Common Mode Rejection Ratio for 741

[pnsakanjankumar]

How we calculate CMRR?
In my lab I used feedback resistor to calculate Common Mode Gain and i got Ac=0.34 and i didn't calculate differential gain.I took Ad=10^5 from the data sheet and therefore I calculated CMRR=10^5/.34 is it right?
but someone said to use feed back gain as Ad.
which is right?
please help me

 

[BradtheRad]

As I understand it, CMRR is of particular interest when, say, 60 cycle hum is reaching input cables. The hum is in phase at both op amp inputs. We want to avoid amplifying it.

Instead we want to amplify differential signals. Which is to say signals of opposite phase.

I would imagine the proper way to evaluate CMRR is to compare how much the device amplifies in-phase signals, as compared to amplifying opposite-phase signals.

To test CMRR, I would first apply an identical AC signal to both inputs. Measure the output. Calculate effective gain. (You may have to adjust the feedback loop in order to get usable output voltage.)

Then split a signal into two opposite-phase signals. Apply the differential signals to the inputs. Again measure output. Calculate gain. You'll probably find you must decrease the feedback resistor in order to reduce gain and avoid clipping.

Calculate the ratio of gains.

Multiply by the ratio of change you made in feedback resistance, if any.

You should end up multiplying two numbers which are both greater than 1. Convert to decibels.

That's how the CMRR spec makes sense to me.

 

[pnsakanjankumar]

Can we calculate Ad by using inverting configuration(with one input)?

 

[FvM]

I'm unable to conclude from your description what you are exactly doing. Suitable measurement circuits for CMRR are suggested in Chapter 1-4 Op Amp Basics - Op Amp Specifications of the Analog Devices Op Amp Applications Handbook:

ADI - Analog Dialogue | Op Amp Applications Handbook

 

[fcfusion]

How we calculate CMRR?
In my lab I used feedback resistor to calculate Common Mode Gain and i got Ac=0.34 and i didn't calculate differential gain.I took Ad=10^5 from the data sheet and therefore I calculated CMRR=10^5/.34 is it right?
but someone said to use feed back gain as Ad.
which is right?
please help me

IMO, you should use a buffer amplifier to measure CMRR. Here is the reason:

When using a buffer, as negleting common mode we have:

Vout = Vin

But if we include the common mode error we have:

Vout = Vin + Ac * CM where CM is input common mode and Ac is the common mode gain

CM = (V1 + V2) / 2 = (Vin + Vout)/ 2

We then rearrange the formula to get:

Ac = (Vout + Vin)/(CM) = 2 (Vout - Vin) / (Vout + Vin)

So when calculating your common mode gain for buffer amplifiers, this is the formula you should use. However common mode rejection ration is usually very high on opamps, and the difference between Vout and Vin is typically very small.

 

[vikstiks]

you did it right bt mathematically we calculate CMRR by th formula

CMRR= 20log(Ad/Ac)

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i have a query..pls reply..

should o/p voltg increase with increase in i/p voltg frequency in case of inverting and non-inverting amplifier?

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no we can not...

 

[vikstiks]

what i did to measure the CMRR is...u pls tell me if i was right...

i first measured common mode gain with the circuitry made on breadboard using 4 resistors of 10 k each. one i kept as feedback resistor on pin no 2-6. and one resistor i grounded with pin no 3. remaining 2 resistors i kept at common mode input. then measured the Acm.

then i made the circuitry for differential amplifier using same 4 resistors. thr i measured Adm.

thn calculated the CMRR as Ad/Acm...

is ths procedure right? pls help...

 

[FvM]

I fear, your circuit doesn't consider the resistor matching requirements to measure usual CMRR numbers. The problem is discussed in detail in the Analog Devices Op Amp Applications Handbook linked above.