Keysight DAQ970A Guia Del Usuario página 249

4 Measurement Tutorials
Sources of error in frequency and period measurements
The internal DMM's AC voltage measurement section performs input signal conditioning. All frequency counters are
susceptible to errors when measuring low-voltage, low-frequency signals. The effects of both internal noise and
external noise pickup are critical when measuring "slow" signals. The error is inversely proportional to frequency.
Measurement errors will also occur if you attempt to measure the frequency (or period) of an input following a DC
offset voltage change. You must allow the internal DMM's input DC blocking capacitor to fully settle before making
frequency measurements.
If the external noise pickup becomes large enough to exceed the hysteresis of the measuring circuitry, the frequency
function may actually become unusable. External shielding and low-pass filtering may help.
Keysight DAQ970A/DAQ973A User's Guide 249