LOg amplifier AD 606

Greetings Hans,

I'm not so familiar with AD606 because it's a very old part. Nonetheless, most log amps such as this have three elements that determine the low-frequency response or cutoff:

1) input coupling capacitors.

2) internal DC offset cancellation loop.

3) output ripple filtering.

To respond flat down to the desired frequency, the affect of all 3 must be considered.

From your comments, it sounds like you've addressed 1) and 2). If any doubt, try larger capacitors to see if it helps.

Item 3) may or may not be addressed in the datasheet. The DC output is derived from low-pass filtering the log'ed sine-wave half-cycles, but the ripple must be filtered out for accurate DC measurement. I'd recommend putting your oscilloscope probe on pins 3, 4, and 6. Make sure the filter is working as effectively at 2MHz as it is at 30MHz.

Also suggest using an independent known-good power meter or terminated oscilloscope to confirm that the signal generator is indeed flat across the frequency band. In other words, make sure this is not an instrumentation problem.

-Bruce

Greetings Hans,

I'm not so familiar with AD606 because it's a very old part. Nonetheless, most log amps such as this have three elements that determine the low-frequency response or cutoff:

1) input coupling capacitors.

2) internal DC offset cancellation loop.

3) output ripple filtering.

To respond flat down to the desired frequency, the affect of all 3 must be considered.

From your comments, it sounds like you've addressed 1) and 2). If any doubt, try larger capacitors to see if it helps.

Item 3) may or may not be addressed in the datasheet. The DC output is derived from low-pass filtering the log'ed sine-wave half-cycles, but the ripple must be filtered out for accurate DC measurement. I'd recommend putting your oscilloscope probe on pins 3, 4, and 6. Make sure the filter is working as effectively at 2MHz as it is at 30MHz.

Also suggest using an independent known-good power meter or terminated oscilloscope to confirm that the signal generator is indeed flat across the frequency band. In other words, make sure this is not an instrumentation problem.

-Bruce