VXi VT1529A/B 사용자 설명서

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.

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.

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.