Fluke 8842a instruction manual Measuring Input Bias Current Error

Measurement Tutorial 4

DC VOLTAGE MEASUREMENT

The input impedance of the 8842A is 10 MΩ in the 200V and 1000V dc ranges, and is greater than 10,000 MΩ in the 20 mV, 200 mV, 2V, and 20V ranges. Therefore, for the 8842A, circuit loading error is less than 0.01% as long as the source impedance is less than 1 MΩ in the 20 mV, 200 mV, 2V, and 20V ranges, and less than 1 kΩ in the 200V and 1000V ranges. The exceptionally high input impedance on the 20V dc range allows high-accuracy readings in CMOS and high-impedance analog circuitry.

NOTE

Input protection circuitry can reduce the input impedance to as low as 100 kΩ when the input is overrange. This may also occur momentarily when the instrument autoranges to a higher range.

4-4. Input Bias Current Error

Input bias current error occurs because a voltmeter’s input bias current always changes the voltage of the circuit under test. However, the error is significant only when measuring voltages in circuits with very high source impedance. The error can be measured as shown in Figure 4-2.

f4-02.wmf

Figure 4-2. Measuring Input Bias Current Error

With the 8842A, it is easy to correct for this error using the OFFSET button:

1.Select the VDC function and the desired range.

2.Connect the 8842A INPUT terminals to a resistor which matches the source impedance of the circuit to be tested.

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