Specifications

Thus far in the discussion, it has been assumed that the low-level channel measured
after a high-level channel has presented a low impedance path to discharge the
A/D’s stray capacitances (path was the thermocouple wire). The combination of a
resistance measurement through a VT1501A Direct Input SCP presents a much
higher impedance path. A very common measurement like this would be the
temperature of a thermistor. If measured through a Direct Input SCP, the source
impedance of the measurement is essentially the value of the thermistor (the output
impedance of the current source is in the gigaohm region). Even though this is a
higher level measurement than the previous example, the settling time can be even
longer due to the slower discharge of the stray capacitances. The simple answer
here is to always use an SCP that presents a low impedance buffered output to the
VT1419A’s Range Amp and A/D. The VT1503A, 08A, 09A, 10A, 12A, and 14A
through 17A SCPs all provide this capability.
Checking for
Problems
The method used to quickly determine if any system channels need more settling
time is to simply apply some settling time to every channel. Use this procedure:
1. First, run the system to make a record of its current measurement
performance.
2. Then, use the SAMPle:TIMer command to add a significant settling delay to
every measurement in the scan list. Take care that the sample time
multiplied by the number of channels in the scan list doesn’t exceed the time
between triggers.
3. Now, run the system and look primarily for low level channel measurements
(like thermocouples) whose dc value changes somewhat. If channels are
found that respond to this increase in sample period, it may be noticed that
these channels are returning 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 some improvement is seen, increase the sample period again and perform
another test. When the sample period is increased and no improvement is
seen, the maximum settling delay has been found 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.
Fixing the Problem If the system scans fast enough with the increased sample period, the problem is
solved. The system is only running as fast as the slowest channel allows but, if it's
fast enough, that’s OK. If on the other hand, getting quality readings has slowed the
scan rate too much, there are two other methods that will, either separately or in
combination, have the system making good measurements as fast as possible.
Use Amplifier SCPs Amplifier SCPs can remove the need to increase settling delays. How? Each gain
factor of four 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 VT1419A are available with gains of 0.5, 8, 16, 64, and 512. Return
now to the earlier difficult measurement example of a where one channel is
measuring 15.5 volts on the 16 volt range and the next a thermocouple on the
0.0625 range. If the 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
Programming the VT1419A Multifunction
Plus
Settling Characteristics
102 Chapter 3
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