Measurement Computing Personal Daq/3000 Series - Manual (Page 16)

1-8 Daq Systems and Device Overviews 988093 Personal Daq/3000 Series User’s Manual
Example 4: Sampling digital inputs for every analog sample in a scan group
The figure below shows another acquisition. The scan is programmed pre-acquisition and is made up of 6
analog channels (Ch0, Ch2, Ch5, Ch11, Ch22, Ch25) and 4 digital channels (16-bits of digital input, 3
counter inputs.) Each of the analog channels can have a different gain and each of the counter channels
can be put into a different mode (totalizing, pulsewidth, encoder, etc.) The acquisition is triggered and the
samples stream to the PC via USB2. Each analog channel requires one microsecond of scan time therefore
the scan period can be no shorter than 6 us for this example. All of the digital channels are sampled at the
start of scan and do not require additional scanning bandwidth as long as there is at least one analog
channel in the scan group. The 16-bits of digital input are sampled for every analog sample in the scan
group. This allows up to 1MHz digital input sampling while the 1MHz analog sampling bandwidth is
aggregated across many analog input channels. The scan period can be made much longer than 6 us, up to
19 hours. The maximum scan frequency is one divided by 6us or 166,666 Hz. Note that digital input
channel sampling is not done during the “dead time” of the scan period where no analog sampling is being
done either.
If the 3 counter channels are all returning 32 bit values and the digital input channel is returning a 16 bit
value, then 18 samples are being returned to the PC every scan period, each sample being 16-bits. 32-bit
counter channels are divided into two 16-bit samples, one for the low word and the other for the high word.
If the maximum scan frequency is 166,666 Hz then the data bandwidth streaming into the PC is
3 MSamples per second. Some slower PCs may have a problem with data bandwidths greater than
6 MSamples per second. All Personal Daq/3000 Series devices have an onboard 1 Msample buffer for
acquired data.
Analog Input & Channel Expansion
The Personal Daq/3000 series has a 16-bit, 1-MHz A/D coupled with 16 single-ended, or 8 differential
analog inputs. Seven software programmable ranges provide inputs from ±10V to ±100 mV full scale.
Each channel can be software-configured for a different range, as well as for single-ended or differential
bipolar input.
Adding additional analog input channels to the /3000 series modules is easy with the optional PDQ30
expansion module. The PDQ30 can connect directly to the Personal Daq’s female DSUB25 connector, or
be connected via cable (CA-96A). PDQ30 adds an additional 48 single-ended or [24 differential-ended]
analog inputs for a total channel capacity of 64 single-ended [or 32 differential] inputs.
Measurement speed of PDQ30 channels is the same 1 Msample/s exhibited by the /3000 module channels.