Measurement Computing USB-1616HS-BNC User Manual page 39

USB-1616HS-BNC User's Guide
Due to the pipelined architecture of the analog-to-digital converter system, the setpoint cannot be evaluated
until 2 µs after the ADC conversion.
Analog outputs have a 3µs delay due to the shifting of the digital data out to the D/A converter which takes
1 µs, plus the actual conversion time of the D/A converter (another 2 µs, worst case). Going back to the above
example, if the setpoint for analog input Channel 2 required a DAC update it would occur 5µs after the ADC
conversion for Channel 2, or 6 µs after the start of the scan.
When using setpoints to control any of the DAC outputs, latencies may increase if attempting to stream data to
DACs or pattern digital output at the same time. The increased latency can be as long as the period of the DAC
pacer clock. For these reasons, avoid streaming outputs on any DAC or pattern digital output when using
setpoints to control DACs.
The detection circuit works on data that is put into the acquisition stream at the scan rate. This data is acquired
according to the pre-acquisition setup (scan group, scan period, etc.) and returned to the PC. Counters are
latched into the acquisition stream at the beginning of every scan. The actual counters may be counting much
faster than the scan rate, and therefore only every 10
As a result, you can set a small detection window on a totalizing counter channel and have the detection setpoint
"stepped over" since the scan period was too long. Even though the counter value stepped into and out of the
detection window, the actual values going back to the PC may not. This is true no matter what mode the counter
channel is in.
When setting a detection window, keep a scan period in mind. This applies to analog inputs and counter inputs.
Quickly changing analog input voltages can step over a setpoint window if not sampled often enough.
There are three possible solutions for overcoming this problem:

Shorten the scan period to give more timing resolution on the counter values or analog values.
 Widen the setpoint window by increasing limit A and/or lowering limit B.
 A combination of both solutions (1 and 2) could be made.
th th th
, 100 , or n count shows up in the acquisition data.
39
Functional Details