JPH0454461Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a voltage measuring device with auto-zero means.

An example of a voltage measuring device having a conventionally known auto-zero means is shown in FIG. In Figure 1, Vin
is the input to be measured, PA is the preamplifier, LP is the low-pass filter, SW is the auto-zero switch, A/D is the analog-to-digital converter (hereinafter simply referred to as the A/D converter), and OP is the arithmetic operation with memory function. The circuit, DI, is the display circuit. The input Vin to be measured is preamp A, low pass filter
A/D via terminal a of LP and changeover switch SW
The output of the A/D converter is measured by a display or recording circuit DI via an arithmetic circuit OP. Terminal b of the changeover switch SW is connected to common.

In order to automatically cancel the circuit offset in this device, as is already known, prior to measurement, connect terminal b of the changeover switch SW to the common
This is done by connecting to COM. but,
Although this method is effective in canceling the offset of the A/D converter in the subsequent stage, it cannot cancel the offset of the preamplifier PA and low-pass filter LP in the previous stage. To solve this,
A configuration in which the changeover switch SW is placed in front of the preamplifier PA as shown in FIG. 2 can be considered, but in this case, the system speed is determined by the response of the low-pass filter LP, so there is a drawback that high-speed operation cannot be performed. This invention solves these problems,
An example thereof is shown in FIG.

In Figure 3, SW is a changeover switch with terminals a and b, PA is a preamplifier, LP is a low-pass filter, A/D is an A/D converter, OP is an arithmetic circuit with a memory function, and DI is a display or recording circuit. It is. The input source to be measured Vin is connected to a low-pass filter via terminal a of the switch SW and the preamplifier PA.
The output terminal of the low-pass filter LP is connected to the display circuit DI via the A/D converter and the arithmetic circuit OP.

In the low pass filter LP, R1, R2
is a resistive element, S1 and S2 are switches, A is an operational amplifier, and C1 and C2 are capacitors. The (+) input terminal of operational amplifier A is connected to the output terminal of preamplifier PA via resistive elements R2 and R1, and
It is connected to the common COM through a series circuit of switch S1 and capacitor C1. Operational amplifier A
The output terminal of is connected to the (-) input terminal of this amplifier, and is also connected to the connection point of resistive elements R1 and R2 via a series circuit of switch S2 and capacitor C2, and is connected to the input terminal of the A/D converter. It is connected.

Figure 4A is a diagram showing the timing at auto-zero of the device in Figure 3, and Figure 4B is a diagram showing the timing at the time of converter operation of the A/D converter, and T is a diagram showing one conversion operation cycle of the A/D converter. It shows. That is, in the apparatus shown in FIG. 3, auto-zero is performed during one conversion operation cycle of the A/D converter. The operation of the apparatus shown in FIG. 3 having such a configuration will be explained as follows.

Prior to the measurement operation, auto-zero is performed during one conversion operation cycle as shown in Figure 4 A. In this case, terminal a of the changeover switch SW is turned off and terminal b is turned on.
and turn off both switches S1 and S2. Then, the operational amplifier A in the low-pass filter LP operates as a voltage follower, and the sum of the offset voltages of the preamplifier PA and operational amplifier A is applied to the A/D converter and converted into digital data, and the converted signal is sent to the operational circuit OP. added and held by its memory function. During this auto-zero execution, the capacitors C1 and C2 maintain the voltage value immediately before auto-zero.

On the other hand, when the measurement operation shown in FIG.
Turn off, and turn on both switches S1 and S2.
Make it. Then, the input to be measured Vin is applied to the low-pass filter LP via the preamplifier PA. In this case, by turning on the switches S1 and S2, the low-pass filter LP operates as a second-order low-pass filter, as is well known.
The input to be measured Vin that has passed through such a low-pass filter LP is applied to the A/D converter, and the converted signal is applied to the arithmetic circuit OP and held by its memory function. The arithmetic circuit OP performs a calculation to subtract the sum of the offset voltages of the preamplifier PA and the operational amplifier OP during auto-zero held previously from the A/D converted value of the input to be measured Vin. As a result, the digital signal output from the arithmetic circuit OP is sent to the preamplifier PA, operational amplifier OP and A/D.
The offset voltage of the converter will be automatically canceled and a digital signal that exactly corresponds to the input to be measured Vin will be applied to the display or recording circuit DI to indicate its value.

In the device of the present invention with such a configuration, (1) Auto zero is performed in the first stage, so
High-performance amplifiers are not required for the preamplifier PA and operational amplifier OP.

(2) During auto-zero, the filter LP operates as a voltage follower, so auto-zero can be performed at high speed regardless of the response time of the low-pass filter LP.

It has the following characteristics.

[Brief explanation of drawings]

1 and 2 each show a conventional example of a voltage measuring device, FIG. 3 shows an embodiment of the voltage measuring device according to the present invention, and FIG. 4 explains the operation of the device shown in FIG. This is a timing chart for Vin...Input under test, PA...Preamplifier, LP
...Low pass filter, A/D...Analog
Digital converter, OP...Arithmetic circuit, DI...Display or recording circuit.

Claims (1)

[Claims for Utility Model Registration] A changeover switch for auto-zero that has a pair of terminals, one terminal connected to the input source under test and the other terminal connected to the common, and connected to this changeover switch via a preamplifier. low pass filter,
An analog-to-digital converter that converts the output of this low-pass filter into a digital signal, an arithmetic circuit that has a memory function and is supplied with the output of the analog-to-digital converter, and a display or record that displays or records the output of this arithmetic circuit. The low-pass filter is configured to operate as a voltage follower during auto-zero, and to operate as a second-order low-pass filter when the analog-to-digital converter converts the input to be measured into a digital signal, and the arithmetic circuit is configured to A voltage measuring device characterized in that it is configured to perform an operation of subtracting an offset voltage output by an analog-to-digital converter during auto-zero from a value of an input to be measured.