How to minimize the voltage offset of the instrument - News - Global IC Trade Starts Here Free Join

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SMD aluminum electrolytic capacitor

1 internal offset

The nanovoltmeter and nanovolt preamplifier rarely indicate a zero reading when no voltage is applied to the input. This is because there is inevitably a voltage bias at the input of the instrument. The input terminals of the instrument are connected by a “short-circuit head”, and the input to the meter is zero. You can also use the zero-zero control on the front panel of the instrument or use the computer control to set the zero. If the short-circuit point at the input has a very low thermoelectromotive force, the input noise and the drift of the zero point over time can be verified in this way. Short-circuiting at the input is usually done with clean, pure copper wires. However, the zero obtained in this way can only be used for verification purposes and is not suitable for use in the final application of the instrument.

If an instrument is used to measure the small voltage drop produced by a current flowing through a resistor, the appropriate zero point can be obtained using the method described below. First, preheat the instrument for a specified period of time, usually for 1-2 hours. During this time, the connection between the device under test and the instrument should be made so that current does not flow through the device under test to minimize and stabilize the temperature gradient. Then the zeroing operation should be performed. On some instruments, this is done by pressing the REL (relative) or ZERO button. The reading of the instrument should now be zero. When the test current is applied, the instrument will indicate the resulting voltage drop.

In some applications, the measured voltage is always present, so the above method cannot be used. For example, the best way to measure the voltage difference between two standard batteries is to reverse the connection of the instrument to the standard battery and then average the readings from the two measurements. The same method can be used to eliminate the bias voltage when measuring the output of the differential thermocouple. This method is the same as the method to eliminate the thermoelectromotive force detailed in the section entitled “Reverse Method to Counteract Thermo Electromotive Force”, see Figure 3-4.

2 zero drift

Zero drift is a change in the reading of the voltmeter over time in the absence of an input signal (measured when the input is shorted). The zero drift of the instrument is almost entirely determined by its input stage. Most nanovoltmeters perform some form of chopping or modulation on the input signal to minimize zero drift of the instrument.

The zero reading of the instrument will also change as the ambient temperature changes. This effect is commonly referred to as the voltage bias temperature coefficient.

3 transient temperature

In addition, the instrument may exhibit transient temperature effects. After a jump in ambient temperature, the voltage offset may change relatively large. The amount of change may exceed the published instrument specifications. The voltage bias then gradually decreases and eventually settles to a value close to its original value. This is due to the fact that the junctions of different metals in the instrument have different thermal time constants. One node has quickly adjusted to the new ambient temperature, while the other node has changed slowly, resulting in a transient voltage bias change.

To minimize voltage bias caused by junction ambient temperature changes, measurements should be taken in a constant temperature environment and the circuit thermally shielded to slow down the temperature change.