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OPA2134 as Musical Instrument input Buffer

Jim Bergantino
Prodigy 40 points
Other Parts Discussed in Thread: OPA2134, OPA1652, OPA1642, OPA2604, TL072

Hi,

My question is regarding matching the source impedance of a non-inverting configured op2134 with the parallel impedance of its feedback resistors.  The circuit is such that the input from the instrument  will be ac coupled into the + terminal followed by a 1Meg resistor to gnd from the + terminal. The output impdance of the instrument will vary (100 ohms or less up to 100k+) due to it being active or passive.  According to page 9 of the OPA2134 data sheet, the Source impedance should ideally match the parallel resistance of the feedback resistors for lowest distortion.

Do you have any recommendations as to what the best value range these feedback resistors should be for this application.

Thank you,

Jim 

  • 0
    TI__Genius 12325 points

    Jim,

    Because the output impedance of the instrument can vary, there won't be any benefit in selecting resistors to match the source impedances at the two inputs. The distortion produced by mismatched source impedance comes from the varying common-mode capacitance of the op amp with the input common-mode voltage. This is very common in older JFET input op amps such as the TL072, OPA2604, and OPA2134 and has to do with how they are fabricated. I have written an article on this that will be published in the 4th quarter Analog Applications Journal this year. Our newer JFET input op amps do not have this issue, for example the OPA1642 does not require matched source impedances for low distortion and has improved performance over the OPA2134. CMOS op amps are also less suceptible to this effect, so the OPA1652 is another option. 

  • 0 in reply to John Caldwell
    Prodigy 40 points

    Hi John,

    Thank you for the quick Response. I was actually looking at the OPA1642 before posting, but it too, on page 13 of its data sheet, mentions the same concerns.  So is it still a concern, but less so, or is the data sheet not correct?

    Thanks again for your help.

    JIm

  • 0 in reply to Jim Bergantino
    TI__Genius 12325 points

    The datasheet of the OPA1642 was released before a major improvement was made to its semiconductor process, so in this case the datasheet does not tell the whole story. Here is a plot of the THD+N of the 1642 a source impedance mismatch of 20 ohms and 10k ohms:

  • 0 in reply to Jim Bergantino
    Prodigy 40 points

    Hi John,

    One other quick question after further reading the spec.  It mentions about placing a 50 ohm resistor on the output for overshoot purposes.  How imperative is this, and if needed, can we get away with  a 220 ohm resistors, as that is the lowest value, audio quality resistor, we're currenty using in the design.

    Regards,

    JIm

  • 0 in reply to Jim Bergantino
    TI__Genius 12325 points

    This resistor is to prevent capacitance on the output (audio cables are known culprits) from causing instability in the part, which often shows up as overshoot in a step response. 220 ohm resistors should work fine (as good if not better than a 50 ohm) for this purpose as long as they don't affect any circuitry after the OPA1642.