National Instruments NI PCI-6110 - Manual (Page 88)

Appendix C Common Questions
© National Instruments Corporation C-3 NI PCI-6110/6111 User Manual
Analog Input and Output
Why is there a minimum sampling rate on the NI PCI-6110/6111?
The NI PCI-6110/6111 makes use of a pipelined ADC in order to achieve
its high sampling rates. Sampling at rates below 20 kS/s can result in
improper digitalization, which would appear as noise in the acquired data.
I connected a DIFF input signal, but the readings are random and drift
rapidly. What’s wrong?
Check the ground reference connections. The signal may be referenced to
a level that is considered floating with reference to the device ground
reference. Even if you are in differential mode, the signal must still be
referenced to the same ground level as the device reference. There are
various methods of achieving this while maintaining a high common-mode
rejection ratio (CMRR). These methods are outlined in Chapter 4,
Connecting Signals .
I’m using the DACs to generate a waveform, but I discovered with a
digital oscilloscope that there are glitches on the output signal. Is this
normal?
When it switches from one voltage to another, any DAC produces glitches
due to released charges. The largest glitches occur when the most
significant bit (MSB) of the D/A code switches. You can build a low-pass
deglitching filter to remove some of these glitches, depending on the
frequency and nature of the output signal.
Can I synchronize a one-channel AI data acquisition with a
one-channel AO waveform generation on the NI PCI-6110/6111?
Yes. One way to accomplish this is to use the waveform generation timing
pulses to control the analog input data acquisition. To do this, follow steps 1
through 4 below, in addition to the usual steps for data acquisition and
waveform generation configuration.
1. Enable the PFI5 line for output, as follows:
• If you are using NI-DAQ, call
Select_Signal(deviceNumber, ND_PFI_5,
ND_OUT_UPDATE, ND_HIGH_TO_LOW) .
• If you are using LabVIEW, invoke Route Signal.vi with signal
name set to PFI5 and signal source set to AO Update.