AD8305: Measuring low frequency signals

Document created by analog-archivist Employee on Feb 23, 2016
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How can we reduce the bandwidth of the AD8305 to get the average power of a
modulated (rectangle signal) source in the kHz range? It is no problem to get
the average power of a HF modulated source (e.g. 52MBit/s fiberoptic
transmission) but what can we do to measure low frequency signals? Low pass
filtering at the log output doesn't make sense, we probably need to reduce the
input bandwidth, e.g. by using a capacitor network between the photodiode and
ground? What would you recommend?


The bandwidth of the AD8305 is dynamic, it varies with input current level. See
TPC13 in the datasheet. This is the result of the effects of junction
capacitance at the logging transistor. More details on this characteristic can
be found in the "Bandwidth and Noise Considerations" section of the AD8304
datasheet, pg.9.

Increasing the capacitance across the diode can be done a few different ways.
The simplest would be to add a capacitor to ground at the photodiode's cathode.
A slightly more complicated solution would be to operate the diode with
zero-bias by connected the anode to INPT and the cathode to VSUM.