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OPA891: OPA891 and OPA892

Abhijit Khadilkar
Intellectual 275 points
Part Number: OPA891
Other Parts Discussed in Thread: OPA892, , OPA2892

Hello Experts,

This board will be used for designing a probe for measuring switching currents of a high-speed SOC.

The input voltage range is expected as 250uV to 7.5mV.

OPA891 and OPA892 have offset nulling pins. The offset voltage drift is 1uV/C.

Two OPA891 will be used as unity gain buffer (stage 1) for P and N differential input. OPA892 will be used as a differential to single ended amplifier with a gain of 20 (stage 2). Another noninverting amplifier of OPA892 with gain of 20 (Stage 3).

 Is OPA891 and OPA892 suitable for my application considering it has input offset voltage null pin?

Following parameters of OPA892 are important for this choice:

1. Output current drive of 200mA.

2. High slew rate.

3. Gain BW product

4.Flat closed loop voltage gain.

thanks, Abhijit

  • 0
    TI__Genius 9850 points

    Hi Abhijit,

    I was wondering if you could share some details on the specs you require. For the BW and slew rate is there a goal or minimum required in the design. The devices you listed could very well work in this application and the benefit of the null pins help reduce error in your application. 

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Intellectual 275 points

    Hi Ignacio,

       Specifications are below:

    • Input voltage dynamic range: 0.25mV to 16mV
    • Flat gain response till 50MHz
    • Should be able to drive 50-ohm output impedance of oscilloscope.
    • Slew rate --- 128V/us
    • Total Gain of two stages of OPA892 is 200 (14 x 14). So output voltage will swing  of stage2 of OPA892 is 50mV to 3.2V.

    Question:

    • Will the OPAMP OPA892 support a gain of 14 for output voltage swing till 3.2V?
    • Given that minimum input voltage is 0.25mV, will the NULL pin cancel the 0.2mV typical offset voltage of OPA891 and OPA892?

    thanks, Abhijit

  • 0 in reply to Abhijit Khadilkar
    TI__Genius 9850 points

    Hi Abhijit,

    Could you also share the supplies you are working with. One consideration with the OPA892 is the minimum supply requirement of 9V. As for your questions, yes, the OPA892 will support that output swing in a gain of 14V/V. The gain of a device wouldn't affect the output swing of the device, other factors such as output current and supply values would affect the output swing. To answer your second question, the null pin feature was meant to reduce the offset beyond the typical Vos spec.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Intellectual 275 points

    Hi Ignacio,

       Will the offset voltage of OPA2892 will be matched in the same direction or it will cancel out each other? There is no null pin for OPA2892.

    thanks, Abhijit

  • 0 in reply to Abhijit Khadilkar
    Guru 271570 points

    The offsets of the two OPA2892 amplifiers are random, i.e., the might go in the same or in different directions, and are likely to have different values.