HP VXI 75000 C Series, User'S And Scpi Programming Manual

Page: 110 / 438

110 Programming the HP E1415 for PID Control Chapter 3
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 you determine if your system presents any of these
problems and how best to eliminate them or reduce their effect.
Background
While the HP E1415 can auto-range, measure, and convert a reading to
engineering units as fast as once every 10
µ
s, 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 HP E1415’s Analog-to-Digital converter
and its Range Amplifier, this situation is the most difficult to measure. For
example lets 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 .0625 volt range. On this
range, the resolution is now 1.91
µ
volt 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, we’ve 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 an HP E1501 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 gigohm 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 HP E1415’s Range Amp and A/D. The HP E1503, 8, 9, 10, 12, and 14
through 17 SCPs all provide this capability.
Checking for
Problems
The method we’ll use to quickly determine if any of your system’s channels
needs more settling time is to simply apply some settling time to every
channel. Use this procedure:
1. First run your 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 your system and look primarily for low level channel
measurements (like thermocouples) who’s DC value changes
somewhat. If you find channels that respond to this increase in sample