Answer Posted / ghost

CMRR (Common Mode Rejection Ratio)
CMRR (Common Mode Rejection Ratio);
it is a parameter normally associated with operational
amplifiers or differential inputs. The importance of a good
CMRR is to cancel unwanted noise signals which are the same
on the two input signals. For any form of signal
conditioning it would be an important parameter, since the
idea of signal conditioning is to be able to provide a pure
signal and as little noise and error as possible.
Basically an operational amplifier has a differential
input. It means that the difference between two inputs is
amplified and that what is similar gets rejected. In a very
simplified way we can say. If I have 5V on one input and 3V
on the other then 3 volt will be rejected and not being
looked at and the 2V difference will continue through the
amplifications stages.
But in real life no operational amplifier or differential
input stage is perfect and there is always some form of
error. One type of error is where not all the common
voltage (the 3V part) is rejected or cancel entirely. A
small amount still go through, allowing noise to be
amplified. The CMRR parameter exist in two forms. ( factor
1) factor2 in the decibels (lowercase!! since deci means
x10-1)
Before we really know what the CMRR factors in their
numerical values mean. Lets look at the composition of the
factor.
Aol= Open Loop Voltage Gain = The amount of times the
output voltage is more than the difference in input
voltage, if no feedback is used. Usually in the order of
100 000 or so but may vary allot depending on the design of
the amplifier
Acm= Common Mode Voltage Gain = The amount of times the
output is more than the input, if both input voltages are
exactly the same. Typically in the order of 0.2
CMRR = Aol/Acl
Example: = 100 000/0.2 = 500 000
Now we understand where the logic of using the decibel
scale come in. It is unnecessary to work with values like
500 000 for a parameter like that. Then we rather say:
CMRR = 20.Log (Aol/Acl ) and it is then 114dB where dB is
the unit of decibel. Remember that 500 000 does not have a
unit, it is only an factor. If you have a factor in dB
form, then:
Aol/Acl=10(CMRR/20)
Example:
5.7=113.98dB / 20
500 000=105.7
The amplifier provides maximum gain to the differential
signal and the amplifier give minimum gain to (unwanted)
common mode voltage and actually reducing its value on the
output by 5 times (x 0.2) this means that common mode noise
may be reduced by 5 times while a differential signal can
be amplified by 100 000 times. All we need to understand is
that CMRR is the ratio between closed loop gain and common
mode gain.
The bigger the CMRR is the better.
CMRR is not the only parameters that effect error and noise.
Consider some more effects, not all the effects will be
shown on datasheets and often we should hope, trust and
assume the parameter will be good enough for our
application. It is likely that the following effects may be
affected by circuit components around the inputs such as
differential inputs, feedback, filters ext. Therefore the
interface to a module may not reflect the same values as
given. But for many that do make use of operational
amplifiers or differential stages will have the following
effects somewhere in the module if not directly at the
input:
*Input offset voltage:
Caused by slight differences between the base-emitter
voltages of the differential input stage and can go up to a
few mV
*Input bias current
The average of both input bias currents of the input stage.
Bias current is required to get input stage transistors
operational and can be about 500nA depending on the design.
The type of signal source and conditioner will determine
what effect it will have on the precision.
*Input offset current
The absolute value of the difference between bias currents.
*Input impedance
The input impedances for differential and common mode may
differ. If transducers with internal resistance are used,
like a load cell for instance. Then the changes in
impedance may have an effect on the amount of error. The
common mode impedance is the impedance of each input to the
ground. Differential impedance is the impedance between the
two inputs. Impedance may be anything depending on the type
of conditioning circuits. But it can go up to the order of
mega ohms and depending on design and application might not
effect the output much.

*Temperature coefficient
The effect temperature have on the device. The parameter
will indicate the amount of offset that may occur due to
the change in ambient temperature. Although the values are
often in the uV/°C bare in mind the application. Some
applications might require precision of up to 12 000
divisions of its full scale deflection or capacity and in
that case, expecting a resolution of 3uV per division is
possible.
* Nonlinearity
The amount of deviation from the proportional input to
output ratio over the entire operating range.
How applicable the above arguments are to the signal
conditioning systems will depend on the type of design and
for what type of signals the conditioner is developed for.
They may differ allot from one application to the next.
Therefore the explanation is more based on the typical
operational amplifier or differential stage what the
question about CMRR would be applicable to. This answer
will apply to more applications than only signal
conditioners.
Some signal conditioners may be time or logic based inputs
and would not require the functionality of operational
amplifiers.

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