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OPA350 DC Offset needed at Input

michael schuermann
Prodigy 20 points
Other Parts Discussed in Thread: OPA350, PCM4202, OPA1632, LM833

When i use the OPA350 in non inverting configuration with Rg = 2k7 and Rf = 1M

when i feed 5mV to the positive Input via Frequency Generator i Need to set an

DC Offset of 2,5mV to get a full swing Output. Without Offset i only get the upper

half wave. Is that normal ? What to do to avoid this ?

Kind regards

Michael

  • 0
    TI__Guru* 93105 points

    Hi Michael,

    You have the OPA350 set up for a high closed-loop voltage gain of about +371 V/V. This is a high gain for a single stage amplifier. The summation of input dc level, ac signal level and voltage offset will all be amplified by 371x and appear at the OPA350 output as a much larger voltage. If the sum of the dc input and ac signal level (negative peak) result in a negative input voltage the output cannot swing below zero volts and the signal will clip. The clipping occurs when the output swing runs out of range, about 10 - 200 mV above ground depending on load.

    You would need to add enough positive offset to keep the input from swinging negative and the output from going into clipping.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 20 points

    Hi Thomas,

    thanks for yout hints. As an absolute newbie in analog design, i have to read and learn alot.

    My goal is to amplify an signal from an moving magnet and switchable an moving coil system

    to the required input signal of ti's PCM4202 ADC. That is from 5mV or 0.5mV to 6V and then

    do an digital RIAA filter with an DSP.

    I think i will have to use a two stage OPA1632 design to get that amount of gain needed.

    Do you have an appnote for me, or some schematics ?

    Kind regards, Michael

  • 0 in reply to michael schuermann
    TI__Guru* 93105 points

    Hi Michael,

    I think it is a better plan to use two stages to attain the high gain your system requires. The OPA1632 is a differential-in, differential-out op amp and as the PCM4202 datasheet shows could serve as a good driver since it has the differential outputs. The OPA1632 usually isn't operated with a high closed-loop gain because it is a wide bandwidth op amp so you will still need to develop most the voltage gain before that stage. Note that the High-Speed Amplifier group supports the product and they are most expert on applying the device. They have an E2E forum as well.

    When I looked for information about moving coil systems phono-cartridge applications come up. The LM833 datasheet shows and RIAA phono preamplifier circuit that has a gain of 30 to 40 dB at 1 kHz. I am not sure if that would be acceptable for the sort of thing you are attempting to do, but it might be worth taking a look at its datasheet for some ideas. The OPA1632 could possibly follow the preamp to get the overall gain up to the required 371 V/V.

    If you have any questions about operational amplifiers and want to learn about them in detail,  we have an excellent on-line training series called TI Precision Labs. It is a great way to come up to speed:

    Regards, Thomas

    PA - Linear Applications Engineering