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OPA1612 power supply sensitivity

Other Parts Discussed in Thread: OPA1612, LP5907

Hi - I am using the OPA1612 for the mic preamps on a new mixer. Two problems - one solved. Solved: low freq THD&N was -92dB compared to -110dB above 1K. Changed the 5V LDO from a MCP1824 to a LP5907. Now -110dB full bandwidth. The line ins use an NJR NJM2737 (basically a low voltage 5532) which showed no sensitivity to 5V from either LDO. Both OPA1612's and NJM2737's are on the same 5V LDO.

Unsolved: Mic circuit is sensitive to the 1.6KHz LED update rate and its harmonics which are about -98dB with no gain. The line ins using the NJR NJM2737 show no artifacts from the LED update rate even at +40dB of gain. The artifacts from the LED update increase with gain on the OPA1612 and are clearly audible.

No amount of shielding helps (much). Thinking it is in 5V supply but not visible on scope. All usual ground planes, decoupling etc are in place and the Lin ins with NJM2737 are dead quiet ( -118dB down on AP)

Why is OPA1612 so supply sensitive? (fixed oscillations at 3-5MHz several weeks ago) What is the actual PSRR of this part? Any suggestions on how to make this go away? 

- Keith

  • Keith,

    OPA1612 PSRR is about 98dB at 1.6kHz (see graphs below) so what you see may be a direct result of the part's power supply rejection ratio limitation.  If this is the case, the only thing you may be able to do is to try to filter out LED ripple by placing a bypass cap (0.1uF) right on the supply pin in addition to having a larger (10uF) bypass cap on the side of your pcb board.  If that's not enough, placing 20ohm to 100ohm Rs resistor is series with the supply together with 10uF bypass supply cap will filter out LED ripple above 800Hz to 160Hz, respectively; however, in such case you must account for the diminished supply voltage headroom due to voltage drop across Rs resistor.