Guide to understand the specifications in controls analog inputs & outputs with regards to signal isolation.

1. There are 2 dimensions to isolation of Analog Input / Output (AIO) signals.
1.Isolation by voltage tolerance. That means the channel is less likely to be
damaged.
2.Isolation by avoidance of signal reference differences.
a. by differential inputs with high Common Mode Rejection Ratio (CMRR)
b. by Galvanic isolation
That means signal pass through correctly.
Controls AIO Isolation
The following are extracts of various control product AIO specifications highlighting CMRR,
isolation, etc. with notes & comparisons.
Here is not discussed the various isolators (products) we can use to create the signal isolation
needed.
The last few slides explains some key terms.
1

2. 2
2

3. 1
2
3

4. This is not isolated AO. It means the AO signal reference are grouped
together & isolated from other circuits. That means, the other end that will
receive this AO signal has to provide signal reference isolation else there will
be an offset in values.
4

5. 1
5

6. Differential input
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7. Note:
AO is isolated by voltage tolerance. Receiving device need isolator to receive the signal properly. 7

8. 8

9. Demand Analog Inputs
9

10. Low AI isolation
External isolators needed
10

11. No AO isolation
External isolators needed
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12. Analog Inputs
Low AI isolation
External isolators needed
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13. Isolator needs to be installed nearer to
the signal receiving device.
Analog Outputs
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14. 14

15. 15

16. Ability of the differential amplifier to eliminate the common-mode voltage from the
output.
The ideal differential amplifier would remove all of the common-mode signal, or
the voltage common to both sides of the differential pair.
Common-mode voltages can come from numerous sources,
• ambient radiated signal coupled into both lines,
• offset from signal common created by the driver circuit,
• ground differential between the two ends of the differential circuit.
It’s not the common-mode voltage that’s of interest, but the differential voltage.
The measure of how good the differential amplifier is at getting rid of common-
mode voltage is its common-mode rejection ratio, or CMRR.
CMRR (Common-mode rejection ratio)
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17. CMRR of an amplifier changes with frequency.
• High-performing differential amplifiers start at very high CMRR ratings of as
much as 100,000:1 and can maintain high CMRR values over a wide
bandwidth.
• High-voltage differential probes will provide good CMRR at dc.
• As signal frequencies rise, CMRR deteriorates.
CMRR and common mode rejection (CMR) measure the ability of a differential
input amplifier, such as an op amp, to reject signals common to both inputs.
Technical definition for CMRR is the ratio of differential gain to common mode
gain expressed in dB.
CMRR (Common-mode rejection ratio)
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18. What is it?
• Method of isolating electrical/electronic systems to prevent unwanted current
flow; no direct conduction path is permitted.
• Energy or information can still be exchanged between the sections by other
means, such as capacitance, induction or electromagnetic waves, or by
optical, acoustic or mechanical means.
Galvanic isolation
A transformer is the most
widespread example of galvanic
isolation.
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19. References:
https://en.wikipedia.org/wiki/Galvanic_isolation
https://en.wikipedia.org/wiki/Common-mode_rejection_ratio
Prepared by:
Chan Poey Lum
Application
• Used where two or more electric circuits must communicate. (grounds
may be at different potentials)
• Effective method of breaking ground loops by preventing unwanted
current from flowing between two devices sharing a ground conductor.
• Used for safety: preventing accidental current from reaching ground
through a person's body.
Galvanic isolation
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