User manual

RT USB3000 Technical Description and User Manual Rev. 4.1. 15 of 18
The analog inputs of the RT USB3000 device are differential. Differential connection of a signal
source reduces the level of common-mode noise. Besides, differential inputs allow connecting signal
sources so that currents of signal circuits do not flow through a single wire, which increases
measurement accuracy.
The correct connection of the sources of analog signal is the most important condition of the
correct operation of acquisition system which allows to avoid a lot of problems during the device
operation. During the connection of the sources of analog signal to the RT USB3000 device it is
necessary to keep to the following recommendations:
1. Differential connection presupposes measuring voltage difference between the inverting
and non-inverting inputs of the channel, i.e. differential voltage. However, it is necessary to
remember that the voltage in relation to the analog ground of the device on both
inputs (common-mode voltage) should not be higher than the acceptable input
signal range.
2. Correct connection of a signal to the differential input is always a three-wire connection. It
is necessary to separate signal wires connected to a high-impedance input from the
common ground wire. Thus, circuit of high current through signal wires, which reduces
measurement accuracy, is eliminated.
3. When several signal sources are connected to the device, it is advisable that their common
wires connect at one point only, on the AGND pin of the Analog Port. This will eliminate
formation of “ground loops” that are a source of extra noise.
4. The unused analog inputs should be grounded — i.e. just to be connected with AGND pins
of the Analog Port. Unused digital inputs could be left disconnected.
On the scheme there are the examples of the correct connection of 1-phase and 2-phase
(differential) signal sources to the device. Note that even single-phase signal sources should be
connected to a differential input with three wires!
The single-phase signal
sources
X1
Y1
...
Xn
Yn
AGND
The differential signal
sources
X1
Y1
...
Xn
Yn
AGND