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INA128: PSRR Test

mp
Prodigy 40 points
Part Number: INA128

Tool/software:

Testing PSRR on the INA12XUA.

I am testing at dc, 

for the circuit, we have in- tied to the output, and in+ tied to ground.  I am measuring Vos between the vin- and vin+.  

gain is set to 5V/V.

for the first measurement Vs+=15V Vs- = -15V, 

and I take the Vos1 measurement

for the second measurement Vs1+=10V Vs1- = -10V,

and we take Vos2 measurement

PSRR=delta Vos/delta Vsupply

I am getting very low psrr values, <40...

the data sheet says +/-2 +-200 / G uV/V

How do I interpret this value and my psrr circuit?

  • +1
    TI__Genius 15390 points

    Hello, 

    In order to test the PSRR for your conditions, you need to connect both inputs to ground as the VCM = Vs/2 = 0V. Please see the top of the Electrical characteristics table where the test conditions are listed: 

    The output will be connected to 10kohms to ground and not tied to the input. The INA128 is an integrated part that does not require a feedback loop. 

    To calculate the PSRR from the datasheet, you take the + or - conditions and calculate based on the gain you are using. So for example, with a gain of 5, the PSRR will  be +0.2+20/5=+4.2 or -0.2-20/5=-4.2. So the PSRR typical is +/-4.2uV/V. 

    Could you clarify why you are testing DC PSRR on the part? Is there a concern for the PSRR performance or have any questions on you application that we can help to troubleshoot? 

    Thank you!
    Regards,
    Ashley

  • +1 in reply to Ashley Kang
    Prodigy 40 points

    okay! I am just learning how CMRR and PSRR differ in different op amp configurations (integrated vs 3 op amp in amp, 2 op amp inamp, single op amp)

    Using the ina128ua for example, gain of 5, PSRR calculations would be:

    +2+200/5=+42uV/V or -2-200/5=-42uV/V

    and these would be max values for PSRR? and no minimum?

  • +1 in reply to mp
    TI__Genius 15390 points

    Hello, 

    Yes, these would the max values for PSRR. Please note this is applicable only for the Vref, load, and temperature condition listed above the table. There is no minimum spec for PSRR. 

    Regards,
    Ashley

  • 0 in reply to Ashley Kang
    Prodigy 40 points

    To measure vos, we are measuring vout to gnd or the voltage between the negative in put and positive input?

  • +1 in reply to mp
    TI__Genius 15390 points

    Hello, 

    You can measure the output and then divide by the gain to see what your Vos referred to input is. It is also recommended to match the datasheet test conditions as close as possible if you want to compare to the datasheet specifications. 

    Regards,
    Ashley