Appendix C
Common Questions
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National Instruments Corporation
C-3
Analog Input and Output
I am using the device in differential AI mode, and I have connected a
differential input signal, but the readings are random and drift rapidly.
What is 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. You can use
one of various methods to achieve ground reference while maintaining
a high common-mode rejection ratio (CMRR). Refer to Chapter 4,
, for more information.
I am 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 lowpass
deglitching filter to remove some of these glitches, depending on the
frequency and nature of the output signal.
Can I programmatically enable channels on the NI 6013/6014 to
acquire in different modes? For example, can I configure ACH0 in
DIFF input mode and ACH1 in NRSE input mode?
Channels on the NI 6013/6014 can be enabled to acquire in different
modes, but different pairs of channels are used in different modes. In the
example configuration given above, ACH0 and ACH8 are configured in
DIFF mode and ACH1 and AISENSE are configured in NRSE mode.
In this configuration, ACH8 is not used in a single-ended configuration.
To enable multimode scanning in LabVIEW, use the coupling and input
configuration cluster input of the AI Config VI. This input has a one-to-one
correspondence with the channel array input of the AI Config VI. You must
list all channels either individually or in groups of channels with the same
input configuration. For example, if you want ACH0 to be differential, and
ACH1 and ACH2 to be NRSE, Figure C-1 demonstrates how to program
this configuration in LabVIEW.
