Practical SCADA for Industry David Bailey - FER-a

SCADA systems, hardware and firmware 25
In the descriptions that follow, these points apply:
• All signals are measured relative to the board’s analog ground point, AGND,
which is 0 V.
• HI and LO refer to the outputs of a signal source, with LO (sometimes called
the signal return) being the source’s reference point and HI being the signal
value. Esn represents the signal values (that is, VHIn – VLOn) in the diagrams,
where n is the signal’s channel number.
• AMP LO is the reference input of the board’s differential amplifier. It is not the
same as AGND but it may be referenced to it.
• Because of lead resistance, etc, the remote signal reference point (or ground) is
at a different potential to AGND. This is called the common mode voltage VCM.
In the ideal situation VCM would be 0 V, but in real-world systems VCM is not
0 V. The voltage at the board’s inputs is therefore Esn + VCM.
Single-ended inputs
Boards that accept single-ended inputs have a single input wire for each signal, the
source’s HI side. All the LO sides of the sources are commoned and connected to the
analog ground AGND pin. This input type suffers from loss of common mode rejection
and is very sensitive to noise. It is not recommended for long leads (longer than ½ m) or
for high gains (greater than 5×). The advantage of this method is that it allows the
maximum number of inputs, is simple to connect (only one common or ground lead
necessary) and it allows for simpler A/D front-end circuitry. We can see from Figure 2.9
that because the amplifier LO (Negative) terminal is connected to AGND, what is
amplified is the difference between Esn + VCM and AGND, and this introduces the
common mode offset as an error into the readings. Some boards do not have an amplifier,
and the multiplexer output is fed straight to the A/D. Single-ended inputs must be used
with these types of boards.
Figure 2.9
Eight single-ended inputs