JP2627499B2 - Switch / capacitor / filter circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] A switched capacitor filter circuit of the present invention comprises a plurality of cascade-connected phase-inverted switched capacitor filter circuit units, and each switched capacitor filter circuit. The input capacitance and the DC feedback capacitance of the circuit section are set so as to satisfy a specific relational expression. As a result, the offset voltage caused by the noise generated when the switch is turned on and off can be canceled,
An appropriate signal can be extracted from the output.

[Industrial applications]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switched capacitor filter circuit, and more particularly to a switched capacitor filter circuit capable of canceling an offset voltage caused by noise generated when a switch is turned on / off to obtain a proper output. The present invention relates to a capacitor / filter circuit.

[Conventional technology]

FIG. 3 shows a first-order phase-inverted switched capacitor capacitor.
FIG. 2 is a circuit diagram of a filter circuit, where 1 is an operational amplifier.
C1 is the input capacitance, C _F is the capacitance that feeds back the output AC, C2
Is the capacity to feed back the (equivalent) DC component of the output, and S1
To S3 are switch circuits that are turned on / off by a clock control signal (not shown).

FIG. 4 is a circuit diagram of a switch circuit constituted by n-channel FETs. When the voltage input to the gate of the switch circuit changes, charges are supplied from the gate to the source due to the parasitic capacitance _CGS existing between the gate (G) and the source (S). The amount of charge supplied at this time is
When the change amount [Delta] V _G of the gate voltage is expressed by _{ΔQ L = C GS × ΔV G} . In particular, the amount of charge supplied by the switch circuit S2 changes the voltage at the input of the operational amplifier 1 and is amplified to appear as an offset voltage of the output voltage.

Now, when the offset voltage when the switched capacitor filter circuit of FIG. 3 is in a steady state is obtained, (Steady state conditions) Becomes

That is, Given by

[Problems to be solved by the invention]

As described above, since the offset voltage shifts a desired voltage, it is desirable to remove the offset voltage.

FIG. 5 shows a circuit in which the primary switched capacitor filter circuits 2 to 4 shown in FIG. 2 are cascaded in three stages.

The output voltage of this circuit is given by _{V 3 = -G A · G B} · G C · V IN + G B · G C · V A -G C · V B + V C.

Here, G _A , G _B , and G _C are the amplification factors of the respective switched capacitor filter circuits, and V _A , V _B , and V _C are offset voltages.

In formulas _{V 3, G B · G C} · V A -G C · V B + V C is because the offset voltage of the final stage, it is desirable to this voltage to zero.

By the way, when the circuit configuration is performed by the inverting type switched capacitor filter circuit, since the amplification factor is negative,
Since the sign of the offset voltage of each stage is different, in certain conditions, can be set to _{G B · G C · V A} -G C · V B + V C = 0.

The present invention has been created focusing on this point,
It is an object of the present invention to provide a switched capacitor filter circuit capable of eliminating an offset voltage due to noise input during switching.

[Means for solving the problem]

FIG. 1 is a diagram showing the principle configuration of a switch capacitor filter circuit of the present invention. In the figure, a first-order switched capacitor filter circuit has a total of n stages, and a second-order switched capacitor filter circuit has a total of n stages.
Phase-inverted switched-capacitor filter in which a plurality of stages or a first-order switched-capacitor filter circuit and a second-order switched-capacitor filter circuit are combined and connected in cascade so as to have a total of n stages In the circuit, the first-order switched capacitor filter circuit connects a first operational amplifier, an input capacitance of the first operational amplifier, and one end of the input capacitance to one of a signal input and a ground line. A first switch circuit for connecting the other end of the input capacitor to either the input of the first operational amplifier or a ground line, and the output of the first operational amplifier. A first DC feedback capacitance that feeds back to an input of the operational amplifier, and one end of the first DC feedback capacitance connected to one of an output of the first operational amplifier or a ground line; A second switched capacitor filter circuit, wherein the second-order switched capacitor filter circuit has a second operational amplifier, an input capacitance of the second operational amplifier, and an input capacitance of the second operational amplifier. A fourth switch circuit for connecting one end of the second operational amplifier to one of the signal input and the ground line, and connecting the other end of the input capacitance of the second operational amplifier to one of the input and the ground line of the second operational amplifier. A fifth switch circuit connected to the first and second operational amplifiers;
An input capacitance of the third operational amplifier, a sixth switch circuit for connecting one end of the input capacitance of the third operational amplifier to one of a signal output of the second operational amplifier or a ground line, A seventh switch circuit for connecting the other end of the input capacitance of the third operational amplifier to either the input of the third operational amplifier or the ground line, and the output of the third operational amplifier to the second operational amplifier. A second DC feedback capacitance that feeds back to the input of the operational amplifier, a third DC feedback capacitance that feeds back the output of the third operational amplifier to the input of the third operational amplifier, and the second and third An eighth switch circuit for connecting one end of the DC feedback capacitance to one of the output of the third operational amplifier and a ground line, wherein the cascade-connected switch of the i-th stage (i = 1 to n) is provided. . Of the first operational amplifier of the capacitor filter circuit The input capacitance or the input capacitance of the second operational amplifier is C1i,
When the first DC feedback capacitance or the second DC feedback capacitance of the i-th stage switched capacitor filter circuit is C2i, the offset voltage of the cascade-connected phase-inverted switched capacitor filter circuit So that the input capacitance C1i and the DC feedback capacitance C
The value of 2i and the following formula, (Where n is a natural number of 2 or more).

In the circuit of [Function] FIG. 1, the offset voltage of the circuit of each stage and V _i, also when the gain and G _i, the output voltage of the final stage, Here, ΔQ _L is the amount of electric charge injected into the input of each stage when the clock control signal is turned on / off, and it is assumed that all the stages are the same.

By the way, according to the circuit configuration of the present invention, the input capacitance C _1i and the DC feedback capacitance C _{2i of} each stage are set so as to satisfy the expression (1). Becomes

That is, the offset voltage can be removed from the output voltage of the final stage.

〔Example〕

Next, examples of the present invention will be described. As described above, FIG. 5 shows a circuit in which three phase-inverted switched-capacitor / filter circuit sections are cascaded.

In this circuit, assuming that the amplification factors of the respective circuit parts are G _A , G _B , G _C and the offset voltages are V _A , V _B , V _C , V ₃ = −G _A G _B G _C V _IN + a _{G B · G C · V a} -G C · V B + V C.

That is, the offset voltage of the final stage is However, C _1A , C _1B , C _1C are the input capacitance of each stage, C _2A ,
_{22 B} and C _2C are DC feedback capacitances of each stage.

By the way, in the example circuit, the input capacity and the DC capacity of each stage are , The final stage offset voltage is zero.

As described above, according to the circuit of the embodiment of the present invention, it is possible to remove the offset voltage generated by turning on / off the switch circuit and obtain a desired output voltage.

The switched capacitor filter circuit of each stage includes a first-order phase-inverted switched filter as shown in FIG.
It may be a capacitor / filter circuit section or a second-order phase-inverted switched-capacitor / filter circuit section as shown in FIG.

Because, in the circuit of FIG. 2, the input capacitance is C3, the input is V5, the DC feedback capacitance is C4, the output of the second stage operational amplifier is V7, and the input of the first stage operational amplifier is Let ΔQL be the leakage from the connected switch circuit, ΔV6 be the output change of the first-stage operational amplifier, and CF1 be the capacitance that feeds back the AC component of the output of the first-stage operational amplifier. From − (C3 · V5 + C4 · V7 + ΔQL) / CF1 = 0 (steady state condition), V7 = − (ΔQL / C4) − (C3 / C4) · V5, and the offset voltage −ΔQL / C4 is the primary of FIG. This is because it is given by the same equation as in the case of the phase inversion type switched capacitor filter circuit section.

As described above, the present invention can be applied not only to the first-order but also to a switched-capacitor filter circuit generally configured by a multi-order phase-inverted switched-capacitor filter circuit unit.

〔The invention's effect〕

As described above, according to the present invention, when a plurality of stages of phase-inverted switched-capacitor / filter circuits are connected, the influence of the noise at the time of switching of each stage on the offset voltage of the final stage output is alternately positive and negative. Can be removed by taking advantage of the fact that As a result, a desired output voltage can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram for explaining the principle configuration of a switched capacitor filter circuit according to the present invention, FIG. 2 is a circuit diagram of a second-order phase-inverted switched capacitor filter circuit section, FIG. FIG. 4 is a circuit diagram of a first-order phase inversion type switched capacitor filter circuit unit. FIG. 4 is a diagram illustrating charge injection by switching of a switch circuit. FIG. 5 is a phase inversion type switch connected in cascade in three stages. To
FIG. 2 is a block diagram of a switched capacitor filter circuit including a capacitor filter circuit unit. (Explanation of reference numerals) In FIG. 1, 1... Operational amplifier, 2, 3, 4... Phase inversion type switched capacitor filter circuit section, C _2i , C ₂₁ , C _2n , C ₂ , C _4. DC feedback capacitance, C _1i , C ₁₁ , C _1n , C ₁ , C ₃ ... input capacitance, C _F , C _F1 ... AC feedback capacitance, C _GD , (C _GS ) ... between gate and drain (source) Parasitic capacitance, V _IN , V ₅ …… Input voltage, V ₀ , V ₁ , V ₂ , V ₃ , V ₆ , V ₇ …… Output voltage, V _A , V _B , V _C …… Offset voltage, V _G: gate voltage, G _A , G _B , G _C: amplification factor, ΔQ _L: injected charge amount, S1 to S3: switch.

 ──────────────────────────────────────────────────続 き Continued on the front page (56) References Miki Takebe, "Switched Capacitor Circuit," Hyundai Kogakusha (February 25, 1985), pp. 189-191

Claims (2)

(57) [Claims] A total of n stages of a primary switched capacitor filter circuit and n stages of a secondary switched capacitor filter circuit, or a primary switched capacitor filter circuit and a secondary A phase-inverted switched capacitor filter circuit in which a plurality of switched capacitor filter circuits are combined and cascaded so as to have n stages in total, wherein the first-order switched capacitor filter circuit comprises: An operational amplifier, an input capacitance of the first operational amplifier, a first switch circuit for connecting one end of the input capacitance to one of a signal input and a ground line, and A second switch circuit connected to either the input of the first operational amplifier or the ground line, and the output of the first operational amplifier to the operational amplifier. A first DC feedback capacitance that returns to a force, and a third switch circuit that connects one end of the first DC feedback capacitance to one of an output of the first operational amplifier or a ground line, A second switched capacitor filter circuit comprising: a second operational amplifier; an input capacitance of the second operational amplifier; and one end of the input capacitance of the second operational amplifier, which is connected to either a signal input or a ground line. A fourth switch circuit connected to one end, a fifth switch circuit connecting the other end of the input capacitance of the second operational amplifier to one of an input of the second operational amplifier or a ground line, A third operational amplifier, an input capacitance of the third operational amplifier, and a sixth terminal for connecting one end of the input capacitance of the third operational amplifier to one of a signal output of the second operational amplifier or a ground line. The switch circuit and A seventh switch circuit for connecting the other end of the input capacitance of the third operational amplifier to either the input of the third operational amplifier or the ground line; and the output of the third operational amplifier to the second operational amplifier. A second DC feedback capacitance that feeds back to the input of the operational amplifier; a third DC feedback capacitance that feeds back the output of the third operational amplifier to the input of the third operational amplifier; An eighth switch circuit for connecting one end of the DC feedback capacitance to one of the output of the third operational amplifier and a ground line, wherein the cascade-connected switch of the i-th stage (i = 1 to n) is provided.・
The input capacitance of the first operational amplifier of the capacitor filter circuit or the input capacitance of the second operational amplifier is C1i, and the first DC feedback capacitance or the second DC feedback capacitance of the i-th stage switched capacitor filter circuit. When the DC feedback capacitance is C2i, the input capacitance C1i and the value of the DC feedback capacitance C2i are set as follows so that the offset voltage of the cascade-connected phase-inverted switched capacitor filter circuit is offset as a whole. formula, (Where n is a natural number of 2 or more).
Capacitor filter circuit. 2. An input capacitor according to claim 1, wherein
A switched capacitor filter circuit characterized in that the values of C1i and the DC feedback capacitance C2i have different values between each switched capacitor filter circuit.