OMC-042 / 043 / 044 Data loggers
Users
’ manual
page 72
Analog Inputs (4..20mA)
The OMC-042 is equipped with four and the OMC-043 with two analog mA 12 bits AD-Conversion inputs.
The input signal must be a 4..20 mA current loop. The impedance of the system is 150 ohms.
Loop Powered Devices
Some devices don’t need a power supply, but take their power from the current loop. But the primary
circuit of the data logger does NOT provide power for this. In this case, you need the power switch to
provide the energy for the current-loop. Consult the manual of the loop-powered device you like to
connect, and use the positive side of the power switch for the power supply. In most cases the data
logger is connected to a device with an active output signal, so there will be no problems connecting it. If
you have questions about interfacing your device with our data logger, contact your Observator
Instruments dealer.
Analog Inputs (0 .. 10 V) (OMC-043/044 only)
The OMC-043 is equipped with two analog 0 .. 10 V 12 bits AD-Conversion inputs. The input signal must
be a dc signal which must not exceed 10 Volts. The user can adopt a higher voltage level if he uses
external resistors. This is done by a simple voltage divider. However this possibility offers a flexible way to
expand the range of the instrument, this is NOT covered in this manual, and the user may not seek for
support from Observator Instruments on this topic.
Analog Differential Inputs (OMC-042 only)
The OMC-042 Data logger has, besides the four 12 bits analog current inputs, also two additional
Differential 16 bits voltage inputs. These inputs are very sensitive and particularly suitable for measuring
signals from load cells.
Differential input ports theory of operation
Differential inputs are very convenient for measuring differential or floating signals. The performance of a
differential input is much better than a normal, single ended one, especially with small mV signals.
Therefore, the differential inputs on the OMC-042 are very suitable for measuring load cells,
pyranometers and other low-level mV output sensors. A differential input consists of a negative (-) and a
positive (+) input. The voltage difference between these two inputs is the signal to be measured.
Common mode noise rejection
One of the major advantages over a single ended input is the common mode noise rejection. It “removes”
practically all noise that is present on the input signal. Especially with long cables, noise is always present
on the leads. Since the input acts like a differential amplifier, the noise on the negative input is subtracted
from the noise on the positive input. What is left is the sensor-signal.
