VXI VT1422A - Manual (Page 159)

Programming the VT1422A for Data Acquisition and Control 157 Chapter 4
Settling Characteristics
Some sequences of input signals, as determined by their order of appearance
in a scan list, can be a challenge to measure accurately. This section is
intended to help determine if a system presents any of these problems and
how best to eliminate them or reduce their effect.
Background
While the VT1422A can auto-range, measure and convert a reading to
engineering units as fast as once every 10 µs, measuring a high-level signal
followed by a very low-level signal may require some extra settling time.
As seen from the point of view of the VT1422A’s Analog-to-Digital
converter and its Range Amplifier, this situation is the most difficult to
measure. For example, let’s look at two consecutive channels; the first
measures a power supply at 15.5 volts, the next measures a thermocouple
temperature. First the input to the Range Amplifier is at 15.5 volts (near its
maximum) with any stray capacitances charged accordingly, then it
immediately is switched to a thermocouple channel and down-ranged to its
0.0625 volt range. On this range, the resolution is now 1.91 µV per Least
Significant Bit (LSB). Because of this sensitivity, the time to discharge these
stray capacitances may have to be considered.
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 VT1422A’s Range Amp and A/D. The VT1503A, 8A, 9A, 10A, 12A,
and 14A through 17A SCPs all provide this capability.
Checking for
Problems
This method can be used to quickly determine if any of the system’s
channels need more settling time by simply applying 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 also be observed that these channels 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.