Keysight DAQ970A User Guide

Additional settling time may be required when the DC level varies from sample to sample.
The internal DMM’s DC blocking circuitry has a settling time constant of 0.2 seconds. This settling time only
affects measurement accuracy when DC offset levels vary from sample to sample. If maximum meas-
urement speed is desired in a scanning system, you may want to add an external DC blocking circuit to
those channels with significant DC voltages present. This circuit can be as simple as a resistor and a capa-
citor.
AC filter Channel delay Settling time
200 Hz (Fast) AUTO 0.12 seconds
20 Hz (Medium) AUTO 1 second
3 Hz (Slow) AUTO 7 seconds
200 Hz (Fast) 0 0.02 seconds
20 Hz (Medium) 0 0.2 seconds
3 Hz (Slow) 0 1.5 seconds
DC blocking settling time (1 time constant) = 0.2 seconds.
Sources of Error in AC Voltage Measurements
Many of the errors associated with DC voltage measurements also apply to AC voltage measurements. Addi-
tional errors unique to AC voltage measurements are described in this section.
Crest Factor Errors (non-sinusoidal inputs)
A common misconception is that “since the internal DMM is true RMS, its sinewave accuracy specifications
apply to all waveforms.” Actually, the shape of the input signal can dramatically affect measurement accur-
acy. A common way to describe signal waveshapes is crest factor. Crest factor is the ratio of the peak value
to the RMS value of a waveform.
In general, the greater the crest factor the greater the energy contained in higher frequency harmonics. All
multimeters exhibit measurement errors that are crest factor dependent. Note that the crest factor errors
do not apply for input signals below 100 Hz when using the slow AC filter.
You can estimate the measurement error due to signal crest factor as shown below:
Total Error = Error
sine
+ Error
crest factor
+ Error
bandwidth
Where:
Error
sine
= DMM's sinewave accuracy
Error
crest factor
= DMM's crest factor
Error
bandwidth
= Estimated bandwidth error as shown below:
Where:
C.F. = Signal crest factor
F = Fundamental input signal frequency
BW = DMM's -3 dB bandwidth (1 MHz for the DAQ970A)
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200 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