Practical SCADA for Industry David Bailey - FER-a

2.3.2.2 Amplifier
SCADA systems, hardware and firmware 21
• Switching time
A similar parameter to settling time, it specifies how long the multiplexer
output takes to settle to the input voltage when the multiplexer is switched
from one channel to another.
• Throughput rate
This relates to the highest rate at which the multiplexer can switch from
channel to channel; it is limited by the settling time or the switching time,
whichever is longer.
• Transfer accuracy
Expresses the input-to-output error as a percentage of the input.
Where low-level voltages need to be digitized, they must be amplified to match the input
range of the board’s A/D converter. If a low-level signal is fed directly into a board
without amplification, a loss of precision will be the result. Some boards provide onboard
amplification (or gain), while those with a PGA make it possible to select from
software, different gains for different channels, for a series of conversions.
The ideal differential input amplifier only responds to the voltage difference between
its two input terminals regardless of what the voltage common to both terminals is doing.
Unfortunately, common mode voltages do produce error outputs in real-world amplifiers.
An important characteristic is the common mode rejection ratio, CMRR, which is
calculated as follows.
CMRR = 20log (Vcm / Vdiff) [dB]
where:
Vcm is the voltage common to both inputs
Vdiff is the output (error) voltage when Vcm is applied to both inputs
An ideal value for CMRR would be 80 dB or greater.
Drift is another important amplifier specification; it depends on time and temperature.
If an amplifier is calibrated to give zero output for zero input at a particular temperature,
the output (still at zero input) will change over time and if the temperature changes.
Time drift and temperature drifts are usually measured in PPM/unit time and PPM/°C,
respectively. For a 12-bit board, 1 LSB is 1 count in 4096 or 244 PPM. Over an operating
range of 0°C to 50°C, a 1 LSB drift is thus:
244 PPM/50°C = 4.88 PPM/°C
In choosing a component, you need to ensure that the board’s time and temperature
drift specifications over the entire operating temperature range are compatible with the
precision you require and don’t forget that it can get quite warm inside the RTUs
enclosure.
2.3.2.3 Sample-and-hold circuit
Most A/D converters require a fixed time during which the input signal remains constant
(the aperture time) in order to perform an A/D conversion. This is a requirement of the
conversion algorithm used by the A/D converter. If the input were to change during this
time, the A/D would return an inaccurate reading. Therefore, a sample-and-hold device is
used on the input to the A/D converter. It samples the output signal from the multiplexer
or gain amplifier very quickly and holds it constant for the A/Ds aperture time.