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Original question:

OPA277: Signal conditiong and offset

OPA277: Offset using- Inverting Vs Non-Invreting

rajesh beesetty
Expert 1240 points
Part Number: OPA277

Dear Experts, I am working on an OP-AMP circuit to scale and offset that converts a +/-2V signal to 0-3V signal. For this I am using the classical circuit (circuit-1) as mentioned in the TI's reference circuits. However, I have recently learned from CMMR VIDEO-1 (07:44) that, Non-Inverting amplifiers suffers distortion. Therefore, it is recommended to use a inverting amplifiers. What would you people recommend for this application (voltages range: less than 3V but frequencies upto 10KHz).

In other-words, If Non-Inverting amplifiers has the CMRR issue, why should Non-Inverting Amplifier exists? One might say it might not be that significant. Then I have the question, WHEN TO IGNORE CMMR distortion problem?

Regards,
Rajesh BN.

  • 0
    TI__Mastermind 23508 points

    Rajesh,

    Reasons for using non-inverting as opposed to inverting topologies.

    1. Main reason:  the input impedance is very high.  The input impedance of an inverting circuit is equal to the input resistor (G = Rf/Rg +.1, the input impedance is Rg).  Generally this impedance will range from 1k to 100k depending on the design.  Op amp input impedances are generally much higher.  This can be important if the input source has a large source impedance.
    2. One reason non-inverting is used is because you don't want an inversion.  For example, a single supply 5V amplifier in a gain of 100V/V could have a 10mV input.  The output will be 1V ( 10mV x 100V/V = 1V).  In the case of the inverting amplifier the output would want to go negative, but a single supply amplifier can't do that. 
    3. One common type of non-inverting amplifier is a unity gain buffer.  In this case no feedback resistors are used.  The gain is set to 1V/V with a short from the output to the inverting input.  This type of amplifier has a very accurate gain of 1V/V.  To achieve the same gain with a non-inverting amplifier you will have gain error associated with the feedback resistors.  

    When does CMRR matter.

    1. Op amps have many error sources.  AC errors include:  Noise, distortion, bandwidth limitations, slew rate limitations. DC errors include: offset, bias current, temperature drift.  All of these errors can be accounted for by simulation and calculation.  The most certain way to know if CMRR is a problem is to do a total error analysis and compair all the errors.
    2. Circuits in high gain is a case where CMRR is generally not a problem.  In this case the input signal and associated common mode will not move by much voltage.  For example, a gain of 10V/V means that the input signal must be 10 times smaller than the power supplies.  The error introduced by CMRR is proportionate to the common mode voltage shift so if the input signal doesn't have a significant common mode shift it will not suffer much from CMRR.
    3. A unity gain configuration is a case where CMRR may be a significant issue.  In this case if your signal source has low output impedance you may consider an inverting amplifier.
    4. Look at the CMRR specification at DC and over frequency.  If your CMRR is 120dB out to wide frequency than you probably don't need to consider this error.  This means that you common mode signal will be attenuated by a factor of 1million.  On the other hand if CMRR is less than 70dB you might want to consider this error source.
    5. The error introduced by CMRR is more significant if higher frequency AC distortion is a concern.  If you are amplifying a DC sensor you probably don't care so much about CMRR errors.

    The non-inverting configuration is probably the most common configuration used.  Nevertheless the inverting does have the common mode rejection advantage.  For systems where the high frequency distortion is important, non-inverting configurations are very useful.

    I hope this helps!, Best regards, Art