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TLV272: Output deviation from expected

Part Number: TLV272
Other Parts Discussed in Thread: INA168

Hi,

I am using TLV272 in Differential opamp configuration. as shown below

 

Point B is Input ground

Point A Voltage varies between (30-40)V W.R.T point B

AB is input

AC is output.

My point C is at ( 30 - 12) V i.e at 18V w.r.t point B (Point B is same as the opamp ground)

so the voltage at Isense+ve and Isense-ve are at 28V w.r.t ground.

Current through the output shunt is 40A.

So the voltage drop across the shunt is 0.08V (40*0.002)

Gain of the opamp is 15

from the data sheet:

Common mode voltage range is 11.65V (VDD - 1.35V)

Now that the voltages at both the +VE and -VE pins of the opamp are way more than this, the opamp output Vout should be very much off.

Question1: Is it allowed for the voltage at the input pins of the opmap to be more than VDD.

Question 2::When my input voltage is 17 my output Vout is still ok( Is it because my common mode voltage is 5V(Which is allowed upto 11.65V)

Question 3: What is the safe possible operating common mode voltage

Question 4: Any alternate part that has a common mode voltage of upto 60V (Since I am measuring DC values here I can compromise with very low bandwidths also)

Thanks,

Rajesh

  • Hi Rajesh,

    to 1 and 2) you're assumption is right. It is not allowed to exceed the common mode range. I have attached the explanation for the input and output limitations from our precision labs. Data sheets usually provide the common mode range relative to the supply voltage.

    1130 - Input and Output Limitations - slides.pdf

    To 3) As long as the input stays in the common mode range, you will have a safe operating voltage. In a difference amplifier configuration the common mode is based on the input and the resistance divider.  In your case you have 30V at the input which has to be attenuated to at least 10.65V (12V-1.35V) by a divider. In your application it is the best to not use one amplifier to meet the common mode range and set the gain.

    To 4) We recommend to use a current shunt monitor like the INA168 at this point which satisfies the desired common mode range.

    I hope this will help you!

    Best,

    Miriam