Keithley DMM7510 Reference Manual page 159

Section 2: General operation Model DMM7510 7½ Digit Graphical Sampling Multimeter Reference Manual
DC and AC coupling
When the function is set to digitize voltage, you can set the signal coupling mode to be DC or AC.
When DC coupling is selected, the instrument measures all AC and DC content of the input signal.
Measure resolution is 1 µV to 1000 V.
If input impedance is set to automatic, DC coupling provides the lowest measurement noise and
highest isolation to measure loading, more than 10 GΩ for the 100 mV to 10 V ranges. The 100 V and
1000 V ranges terminate a 10 MΩ divider across HI and LO input terminals.
If input impedance is set to 10 MΩ, the 100 mV to 1000 V ranges are terminated with a 10 MΩ divider
across the HI and LO input terminals. This provides stable readings with open input terminals
(approximately 1 mV). The 100 V and 1000 V ranges have the best bandwidth (approximately
20 kHz). The 100 mV to 10 V ranges have consistent 600 kHz bandwidth with either input impedance
setting.
When AC coupling is selected, the instrument only measures the AC content of the input signal. For
AC coupling, the Model DMM7510 is optimized for all ranges with a consistent 600 kHz bandwidth.
AC coupling is terminated with an RC filter that can be set to fast or slow. Internally, R is fixed at
1.1 MΩ across the HI and LO terminals. This optimizes bandwidth and measure loading.
When the fast filter is selected, the Model DMM7510 is optimized to settle in 85 ms with AC
waveforms that contain less than or equal to 3 % DC content. You can achieve additional DC content
up to 400 V, but additional settling time may be needed to achieve rated accuracies.
Fast AC coupling is recommended for signals that are more than or equal to 1 kHz.
When the slow filter is selected, the Model DMM7510 is optimized to settle in 850 ms with AC
waveforms that contain less than or equal to 3 % DC content. You can achieve additional DC content
up to 400 V, but additional settling time may be needed to achieve rated accuracies.
Slow AC coupling is recommended for signals that are less than or equal to 3 Hz to 600 kHz.
You can properly amplitude-compensate the digitize voltage AC coupling readings by entering the
frequency of the input signal. For example, to digitize a 2 V peak-to-peak signal at 3 Hz, set the AC
coupling filter to slow and enter 3 Hz for the AC coupling frequency. This properly compensates the
attenuation from a low frequency signal across the internal RC filter.
You can compensate the reading for both slow and fast AC coupling filters. The allowed frequencies
are from 3 Hz to 1 MHz.
2-132 DMM7510-901-01 Rev. B / May 2015