as a Radio Amateur I am doing some developmentin controlling output signalof radio equipment.I made a circuit by using AD´s log amp AD 606. Recently I checked the flatness of Vlog versa frequency between 2 to 30 MHz.Here I realized a variation of about 1.5 db. As shown in the amplifiers data sheet, one could get it flat by connecting a capacitor of 150 pF at pins 11 and 12. Trying this I had no success. The Vlog did not alter at all at the corner frequency of about 3 mc. At the Input FIL pins I use capacitors with 150 pF closely tolerated. Grounding appears to be ok. Are these chips maybe defective or whatsoever? What can be done to get a flat response over a limited frequency band?
Thank you very much indeed.
Sen. Eng. of University of Munich
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.