Calculating DMM Loading on a High Resistance Divider

A high-resistance divider may produce the expected ADC result but a lower voltage when touched with a handheld meter. The meter is a load connected across the lower resistor. Its input resistance changes the divider ratio, and any input capacitance also changes how quickly the reading settles after connection.

Calculate the loaded circuit

Use the documented input resistance for the exact voltage function and range. The NI ELVIS II Series manual explains source-resistance loading in its DMM theory of operation. Some instruments provide different resistance modes or change input behavior across ranges. A generic assumption of infinite impedance defeats the purpose of the calculation.

For an illustrative divider made from two 1-megohm resistors across 5 V, the unloaded midpoint is 2.5 V. Connecting a 10-megohm meter places 10 megohms in parallel with the lower 1 megohm, reducing that leg to approximately 0.909 megohm. The measured midpoint is then about 2.381 V, a roughly 4.8-percent reduction. This error is much larger than typical precision resistor tolerance.

Check other connected loads

Include the ADC input, oscilloscope probe, protection network, leakage across the board, and any test fixture. Two instruments attached simultaneously load the node together. If a buffer is present, measure on the correct side of it; measuring the raw divider and the buffered output answers different questions.

• Record the meter model, function, range, and impedance setting.
• Calculate the node’s Thevenin resistance before adding the instrument.
• Repeat with an independently known input resistance when available.
• Allow the divider and instrument capacitance to settle after connection.
• Check that meter loading does not change a comparator or wakeup state.

Choose a measurement-compatible design

Reducing both divider resistances reduces loading sensitivity but increases continuous power consumption. A buffer can preserve a high divider resistance, although its bias current, leakage, input range, and quiescent current must be budgeted. A switched divider saves average power but creates a separate acquisition-settling problem.

Do not calibrate the divider against a loaded voltage and then remove the meter without accounting for the resulting change. Combine this calculation with divider ratio tracking and ADC input leakage and settling. Store the predicted loaded voltage alongside the bench result so a correct circuit is not mistakenly reworked to satisfy an intrusive measurement.