Measurement Computing CIO-DAS16/M1 - Manual (Page 32)

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For example, the CIO-DAS16 has a common mode plus signal range of 11.5 volts, common mode not to
exceed 10 volts. This specification is illustrated graphically in Figure 6-2 and is referred to as Cumulative
Signal Range (CSR).
Most manufactures of A/D boards specify the CMR directly from the component data sheet, ignoring the
effect of the board level system on that specification. A data sheet of that type might claim 10 volts of
CMR. Although this is a factual specification and the designer of the board (or other EE) would be able to
translate that into a systems specification, most A/D board owners are confused or mislead by such specs.
6.2 COMMON MISUNDERSTANDINGS
The CMR specification of a differential input is often confused with an isolation specification, which it is
not. It makes sense. doesn't it, that 10 volts of CMR is the same as 10 volts of isolation? No. The graph
above shows why.
Failure to specify the common mode plus signal system specification leads people to believe that a DC
offset equal to the component CMR can be rejected regardless of the input signal voltage. It cannot, as the
graph above illustrates.
When is a differential input useful? The best answer is whenever electromagnetic interference (EMI) or
radio frequency interference (RFI) can be present in the path of the signal wires. EMI and RFI can induce
voltages on both signal wires and the effect on single ended inputs is generally a voltage fluctuation
between signal high and signal ground.
A differential input is not affected in that way. When the signal high and signal low of a differential input
have EMI or RFI voltage induced on them, that common mode voltage is rejected. This is subject to the
system constraint that common mode plus signal not exceed the A/D board's CSR specification.
6.3 GROUND LOOPS
Ground loops are circuits created when the signal ground and the PC ground are not at the same voltage.
Ground loop inducing voltage differential can be a few volts or hundreds of volts. They may be constant
or transient (spikes). A differential input will prevent a ground loop as long as the CSR specifications is
not exceeded. If ground voltages greater than the CMR are encountered, isolation is required.
6.4 PULL UP & PULL DOWN RESISTORS
IMPORTANT NOTE: WHEN THE 82C55 IS POWERED ON OR RESET, ALL PINS ARE SET TO
HIGH IMPEDANCE INPUT.
The implications of this is that if you have output devices such as solid state relays, they may be switched
on whenever the computer is powered-ON or reset. To prevent unwanted switching, drive all outputs to a
known safe state after power-on or reset. To do this, pull all pins to either high or low voltage through a
2.2Kohm resistor.
When the 82C55 is powered-on or reset, the control register is set to mode 0 and all ports are set to input
mode.
When used as an output device to control other TTL input devices, the 82C55 applies a voltage level of
0V (nominal) for low and 2.5V to 5V for high. It is the output voltage level of the 82C55 that the device
being controlled responds to. The concept of voltage level of an 82C55 in input mode is meaningless. Do