want to measure the voltage in a circuit that has an impedance of, say, 1 Mohm.
connect this signal to a handheld DVM, such as a Wavetek 25XT DVM that has an input impedance of 10 Mohms, the
instrument's internal resistance will affect the voltage measured, as shown in
the figure. Use the ratio of the DVM's resistance to the total resistance to
determine the voltage you see on the meter.
Vmeter = Vunknown * [Rmeter / (Rmeter + Rsensor)]
I put a 1.04
Mohm resistor in series with a power supply and measured the voltage across the
supply output (1.96V) and then with the DVM in series (1.75V). That's about a
10-percent error. When I ran the test with an HP 3478A digital multimeter,
which has a 1010
-ohm input resistance, the reading from the power
supply (1.905V) matched the reading from the power supply with the resistor in
series. To reduce this type of error, Keithley Instruments recommends a
measurement instrument have an input resistance at least 100 times the
resistance of the voltage source.
high-quality DVM and source-measure units have an input resistance of 109
or more ohms. Oscilloscopes have input resistances of 1 Mohm. Unfortunately, specs for some plug-in data-acquisition boards and USB-plug-in
modules lack this information. Ask before you buy.
must include all shunt resistances when you consider a meter's input
resistance. These resistances can arise from leakage in test leads, dirty
connectors and contacts, closely spaced conductors, and even solder-flux
residue on a circuit board, which is particularly a problem on prototype PCBs.
Many manufacturers of instrument cables do not provide a cable-leakage
specification except for radio-frequency signals.
instruments also have a small bias current that flows in or out due to the
nature of internal circuits. This bias current flows through the circuit you
want to test and causes a small voltage error. The Keithley 2182A
nanovoltmeter, for example, specifies a bias current below 60 pA for -10 to +5V
and 120 pA for 5 to 10V. You can use Ohm's law to calculate the voltage that
can add to or subtract from the voltage you want to measure. If you still have
a 1 Mohm resistance in your circuit, the 60 pA bias current can cause an error
of 60 ΅V. For a 2V signal, that might seem minor, but keep in mind that offset
remains constant across a voltage range, so the error becomes more noticeable
on lower-voltage meter readings.