Cappels' project pages
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.
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
Over-voltage protected on the input
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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.
How Well Does It Work?
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
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.
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
Contents © 2017, 2018 Richard Cappels All Rights Reserved. Find updates
First posted in July, 2018
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