Filter - Keithley 2182 User Manual

3-8 Range, Digits, Rate, and Filter

Filter

The Model 2182 has an analog filter and a digital filter. When Filter is enabled by pressing
the FILT key (FILT annunciator on), it assumes the combination of analog and digital filter
configuration for the present measurement function (DCV1, DCV2, TEMP, TEMP2). Filter
state (enabled or disabled) and configuration is saved by each function.
Analog filter
With the low-pass Analog Filter ON, the normal-mode noise rejection ratio of the instrument
is increased at 60Hz. This filters out noise induced by the power line. The Analog Filter
attenuates frequency at 20dB/decade starting at 18Hz.
A primary use of the Analog Filter is to keep the high-gain input stage of the Model 2182
from saturating due to the presence of high AC and DC voltage. Note, however, that the filter
only attenuates AC voltages for the 10mV range of the Model 2182.
The Analog Filter adds approximately 125msec of settling between A/D conversions. The
additional settling time may be required when using a high-impedance (≥100kΩ) source in the
test circuit.
The increased settling time causes the reading rate of the Model 2182 to be greatly reduced.
Therefore, if the Analog Filter is not needed, turn it OFF.
Digital filter
The digital filter is used to stabilize noisy measurements. The displayed, stored or transmitted
reading is a windowed-average of a number of reading conversions (from 1 to 100).
Digital filter characteristics
In general, the digital filter places a specified number of A/D conversions (Filter Count) into
a memory stack. These A/D conversions must occur consecutively within a selected reading
window (Filter Window). The readings in the stack are then averaged to yield a single filtered
reading. The stack can be filled in two ways (Filter Type); moving or repeating. The moving filter
keeps adding and removing a single A/D conversion from the stack before taking the average,
while the repeating filter only averages a stack that is filled with new A/D conversions.
Details on digital filter characteristics are provided as follow:
Filter count — The filter count specifies how many consecutive A/D conversions (within the
filter window) to place in the memory stack. When the stack is full, the A/D conversions are
averaged to calculate the final filtered reading. The filter count can be set from 1 to 100. Note
that with a filter count of 1, no averaging is done. However, only readings within the filter
window will be displayed, stored or transmitted.