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OPA1612: OPA1612 distortion

Paul Rebers
Prodigy 110 points
Part Number: OPA1612

I made a simple measure-amplifier with 20dB gain,

Straight feedback is with 200 Ohms parallel to -input, and 1800 n series, supply: 2x15V

Harmonic Distortion (pure D3: 3kHz from 1kHz) is much higher than specs: -100dBc (200uV) at 20Vpp rising to -96 dBc (400uVpp) at 25vpp.

What is origin of this D3-distortion ??

Remarkable: Halving impedance feedback-network to 100 and 900 Ohms, reduced the distortion w -6dB,
but no other components parallel to -input can inject currents..

  • 0
    Prodigy 110 points

    I found origin D3-distortion: Thermal modulation FB-network
    1k8 and 2k- resistors in 0805 are only linear (D<120dBc) at low voltage: till 5Vp (8mWp)
    Smaller SMD-resistors in 0603 are worse.

  • 0 in reply to Paul Rebers
    TI__Expert 6400 points

    Hi Paul,

    I would like to do double check another root cause of the distortion.
    At first, I'd like to confirm your test circuit schematic. Is it same as the attached schematic?

    If there were any load condition, please let me know.

    Thank you and Best regards,
    Iwata Etsuji

  • 0 in reply to Iwata Etsuji
    Prodigy 110 points

    This looks almost my circuit, except:
    I use no capacitors, C1, in the feedback-network w R1, and R2.
    - output opamp had 50 ohm in series (behind feedback), to prevent capacitive load from cables to D-meter.
    - At +-input: RF-filter with 100 ohm in series, and 47pF parallel.
    (frequency-gain is flat to > 10MHz, and no RF-oscillations are found with 3GHz-SA.)

  • 0 in reply to Paul Rebers
    TI__Expert 6400 points

    Hi Paul,

    Thank you for your clarification.

    I understand your schematic is in as below.

    As you can see in this simulation result, we can see the gain peaking in frequency - gain curve without additional load.

    If you could not see any gain peaking with the actual load condition such as the cable impedance and D-meter input impedance, I agree with you in regard to the origin of 3rd order distortion.

    While, you can also choose resistors with respect to not only its wattage but also resistor material.
    Are you using the thin film SMD resistors?
    Considering with the below application note, the thin film resistor has better THD+N performance than the thick film resistors.
    https://www.ti.com/lit/pdf/sboa312

    Thank you and Best regards,
    Iwata Etsuji

  • 0 in reply to Iwata Etsuji
    Prodigy 110 points

    Hi Iwata, thanks for reply.

    I used standard thick-film resistors 0603 from Yageo, indeed not OK for high performance audio.
    Lower signal-levels (< 3Vpp as in R1) are no problem..
    Looking for more info about the exact specs, like distortion or modulation vs signal-power or temperature-change.

    Regards,
    Paul Rebers