Increasing the input resistance of the voltmeter to 1 GΩ. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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Sometimes in amateur radio practice it is required to measure the voltage with a very small current - less than 1 μA. Similar measurements are also required in electrochemistry, when it is necessary to measure the potential difference across any electrodes. Direct connection of a digital multimeter, which is available for many radio amateurs, is unacceptable in this case, since the input resistance of most multimeters does not exceed 1 ... 10 MΩ. In some cases, it significantly affects the measurement accuracy.
In this case, a simple buffer amplifier with a high-resistance input divider will help. Naturally, such a divider will also require an amplifier with a very low input current, for example, an op-amp of the KR1409UD1 series with MOS transistors at the input (the input current of the KR1409UD1B is not more than 10 pA). Small input current and imported op-amps of the SA3140 series, also made using BiFET technology.
The use of the SA3140E op amp made it possible to assemble a high-precision amplifier (see the diagram in the figure), which is quite stable with temperature changes, with an input divider with a resistance of 1 GΩ. It allows you to measure voltages from a few millivolts to 10 V with a constant input resistance. To measure a higher voltage, you can further increase the resistance of the resistor R1. The use of other series of op amps may cause problems, in particular with zeroing the output.
(click to enlarge)
The buffer stage is assembled according to the scheme of a non-inverting amplifier with a transfer coefficient of about 20, almost equal to the division ratio of the input voltage divider. Setting up the device consists in setting the "zero" at the output of the op-amp with the input probes closed. The use of the CA3140E chip made it possible to balance the amplifier output with an accuracy of 1 mV. With a trimmer resistor R6, you can slightly change the gain and set the output of the op-amp to exactly the same voltage as at the input of the resistive divider.
Almost any DC voltmeter can be connected to the output of the amplifier. You can also connect a pointer magnetoelectric head with an arrow located in the middle of the scale, picking up a series resistor. Through the buffer amplifier, you can also observe a low-frequency signal with an amplitude of up to 10 V on the oscilloscope (this requires disconnecting the smoothing capacitor C1).
If the transfer coefficient of the cascade after the divider is set equal to one (op-amp in repeater mode), then it is permissible to apply voltage up to 250 V to such a high-resistance buffer cascade; in this case, the voltage at the input of the microcircuit will not exceed the maximum allowable value.
The author scored the resistance of the resistor R1 by connecting three high-resistance resistors with a resistance of 330 MΩ in series (for example, CMM, C3-14-0,125, etc.). It is desirable to mount these high-resistance resistors on the reference contacts with fluoroplastic insulation, and to minimize leakage at the input of the op-amp, it is advisable to surround pin 3 DA1 on the printed circuit board (made of fiberglass) with a protective foil ring connected to pin 2 of the microcircuit.
In the divider of the OOS OU circuit, you can use ordinary resistors - C2-23 or similar. Trimmer resistor R5 - SP5-2 (multi-turn), R6 - SP5-16. Capacitors can be used any, preferably small.
The buffer amplifier and divider are sensitive to interference, so they must be placed in a metal shield, which is connected to a common wire. The design and materials of the divider probe must provide high insulation resistance to minimize leakage current in this circuit.
Author: I.Korotkov, Bucha village, Kyiv region, Ukraine
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