What is the CMRR measuring circuit for signal conditioning module with current output?

Answer 1

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. 1) factor 2) factor 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.