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Original question:

OPA170 Abnormal Behaviour

OPA170: Increasing opamp gain

Fairuz
Prodigy 200 points
Part Number: OPA170
Other Parts Discussed in Thread: TLV9101, OPA990, TL081H

Hi,  this is my new thread in relation of my other thread (OPA170 Abnormal Behaviour).

My setup (use as current sensor : V2 connecting V3 with ~0.1 ohm;150C test)

V1 = 12V, V3=13.5V and if there is a short circuit, V2 will reduce until the different ~120mV. this voltage will feed into my  next comparator set at 0.5V to trigger next circuit.

based on differential calculation, Vout should be ~0.6V however this is not the case, based on previous thread.

Since Vin for OPA170 unable to extend from (V+)-2V and V+, the new Rf will be drop to 80k with gain ~2 @ Vout=0.24V. 

so the question are :

1. Is there any recommendation to increase the gain further back to > 5 ? need simple solution as i have comparator at next stage.

2. OPA990 or TLV9101 will be good for rail-to-rail Vin but it did not fit temperature requirement,150C. Any other 150C rail-to-rail Vin suggestion?

  • 0
    Guru 140180 points

    Hi Fairuz,

    as being a differential amplifier R4 / R2 must equal R1 / R3 and R1 / R3 is dictated by your common mode input signal and the common mode input voltage range of OPAmp. So you cannot increase the gain of differential amplifier arbitrarily.

    But why not adding an additional OPA170 wired as non-inverting amplifier providing some additional gain? Or modify the threshold voltage of following comparator.

    Kai

  • 0
    TI__Guru*** 132416 points

    Fairuz,

    Have you consider using an op amp that likes the positive power rail. TL081H is an option. Vos is a little wider, but your high voltage common mode is likely a bigger error.

  • 0 in reply to Ron Michallick
    TI__Genius 16260 points

    Hi Ron,

    Fairuz mentioned they needed the operating temperature up to 150C in the previous thread here: https://e2e.ti.com/support/amplifiers-group/amplifiers-internal/f/amplifiers-internal-forum/1122396/opa2990-input-offset-variation. TL081H is a good alternative for end equipment's that don't require operating above 125C. 

    Fairuz,

    If it wasn't for having to operate at above 150C, you would have a lot more options as Ron had mentioned. I think Kai's suggestion of adding another gain stage would be the most practical solution. 

    Best Regards,

    Robert Clifton 

  • 0 in reply to kai klaas69
    Prodigy 200 points

    Hi Kai, i try to make circuit as simple as possible as i will need to have similar circuit for >40 DUT per board. the circuit was inherited from previous design which having lower voltage rating. with current supply 12V and i will need to make voltage divider drop from 0.5V to 0.2V, which too low but i can try. adding more opamp will increase the component count/board as i have next stage comparator and other circuit. if no other suggestion i guess i have to leave the circuit as is and do some tweaking.

  • 0 in reply to Robert Clifton56
    Prodigy 200 points

    thanks, i guess i have to leave the circuit as is for now since the board already fabricated. will need to play around on the comparator threshold voltage and see the actual results. any suggestion appreciated. any other component replacement also appreciated so i could change in next design. and too bad i have to test it at 150C.

  • 0 in reply to Fairuz
    TI__Genius 16260 points

    Hi Fairuz,

    Is there a reason that this system requires the device to be operating at 150C? This is a very unusual requirement. 

    Best Regards, 

    Robert Clifton 

  • 0 in reply to Robert Clifton56
    Prodigy 200 points

    Hi Robert, the device is use to monitor current for Reliability test at 150C. its a bit late to change the design as it inherited from previous design but i am open for next.

  • 0 in reply to Fairuz
    TI__Guru*** 132416 points

    Fairuz,

    Is there a way to cool the measurement circuity while keeping the thing under test at 150C. Does this measurement need to be made inside the 150C oven? Perhaps I misunderstand and the op amp is part system that needs to be tested at 150C.