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, you may want to add an external DC blocking circuit for circuitry 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.
If you have the DIG (digitizing) option, simply select [Acquire] > Digitize to select the fastest DCV or DCI measurements. If you do not have the DIG option, or for resistance measurements, to make the fastest (but least accurate) DC or resistance measurements:
Refer to the particular measurement type in Measurements for more information on the above functions.