86
c
HAPTER
4:
Operation
Model 372 AC Resistance Bridge and Temperature Controller
4.4.7 Filter
The firmware filter is an integral part of the Model 372 resistance measurement. It is
directly related to measurement resolution and frequency response. The instrument
calculates readings 10 times per second no matter how the filter is configured. The
filter algorithm takes a running, linear average of 10 to 2000 readings depending on
settling time value. A new filtered reading is available over the interface ten times a
second and displayed at the normal display update rate. The linear average filter gives
faster settling times than other filter methods, but it has a poor frequency response.
If you are tracking periodic changes in resistance, be aware that using this filter at its
maximum settle time could mask those periodic changes.
When the firmware filter is disabled, the 200 ms time constant digital filter that is
used to generate unfiltered readings remains active.
Menu navigation:
Input Setup
Q
Input
(A, 1-16)
Q
Filter
(Off or On)
Default: Off
Interface command:
FILTER
4.4.7.1 Filter Settle Time
Settling time is a filter setting that drives the filter noise attenuation performance.
Filter settling time can be set from 1 to 200 s. TABLE 4-6 shows how the settling time
is related to the number of readings averaged and nominal noise attenuation. Filter
settling time begins after the pause time finishes on range or channel change.
Because the Model 372 uses a linear average, the measurement bandwidth is not a
simple function of frequency.
The settle time can be set between 0 s and 200 s. When the filter is on and the settle
time is zero, the instrument continues to calculate the filtered reading, despite the
unfiltered readings being displayed. This feature allows the user to quickly toggle
between 0 s and the desired settle time to observe the filtering effects on the reading.
Previously on the Model 370, the only way to do this was to turn the filter off then on.
This required the filter to re-populate with enough readings to meet the settle time
requirement before a settled reading was presented.
The Model 372 also contains a new feature that allows the filter settle time to be
adjusted without having to reset the filter. For example, if the reading is settled and
the settle time is changed from 60 s to 30 s, the newly configured filtered reading is
instantly provided. This is also applicable when increasing the settle time, assuming
enough time has elapsed from the time the reading originally settled to account for
the increase in settle time.
Menu navigation:
Input Setup
Q
Input
(A, 1 – 16)
Q
Filter Settle Time
Q
(0 – 200)
Settling times
Number of readings
Attenuation of noise
6 s
60
–10 dB
12 s
120
–16 dB
25 s
250
–22 dB
50 s
500
–28 dB
100 s
1000
–34 dB
200 s
2000
–40 dB
TABLE 4-6
Example of settling time as it relates to the number of
readings and attenuation of noise