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AD8307 as audio log detector for testing frequency response.

Question asked by timc2 on Aug 18, 2017
Latest reply on Aug 23, 2017 by jdobler

I am working on a project using the Analog Devices AD 8307 Logarithmic Amplifier. The ultimate goal is to have it operate in the audio frequency range 20Hz to 20KHz.  The output from my audio sweep generator is connected to the input of the DUT.  The audio generator sweep voltage output is connected to the X input on an oscilloscope.  The output of the DUT is connected to the input of the log amp.  The DC output of the log amp is connected to the Y input of the oscilloscope.  The setup will allow me to see a real time trace of the DUT frequency response.  I used the design in your data sheet that has the 4 pole LPF. The system works well at slow sweep speeds (>30mS,) but as I increase the sweep speed to a point where it’s a continual trace with no noticeable flicker, the output from the log amp starts to flatten out.  I looked into the LPF and see that it’s rolling off at 10Hz. I tried altering the roll of point up and down (5Hz & 100Hz,) to no avail. Changing to 100Hz caused an increase in ripple on the log amp DC output.  It appears that at the higher sweep speeds (<20mS) the system start to integrate the detected AC signal thus changing the resolution of the DC output.  I am not concerned about absolute accuracy. The input has a 5k series resistor in each leg (pins 1 & 8) with a 1320 ohm and 300pf across the two input pins to give me a 600 ohm input with some HF rejection.   Any ideas how to solve this issue or can you suggest an alternative device that will produce the results I am seeking?