Resistances Affect Voltage Measurements

DN Staff

June 2, 2011

2 Min Read
Resistances Affect Voltage Measurements

Suppose youwant to measure the voltage in a circuit that has an impedance of, say, 1 Mohm.?If youconnect this signal to a handheld DVM, such as a Wavetek 25XT DVM that has an input impedance of 10 Mohms, theinstrument's internal resistance will affect the voltage measured, as shown inthe figure. Use the ratio of the DVM's resistance to the total resistance todetermine the voltage you see on the meter.

Vmeter = Vunknown * [Rmeter / (Rmeter + Rsensor)]

I put a 1.04Mohm resistor in series with a power supply and measured the voltage across thesupply output (1.96V) and then with the DVM in series (1.75V). That's about a10-percent error. When I ran the test with an HP 3478A digital multimeter,which has a 1010-ohm input resistance, the reading from the powersupply (1.905V) matched the reading from the power supply with the resistor inseries. To reduce this type of error, Keithley Instruments recommends ameasurement instrument have an input resistance at least 100 times theresistance of the voltage source.

Mosthigh-quality DVM and source-measure units have an input resistance of 109or more ohms. Oscilloscopes have input resistances of 1 Mohm. Unfortunately, specs for some plug-in data-acquisition boards and USB-plug-inmodules lack this information. Ask before you buy.

You alsomust include all shunt resistances when you consider a meter's inputresistance. These resistances can arise from leakage in test leads, dirtyconnectors and contacts, closely spaced conductors, and even solder-fluxresidue on a circuit board, which is particularly a problem on prototype PCBs.Many manufacturers of instrument cables do not provide a cable-leakagespecification except for radio-frequency signals.

Voltage-measuringinstruments also have a small bias current that flows in or out due to thenature of internal circuits. This bias current flows through the circuit youwant to test and causes a small voltage error. The Keithley 2182Ananovoltmeter, for example, specifies a bias current below 60 pA for -10 to +5Vand 120 pA for 5 to 10V. You can use Ohm's law to calculate the voltage thatcan add to or subtract from the voltage you want to measure. If you still havea 1 Mohm resistance in your circuit, the 60 pA bias current can cause an errorof 60 muV. For a 2V signal, that might seem minor, but keep in mind that offsetremains constant across a voltage range, so the error becomes more noticeableon lower-voltage meter readings.

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