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Fixing the Problem
Use Amplifier SCPs
Chapter 3
period, you may also notice that these channels may return slightly
quieter measurements as well. The extra sample period reduces or
removes the affected channels coupling to the value of the channel
measured just before it.
4. If you see some improvement, increase the sample period again and
perform another test. When you increase the sample period and no
improvement is seen, you have found the maximum settling delay
that any single channel requires.
5. If the quality of the measurements does not respond to this increase in
sample period, then inadequate settling time is not likely to be
causing measurement problems.
If your system scans fast enough with the increased sample period, your
problem is solved. Your system is only running as fast as the slowest channel
allows but if its fast enough that's OK. If on the other hand, getting quality
readings has slowed your scan rate too much, there are two other methods
that will, either separately or in combination, have your system making good
measurements as fast as possible.
Amplifier SCPs can remove the need to increase settling delays. How? Each
gain factor of 4 provided by the SCP amplifier allows the Range Amplifier
to be set one range higher and still provide the same measurement
resolution. Amplifier SCPs for the HP E1415 are available with gains of .5,
8, 16, 64, and 512. Lets return to our earlier example of a difficult
measurement where one channel is measuring 15.5 volts on the 16 volt
range, and the next a thermocouple on the .0625 range. If our thermocouple
channel is amplified through an SCP with a gain of 16, the Range Amplifier
can be set to the 1 volt range. On this range the A/D resolution drops to
around 31 µvolt per LSB so the stray capacitances discharging after the 15.5
volt measurement are now only one sixteenth as significant and thus reduce
any required settling delay. Of course for most thermocouple measurements
we can use a gain of 64 and set the Range Amplifier to the 4 volt range. At
this setting the A/D resolution for one LSB drops to about 122 µvolts and
further reduces or removes any need for additional settling delay. This
improvement is accomplished without any reduction of the overall
measurement resolution.
NOTE
Filter-amplifier SCPs can provide improvements in low-level signal
measurements that go beyond just settling delay reduction. Amplifying the
input signal at the SCP allows using less gain at the Range Amplifier (higher
range) for the same measurement resolution. Since the Range Amplifier has
to track signal level changes (from the multiplexer) at up to 100 KHz, its
bandwidth must be much higher than the bandwidth of individual
filter-amplifier SCP channels. Using higher SCP gain along with lower
Range Amplifier gain can significantly increase normal-mode noise
rejection.
Programming the HP E1415 for PID Control
111
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