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OPA350: Regarding using alternate opamp having lesser bandwidth

Caliber
Intellectual 825 points
Part Number: OPA350
Other Parts Discussed in Thread: TLV9061, OPA320, OPA325, LMP7731

Dear TI team,

Good evening.

We are using OPA350 in sensing circuits in grid tie inverter.

In our sensing circuit,we are using differential isolation opamp and isolating high voltage and low voltage sections.

After differential opamp,to convert differential output to single ended ADC,we are using OPA350 opamp.

OPA350 is a 38MHz band width opamap,

can we use lesser bandwidth opamp like OPA 320,TLV9061?

Is using lesser bandwidth opamp helps in reducing noice at ADC/output of opamp?

Our inverter switching frequency is 50KHz,is there any advantages of using lesser bandwidth opamps?

Kindly reply.

Thank you,

Caliber

  • 0
    TI__Guru 63700 points

    Caliber,

    The bandwidth of the op amp has to do with the response time and thus ability to take quick readings.  Therefore, if the gain is relatively low (G<20), there should be no significant difference in performance between OPA320, OPA325, or TLV9061 and OPA350.  A lower bandwidth op amp would reduce the overall noise ONLY if its input noise spectral density was the same or lower than the op amp with higher bandwidth. However, since the lower bandwidth typically means lower quiescent current and thus higher input noise spectral density, the total noise may be lower, higher, or the same.  The advantage of using a lower bandwidth part typically has to do with lower IQ while a disadvantage is a slower response. Having said that, OPA320 is one of our best op amp to drive ADC input.

    If you need further assistance, please provide your schematic - a picture is worth thousand words.

  • 0
    Guru 279520 points

    If your goal is to reduce noise, consider the LMP7731.

  • 0 in reply to Clemens Ladisch
    TI__Guru 63700 points

    Clemens,

    Even though LMP7731 (GBW of 22MHz) has lower input voltage noise spectral density than comparable speed OPA320 (GBW of 20MHz), being a bipolar input op amp LMP7731 typical input current noise spectral density is more than three-orders of magnitude higher, 2.2pA/rt-Hz vs 0.6fA/rt-Hz, respectively. Thus, without knowing the actual resistor values used for converting differential signal into single-ended, one may NOT conclude that use of LMP7731 would result in the lower overall noise.

  • 0 in reply to Marek Lis
    Intellectual 825 points

    Dear Marek,

    Good afternoon.

    Thank you for explanation and suggestions.

    Thank you,

    Caliber

  • 0 in reply to Caliber
    TI__Guru 55441 points

    Hi Caliber,

    Our inverter switching frequency is 50KHz,is there any advantages of using lesser bandwidth op amps?

    If inverter's switching frequency is configured at 50kHz, the op amps and voltage/current feedback loop need to be 1/10 to 1/20th lower than the switching frequency. In other words, the op amp's BW should be kept below 5kHz. If not, the inverter's switching frequency and its harmonics will increase the system's noise floor. One decade earlier of 50kHz will give you a minimum -20dB to -40dB reduction from the switching frequency noise and its harmonics. 

    This is the reasons many inverter switchers are operating above 100kHz in range (you also need to consider your magnetic core loss in your design. If you are using Transformer Stamping in material, then you have to keep below 50kHz).  It will be good to move the switching frequency and noises at higher frequency.

    If you are doing this for solar DC to AC power conversion in 50Hz/60Hz application, BW of 5kHz or slightly lower may be acceptable. Therefore, you do not need high BW op amps.  

    If you have additional questions, please let us know. 

    Best,

    Raymond  

  • 0 in reply to Raymond Zhang1
    Intellectual 825 points

    Dear Raymond,

    Good morning.

    Thank you very much for your response and suggestions.

    Thank you,

    Caliber

  • 0
    Guru 140200 points

    Hi Caliber,

    from my point of view there's no simple answer. One would need to see the schematic.

    Kai

  • 0 in reply to kai klaas69
    Intellectual 825 points

    Dear Kai,

    Thank you for your reply.

    Our team is analyzing the suitable component replacement to reduce noise.

    We will get back to you if any doubts are there.

    Thank you for your support.

    Thank you,

    Girish K

    Caliber

  • 0 in reply to Caliber
    TI__Guru 55441 points

    Hi Caliber,

    I am going to close this inquiry. If you have additional questions, you may reopen this thread or create a new one via E2E forum. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Intellectual 825 points

    Hi Raymond,

    Thank you.

    You can close this thread.

    Thank you,

    caliber