Keithley 6485, User Manual

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4-4 Range, Units, Digits, Rate, and Filters Model 6485 and 6487 User’s Manual
Rate
The RATE key selects the integration time of the A/D converter, which is the period of time
the input signal is measured. The integration time affects the amount of reading noise, as
well as the ultimate reading rate of the instrument. The integration time is specified in
parameters based on a number of power line cycles (NPLC), where 1 PLC for 60Hz is
16.67msec (1/60) and 1 PLC for 50Hz (and 400Hz) is 20msec (1/50). The rate setting is glo-
bal for all ranges. Therefore, it does not matter what range is presently selected when you set
rate.
To select the rate, either press the RATE key to select SLOW (6 PLC, 60Hz; 5 PLC,
50Hz), MED (1 PLC), or FAST (0.1 PLC), or press CONFIG and then RATE, and then
enter the desired number of PLCs (0.01 to 60, 60Hz; 0.01 to 50, 50Hz).
To set the rate by remote, send :NPLC <plc> with an allowable range, or 0.01 to 60
(60Hz), or 0.01 to 50 (50Hz).
Model 6487 damping
High capacitance seen at the input will increase reading noise. This capacitance can be
attributed to a long input cable or to the capacitance of the source, or a combination of both.
Enabling damping (analog filtering) will reduce noise caused by high capacitance for current
measurements. However, damping will also slow down the response of the measurement.
To enable or disable damping from the front panel, simply press the Model 6487 DAMP
key. Note that the FILT annunciator will be on when damping is enabled. Via remote,
send DAMP ON or DAMP OFF. Note that the FILT annunciator is used for both the ana-
log damping filter and the two types of digital filters.
Filters
Filtering stabilizes noisy measurements caused by noisy input signals. The Model 6485/
6487 uses two types of filters: median and digital. The displayed, stored, or transmitted
reading is simply the result of the filtering processes. Note that both the median and digital
filters can be in effect at the same time.
With both filters enabled, the median filter operation is performed first. After the median
filter yields a reading, it is sent to the stack of the digital filter. Therefore, a filtered read-
ing will not be displayed until both filter operations are completed.
The settings for the filter are global. For the Model 6485, the MEDN key controls the
median filter and the AVG key controls the average filter. For the Model 6487 both filters
are controlled with the FILT key. When either the median or digital filter is enabled, the
FILT annunciator is on. Note that the FILT annunciator is used for both the digital filters
and the Model 6487 analog damping filter.