High-Speed Measurements
Making High–Speed AC Measurements
The multimeter's AC voltage and AC current functions implement three low–frequency filters. These filters allow you to
trade–off minimum measured frequency for faster reading speed. The FAST filter settles in 0.025 seconds, and is
useful for frequencies above 200 Hz. The MEDIUM filter settles in 0.625 seconds for voltage and 0.25 seconds for
current, and is useful for measurements above 20 Hz. The SLOW filter settles in 2.5 seconds for voltage and 1.66
seconds for current, and is useful for frequencies above 3 Hz.
With a few precautions, you can perform AC measurements at speeds up to 500 readings per second. Use manual
ranging to eliminate autoranging delays. By setting the trigger delay to 0, the FAST, MEDIUM, and SLOW filters allow
up to 500, 150, and 50 readings per second, but with reduced accuracy because the filter may not fully settle. If the
sample–to–sample levels are similar, little settling time is required for each new reading. Under this specialized
condition, the MEDIUM filter provides reduced accuracy results at 20 readings per second, and the FAST filter provides
reduced accuracy results at 200 readings per second.
AC Filter Filter Bandwidth Settling Time (s) Full Settling (rdgs/s) Partial Settling Maximum rdgs/s
ACV ACI ACV ACI ACV/ACI ACV/ACI
Slow 3 Hz 2.5 1.67 0.4 0.6 2 50
Medium 20 Hz 0.63 0.25 1.6 4 20 150
Fast 200 Hz 0.025 0.025 40 40 200 500
In applications where sample–to–sample levels vary widely, but the DC offset level does not change, the MEDIUM filter
settles at 2 to 4 readings per second (depending upon the lowest frequency component in the waveform) as shown in
the following table:
Lowest Frequency Component 20 Hz 50 Hz 100 Hz 200 Hz
AC Current (allowable rate in rdgs/s) 4 4 4 4
AC Voltage (allowable rate in rdgs/s) 2 3 4 4
For AC voltage, additional settling time may be required when the DC level varies from sample to sample. The default
sample delays allow for a DC level change of 3% of range for all filters. If the DC level change exceeds these levels,
additional settling time is required. The multimeter'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
measurement 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 capacitor.
For AC current, additional settling time is not required when the DC level varies sample to sample.
Making High–Speed DC and Resistance Measurements
The multimeter incorporates an automatic zero measurement procedure (autozero) to eliminate internal thermal EMF
and bias current errors. Each measurement actually consists of a measurement of the input terminals followed by a
measurement of the internal offset voltage. The internal offset voltage error is subtracted from the input for improved
accuracy. This compensates for offset voltage changes due to temperature. For maximum reading speed, turn
autozero off. This will more than double your reading speeds for DC voltage, resistance, and DC current functions.
Autozero does not apply to other measurement functions.
96 Agilent Truevolt Series DMM Operating and Service Guide
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