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