Keysight DAQ970A User Guide

Where:
R
L
= Lead resistance
R
i
= DMMisolation resistance
C
i
= DMMisolation capacitance
V
ground
= Ground noise voltage
I = Current flow caused by
To minimize ground loop errors:
If V
ground
is a DC voltage, keep R
L
small compared to R
i
.
If V
ground
is an AC voltage, keep R
L
small compared to Z, and set the DMM's integration time to 1 PLC or
greater.
Low-level AC measurement errors
When measuring AC voltages less than 100 mV, be aware that these measurements are especially sus-
ceptible to errors introduced by extraneous noise sources. An exposed test lead will act as an antenna and
the internal DMM will measure the signals received. The entire measurement path, including the power line,
act as a loop antenna. Circulating currents in the loop will create error voltages across any impedances in
series with the instrument’s input. For this reason, you should apply low-level AC voltages to the instrument
through shielded cables. You should also connect the shield to the input LO terminal.
Be sure to minimize the area of any ground loops that cannot be avoided. A high-impedance source is
more susceptible to noise pickup than a low-impedance source. You can reduce the high-frequency imped-
ance of a source by placing a capacitor in parallel with the instrument’s input terminals. You may have to
experiment to determine the correct capacitance value for your application.
Most extraneous noise is not correlated with the input signal. You can determine the error as shown below:
Correlated noise, while rare, is especially detrimental. Correlated noise will always add directly to the input
signal. Measuring a low-level signal with the same frequency as the local power line is a common situation
that is prone to this error.
You should use caution when switching high-level and low-level signals on the same module. It is possible
that high-level charged voltages may be discharged onto a low-level channel. It is recommended that you
either use two different modules or separate the high-level signals from the low-level signals with an
unused channel connected to ground.
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182 Keysight DAQ970A User's Guide

Keysight DAQ970A Specifications

General

Scan Rate	Up to 450 channels/sec
Connectivity	USB, LAN, GPIB (optional)
Display	4.3-inch color LCD
Operating Temperature	0 °C to 50 °C
Number of slots	3 slots
Measurement Types	DCV, ACV, DCI, ACI, 2-wire resistance, 4-wire resistance, frequency, temperature, capacitance
Voltmeter	6.5 digit
Resolution	22 bits
Memory	Non-volatile memory