make up the switch and the direct offset of the operational amplifier.
The smaller the output offset becomes, the larger the output offset becomes.

In the biquad type SCF shown in Fig. 2, the unit capacity C
When o is made small, there is a drawback that the output offset increases due to the same principle as the primary value divider.

[Means for deciding issues]

The sui-tito capacitor filter of the present invention is configured by a plurality of switches, capacitors, and two operational amplifiers, and connects the capacitor to the inverting input terminal of each operational amplifier via or without the switch to the input terminal 11411. Divide the capacitance group 1 connected to the first operational term 1 and the second capacitor connected to the second operational amplifier into groups, and divide the unit valley 1 of the capacitance group 1 connected to the second operational amplifier into groups. It is characterized by the fact that the capacity of the capacity group is also large.

[Central martyrdom]

Next, the present invention will be explained with reference to the drawings. [0] Figure is a +ol path diagram of a pi-force, de-type SCF, which is an example of the present invention. @nn Amplifier A10 inverting input terminal, switches 1, 2, 3', 4 and capacitance values GC,, HC, 3+d
15.16 is connected to the input terminal, and the capacitance value DC
, is connected to the output terminal of the arithmetic amplifier A1 through the valley type 18 of
, are connected to the output terminal of the operational amplifier A2 via the valleys jd21.22 of ECo. Also, tJA, operational amplifier A
20 inverting input terminal connects switch 5.6 and capacitance value AC
It is connected to the output terminal of the operational amplifier 5A1 via the valley jiL17 of 1.

Chi9.10 and Yongjt(l11)'Ct e Bet
is connected to the output terminal of the amplifier A2 through the terminal 20.19 of the switch 11, 12.13.14 and the output terminal of the amplifier A2.
Value IC,,JC! It is connected to the input terminal via the capacitors 23 and 24. The transfer function of this SCF is the input voltage Vl
, output 1! ! ,! Letting E be v2, then the equation becomes, which is the same as the SCF in FIG.

Here, the capacity weight is the capacity 17, 20, 23.24.19 of the group whose unit is capacity 1 value BC1, and the valley -jii2] of the group whose unit is capacity value DCo, 22° 15.16
．． In this case, the transfer function can be expressed as follows, and it can be seen that the unit volume weight can be set independently for each valley group. Direct offset △V is Δv=C+G-HV. .

becomes. The voltage VOI obtained by multiplying the output offset voltage of the operational amplifier by the input voltage VOI is inversely proportional to the unit capacitance Co. Here, let the unit party values Co and Ct' be Co
>CI.

First, consider the case where the value of each unit jtflllUCo is made equal to the conventional value. At this time, DC offset. The transfer function is the same as before. The total capacitance value is: Total capacitance value = (A + F + I + J + B) C, + (C + E + G
+H+D)C0 becomes (A+F+I+J+B)(Co
−C,).

Second, when the total capacitance value is made equal to the conventional value, Co>CI
Therefore, the unit capacitance value Co increases, and the DC offset decreases in proportion to the ratio of the conventional value and the changed value of the unit capacitance value Co. In this case, the transfer function remains unchanged. Coefficient A-J of the capacitance of a bandpass filter with a center frequency of 1633 Hz
is shown in Figure 5. With this configuration, each party's unit value is C! Assuming that the unit capacitance value C0 is 1, each cycle is Co X36.489
A filter with the same characteristics can be constructed with a capacitance value of 70 cm compared to the conventional capacitance value of 9.

] Also, if the unit capacity 1 [CI is 100 of the unit capacity value Co, and the total capacity value is equal to the conventional capacity value, then the unit capacity weight 1 direct co is 1.7 times the conventional capacity value, and the DC offset is It will be 60 inches before the change.

〔Effect of the invention〕

As explained above, the present invention divides the capacitance of the pi-force and de-type SCF into two groups and independently changes the unit capacitance of each group.
The DC offset is set to 11 without changing the transmission information.
There is a young fruit in that it makes it possible to do r:@, and it also makes it possible to reduce the total trough and 1 shift without changing the direct light offset and transmission characteristics.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a circuit diagram showing a closed-loop type switched capacitor filter, and FIG. A time chart of φ, FIG. 4 is a circuit diagram of a first-order divider explaining the offset in the SCF, and FIG. 5 is a diagram showing the coefficients of each award of the bandpass filter. 1-14...Switch, 15-24...
・Volume weight, co...First unit volume weight, CI...
...Second unit weight, AI, A2...Operation amplifier, v! ...Input voltage, V2...Output voltage, V, , V.鵞・・・Input main offset voltage, φ, φ・・・Clock that drives the switch s 01
11. CO! '...Gate and Source, Dray 71
m'J (7) 'If 8th grade, Ca, Cb
...-Volume'11t%SW]. SW2. S~V3.5W4--Suiyf. Ding two

Claims (1)

[Claims] The capacitor, which is composed of a plurality of switches, capacitors, and two operational amplifiers and is connected to the inverting input terminal of each operational amplifier via or without the switch, is connected to the first operational amplifier on the input terminal side. and a second capacitance group connected to the second operational amplifier, and the unit capacitance of the first capacitance group is set to a larger value than the unit capacitance of the second capacitance group. Biquad switched capacitor filter.