Measurement Computing Personal Daq/3000 Series - Manual (Page 14)

1-6 Daq Systems and Device Overviews 988093 Personal Daq/3000 Series User’s Manual
Example 2: Analog channel scanning of voltage and temperature inputs
The figure below shows a more complicated acquisition. The scan is programmed pre-acquisition and is
made up of 6 analog channels (Ch0, Ch2, Ch5, Ch11, Ch22, Ch23.) Each of these analog channels can
have a different gain. Two of the channels (22 and 23) are from a PDQ30 expansion module. These two
channels can be programmed to directly measure thermocouples. In this mode, oversampling is
programmable up to 16384 oversamples per channel in the scan group. When oversampling is applied, it is
applied to all analog channels in the scan group, including temperature and voltage channels. (Digital
channels are not oversampled.) If the desired number of oversamples is 256 then each analog channel in
the scan group will take 256 microseconds, the returned 16-bit value represents an average of 256
consecutive 1us samples of that channel. The acquisition is triggered and 16-bit values (each representing
an average of 256) stream to the PC via USB2.
Since two of the channels in the scan group are temperature channels, the acquisition engine will be
required to read a cold-junction-compensation (CJC) temperature every scan. In fact, depending upon
which PDQ30 channels are being used for temperature, there may be a CJC temperature required for each
temperature channel in the scan. Each 4 channel terminal block of the PDQ30 shares one CJC so if all
temperature channels are grouped on one (of the six) terminal blocks, then only one CJC temperature
measurement will need to be made per scan. For every PDQ30 terminal block that is measuring at least
one temperature channel, one additional CJC temperature measurement will be automatically added to the
scan group. This increases the scan period and reduces the maximum scanning frequency.
Programmable
Averaging
up to 16384
In this example, the desired number of oversamples is 256, therefore each analog channel in the scan group
requires 256 microseconds to return one 16-bit value. The oversampling is also done for CJC temperature
measurement channels. The minimum scan period for this example is therefore 7 X 256 µs, or 1792
microseconds. The maximum scan frequency is the inverse of this number, 558 Hz.
Channels 0 through 7 of the Personal Daq/3000 can be used to measure temperature in place of voltage.
There are three CJC channels per analog input terminal block. When all 8 differential analog inputs are
used for temperature 6 CJC channels must be included as part of the scan group. This means when the
device is measuring all 8 temperature inputs and using oversampling of 256, the minimum scan period is
14 x 256 µs, or 3584 µs.
Autozero may also be employed. This adds more channels to the scan group and further reduces the
maximum scan frequency. Auto zero channels read a shorted analog input that is internal to the PDQ30 or
PersonalDaq/3000 Series module. Auto zeroing reduces drift due to fluctuating ambient temperatures or
ambient temperatures outside the DC specifications.
Reference Note:
Appendix A includes detailed information regarding signal modes, methods of noise
reduction, and averaging techniques.