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A X1,000 DC Amplifier With Submicrovolt Input Offset adjustment
This was designed to act as a preamp to extend the resolution of a DVM with 100 microvolt resolution to 100 nanovolts resolution and allows for offset adjustment.


 
Figure 1. The X10 output stage with
offset adjustment can be seen behind the battery.


Features
Gain is 1,000X DC
Battery powered -Change the battery when it gets down to 2.3 volts
Input range 100 unV to 2.5 mV
Offset Adjustable
Over-voltage protected on the input


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Motivation

I was dissatisfied with my capability for adjusting the output offset voltage of an adjustable voltage reference because the most sensitive scales on my digital voltmeters only went down to 100 microvolts in the last digit. Having some ICL7650 chopper stabilized opamps that somebody gave to me some decades ago I decided to press one of them into use by making a preamp for my voltmeters.


Figure 2. A simple X 1,000 DC amplifier.
The Circuit

The circuit is very straight forward. Since the amplifier's input and output was going to be working near ground I was concerned that if I use a single supply opamp, noise clipping at ground would cause an offset of the output when averaged by a digital voltmeter.  That meant a negative power supply is necessary. An ICL7660 positive to negative voltage converter handled that need well. The power supply is no more than a 3V button cell, the ICL7660 and some capacitors. Current drain with the particular parts I used was lower than I expected, measuring only 1.05 milliamps. With the battery rated at 225 milliamp-hours, and that I will not use this frequently, I should be able to use a single battery for nearly the shelf life of the battery.

The amplifier itself is configured as a classic non-inverting amplifier so as to have a high input impedance. The reason for the funny values on the approximately 10:1 divider is because I could not find the 100k 1% resistors I was sure I had bought and I don't have any 1.06k resistors. A more rational looking divider would be 100K and 1.02K.That gets amplification of X100. The second stage provides another X10 gain so that 1 microvolt input is amplified to 1 millivolt on the output.

The input circuit is composed of a single pole low pass filter at 3 kHz, this is not meant to be a noise filter but is intended to slow any high voltage pulses that might be accidentally connected to the input, giving the 1N916 clipping diodes time to work. Note: in the case of electrostatic discharge measurement of pulse risetime is limited by the test equipment.  Since the full scale input voltage is 2.5 millivolts the diodes do not measurably affect the linearity of the input signal.

A clamp signal on pin 9 of the ICL7650 helps the amplifier recover quickly from overloads. The two capacitors connected to the ICL7650 pins labeled C1 and C2 Store the offset nulling values for the internal main amplifier and the nulling amplifier.

There is about 5 millivolts of sawtooth from the power supply on the output of the ICL7650 on the amplifier I made, and since the amplifier is linear over its range of operation the noise is averaged out by the low pass input filter on the digital voltmeter.
 
How Well Does It Work?

It works fine. The output offset can be set to with 100 nanovolts of ground on average but the adjustment is sensitive and is easily thrown off.

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First posted in July, 2018

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