JPH0334683B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application This invention relates to a DC voltage amplification circuit used in a two-wire transmitter or the like.

(b) Conventional technology Operates with low power consumption, such as a two-wire transmitter (e.g., power supply voltage DC24V, signal current DC4~20V)
mA), a circuit is required to amplify the minute DC voltage. The method of amplifying DC voltage using an operational amplifier is generally well known, but there is a limit to the input voltage value that can be amplified due to temperature changes such as offset voltage and offset current, and it is not possible to amplify extremely small voltages. . For the purpose of amplifying minute DC voltages, a chip-pick type operational amplifier can be used, but this increases power consumption and
The drawback is that it does not operate with low power consumption.

On the other hand, when amplifying a signal from a Wheatstone bridge or the like that converts a state quantity into an electrical signal, for example, an operational amplifier with a relatively high input impedance must be used because an input current of the amplifier causes an error. However, operational amplifiers with high input impedance have a large offset voltage and a large temperature change, so there is a problem that they are not suitable for amplifying minute DC voltages.

(c) Purpose In view of the above, the purpose of the present invention is to provide a system that operates with low power consumption in devices that require a small output voltage and high input impedance, such as the output amplification of a Wheatstone bridge. Another object of the present invention is to provide a DC voltage amplification circuit that amplifies minute DC voltages.

(d) Configuration In order to achieve the above object, the DC voltage amplifier circuit of the present invention transforms the impedance of an input voltage using a pair of voltage follower operational amplifiers, and amplifies the output using a second operational amplifier. High to a pair of voltage follower operational amplifiers
and Low inputs are alternately switched and inputted by a first switching means, and the second operational amplifier output is synchronized with the switching of the first switching means by a second switching output, The first and second capacitors are alternately connected, and the voltage charged and held in these two capacitors is differentially amplified by the third operational amplifier, and the offset voltage and offset current generated in each operational amplifier are I'm trying to cancel it.

(E) Examples The present invention will be explained in more detail below with reference to Examples.

FIG. 1 is a connection diagram of a DC voltage amplification circuit showing one embodiment of the present invention. In the same figure, 1 is 4
This is a Wheatstone bridge consisting of resistors R1, R2, R3, and R4, and a minute voltage to be amplified is output from points H and L.

2 and 3 are a pair of voltage follower operational amplifiers provided for impedance conversion of input voltage. Point H of the Wheatstone bridge 1, the (+) input terminal of the voltage follower operational amplifier 2, and the (+) terminal of the voltage follower operational amplifier 3.
Analog switches S11 and S21 are connected between the input terminals, and the L point of the Wheatstone bridge 1 and the (+) point of the voltage follower operational amplifiers 2 and 3 are connected between the input terminals.
Analog switches S12 and S22 are connected between the input terminals. Analog switch S11, S12
and analog switches S21 and S22 (first switching means) are turned on alternately by pulse signals P1 and P2 from the pulse signal generator 4, and the high and low voltages at points H and L are applied to the voltage follower operational amplifier 2. , 3 are input alternately.

5 is an operational amplifier (second operational amplifier) that amplifies the outputs of voltage follower operational amplifiers 2 and 3;
The output terminals of the voltage follower operational amplifiers 2 and 3 are connected to both (+) and (-) input terminals of the operational amplifier 5 via input resistors Ri, respectively. The (+) input terminal of the operational amplifier 5 is connected to the power supply VS via a resistor Rf, and the resistor Rf is connected to its output terminal and (-) input terminal.

C1 and C2 are capacitors (first
Analog switches S13 and S23 (second switching means) are connected to the output terminal of the operational amplifier 5 and one end of the capacitors C1 and C2, and the other end of the capacitors C1 and C2 is connected to a common terminal. connected to electrical potential. Analog switch S1
3 and S23 are turned on alternately by pulse signals P1 and P2 from the pulse signal generator 4. That is, the on/off state is switched in synchronization with the switching of analog switches S11 and S12 and analog switches S21 and S22.

6 is an operational amplifier (third operational amplifier) that differentially amplifies the voltage held in capacitors C1 and C2, and the non-common side of capacitors C1 and C2 is connected to the (+ ),
(-) are individually connected to the input terminals.
The (+) input end of the operational amplifier 6 is connected to the power supply VS via a resistor Rs, and the output end is connected to the (-) input end via a resistor Rs.

As shown in FIG. 2, the pulse signals P1 and P2 generated by the pulse signal generator 4 are ones in which high and low are alternately repeated in opposite phases, and the signal P1
is high, analog switches S11, S1
2, S13 is turned on, and when the signal P2 is high, the analog switches S21, S22, and S23 are turned on.

In the example circuit configured as above,
When a state quantity (for example, pressure) is applied to the Wheatstone bridge 1, the resistance value of the bridge changes according to the state quantity, and a minute voltage corresponding to the state quantity is output between points H and L. The potential at point H is high, L
Assume that the potential at the point is low. Considering the timing when the signal P1 of the pulse signal generator 4 is high, analog switches S11, S12, and S13 are on, so the potential VH at point H is the same as that of the analog switch S1.
1, it is input to a voltage follower operational amplifier 2, and the potential VH at point H is impedance-converted to its output terminal and output as is. Further, the potential VL at point L is output to the voltage follower operational amplifier 3 via the analog switch S12, and the potential VL is output as is to the output terminal thereof. The outputs of the voltage follower operational amplifiers 2 and 3 are further amplified by the operational amplifier 5, and the output is applied to the capacitor C1 via an analog switch S13 to charge the capacitor C1. The output voltage of the operational amplifier 5 is then held in the capacitor C1. This held voltage is generated by inputting the potentials VH and VL at the H point and L point of the Wheatstone bridge 1 to the (+) and (-) input terminals of the operational amplifier 5 via the voltage follower operational amplifiers 2 and 3. This is the output voltage of operational amplifier 5 when

Next, when the signal P2 of the pulse signal generator 4 becomes high, the analog switches S11 and S
12, S13 turns off and analog switch S2
1, S22, and S23 are turned on, the potential VH at the H point is input to the voltage follower operational amplifier 3 via the analog switch S21, and the potential VL at the L point is input to the voltage follower operational amplifier 3, contrary to the case where the signal P1 is high. The signal is input to the voltage follower operational amplifier 2 via the . Therefore, the outputs of the voltage follower operational amplifiers 2 and 3 are opposite to those when the signal P1 is high. That is, the potential VL is directly derived from the output of the voltage follower operational amplifier 2, and the potential VH is directly derived from the output of the voltage follower operational amplifier 3. This output is further amplified by an operational amplifier 5,
The output is then applied to capacitor C2 via analog switch S23, and capacitor C2
to charge. The output voltage of the operational amplifier 5 is then held in the capacitor C2. This held voltage is the output potential when the potential VL at point L passes through the voltage follower operational amplifier 2 and is input to the (+) input terminal of the operational amplifier 5; , 3 and 5 are included. However, when the voltages held in these capacitors C1 and C2 are applied to the operational amplifier 6 and both held voltages are differentially amplified, the offset voltage is canceled at the output terminal of the operational amplifier 6, corresponding to the potential difference between the H point and the L point. The output potential is derived.

Here, the point in which the offset voltage is canceled will be briefly explained using equations.

The output voltages of voltage follower operational amplifiers 2, 3 and operational amplifier 5 are V1, V2, V3, and the offset voltages of these operational amplifiers are V10, V20,
If V30, analog switch S11, S1
2. Each output voltage when S13 is on can be expressed as V1 = VH + V10 V2 = VL + V20 V3 = VS + Rf (VH + V10 - VL - V20) / Ri - (Ri + Rf) V30 / Ri. V3 in the above equation is held in capacitor C1 as Vc1.

On the other hand, analog switches S21, S22, S23
Each output voltage when is on can be expressed as V1 = VL + V10 V2 = VH + V20 V3 = VS + Rf (VL + V10 - VH - V20) / Ri - (Ri + Rf) V30 / Ri. In this case, V3 is connected to capacitor C2 by Vc
2.

Since the operational amplifier 6 differentially amplifies the above Vc1 and Vc2, its output voltage VO is VO=VS+2Rf・Rs(VH−VL)/Ri・R As is clear from this equation, the output voltage VO is ,
Offset voltages V10, V20, and V30 are canceled and do not appear, and the change in output voltage is due to the change in VH - VL,
That is, it is proportional to the output voltage of the Wheatstone bridge.

(F) Effects According to the DC voltage amplification circuit of the present invention, offset voltage and offset current can be canceled, so that it is possible to amplify a minute voltage that requires a large input impedance of the input circuit. Therefore, since there is no need to use a chopping type operational amplifier, a DC voltage amplification circuit with low power consumption can be achieved. Furthermore, it is possible to obtain a circuit that is resistant to input voltage common mode noise (DC input voltage level fluctuation).

[Brief explanation of the drawing]

FIG. 1 is a connection diagram of a DC voltage amplification circuit showing one embodiment of the present invention, and FIG. 2 is a diagram showing a pulse signal generated by a pulse signal generator of the DC voltage amplification circuit. 2, 3: Voltage follower operational amplifier, S1
1, S12, S13, S21, S22, S23:
Analog switch, 4: Pulse signal generator, 5,
6: Operational amplifier, C1, C2: Voltage holding capacitor.

Claims (1)

[Claims] 1. A pair of voltage follower operational amplifiers into which high and low signals of minute DC voltage are input and perform impedance conversion, and a first switching device that switches the high and low signals and inputs them to the pair of voltage follower operational amplifiers. a switching control circuit for controlling the switching operation of the first switching means; a second operational amplifier that receives and amplifies the output of the pair of voltage follower operational amplifiers; and the second operational amplifier. first and second capacitors that are charged by the output of the circuit and hold the voltage thereof, and are controlled to be switched by the switching control circuit, and the output of the second operational amplifier is connected to the switching operation of the first switching means. a second switching means that synchronously switches and connects the first and second capacitors; and a third operational amplifier that differentially amplifies the voltage held in the first and second capacitors. A DC voltage amplification circuit.