Campbell Scientific SDM-CVO4 4-Channel Current/Voltage Output Module - Manual (Page 17)

SDM-CVO4 4-Channel Current/Voltage Output Module
problem is where the sensor outputs are not within the common-mode voltage
that the datalogger will accept (±2.5 V for the CR10X).
Careful consideration should be made before using an SDM-CVO4 for such a
function. Not only is it a relatively expensive method, but it can also result in a
less accurate method of sensor measurement (compared to when the datalogger
excites the sensors directly) if the precision of the output is critical to the
sensor accuracy.
The reason for this is that direct datalogger excitation is a ratiometric
measurement whereby any drift in the excitation output of the datalogger is
compensated for. With the SDM-CVO4 as an excitation source, any drift in its
output accuracy, or of the logger measurement accuracy, can result in a
combination of those errors. It is worth investigating, therefore, the exact
requirements of the sensors you wish to use. For instance, on closer inspection
of the specification of many pressure sensors you will often find the
recommendation of 10V excitation, but in reality many will accept a lower
voltage. If new sensors are to be bought for a specific project, it can be worth
checking if versions of sensors are available that will accept a low voltage
excitation.
There are, however, some sensors that have active components or that have
large common mode output voltages that require an isolated and/or precise
high voltage supply within the current output capability of the SDM-CVO4. In
this context the SDM-CVO4 can be used to provide an isolated supply which
can be regulated in the range of 0-10,000 mV, a regulated current (0-20 mA)
or an unregulated 15 V (nominal) supply (by setting current output mode and
using the V
o
terminal).
In these applications, if all channels of the SDM-CVO4 are being used for
exciting sensors, you can use the standby mode between measurements to save
power. To do this you would send a command (instruction 103 with zero reps -
see below) to the SDM-CVO4 to shut it down after making the measurements.
When using this mode you need to allow at least 100ms after turning the
SDM-CVO4 on again (by using instruction 103 with a non-zero number of
reps) for the outputs to stabilize, before starting your measurement sequence.
It is possible that you can power more than one sensor from each output of the
SDM-CVO4, either by parallel connection in voltage output mode or serial
connection in current output mode. Make sure, however, that you do not try to
take more current than available in voltage mode. Additionally you also need
to check that all outputs from the sensors powered by one channel do not have
a spread which exceeds the common mode range of the datalogger inputs.
6. Datalogger Programming
Our CR800, CR850, CR1000, CR3000, and CR5000 use the SDMCVO4
instruction in CRBasic. Our CR7, CR10(X), 21X, and CR23X dataloggers use
Edlog Instruction 103. Both CRBasic and Edlog are provided in PC400,
LoggerNet, and RTDAQ software.
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