A question regarding Log Amplifier AD8305

[steadyj]

Hi,

I once used AD8305 as an optical power monitor connected with a photo diode. As in AD8305 datasheet, the photo diode anode, i.e. the INPT pin of AD8305, should be biased at 0.5V natuarally due to the inside structure of chip. But during measurement, I found the voltage at INPT is 0.5V only when input optical power is strong enough. If the input optical power is weaker and weaker, the voltage will be lower and lower.

Does anybody have some clue about this phenomenon?

 

[erikl]

Hi,
... But during measurement, I found the voltage at INPT is 0.5V only when input optical power is strong enough. If the input optical power is weaker and weaker, the voltage will be lower and lower.

Does anybody have some clue about this phenomenon?

Hi,
are you aware that the input impedance at this node at no or very low input current is in the order of 0.5GOhms (0.5V/1nA)? The input impedance of your measuring instrument (DVM?) might well corrupt your measurement.

 

[steadyj]

Hi erikl,

Thank you very much! I think at least it is a point. Yes I used a Fluke multimeter. But as my understanding, the impedance of DVM will shunt the photo current. As a result the output of Log Amplifier will abate, rather than the bias voltage at INPT decreased.

 

[erikl]

Hi erikl,
... As a result the output of Log Amplifier will abate, rather than the bias voltage at INPT decreased.

Hi steadyj,
may be both? I guess you could measure this voltage rather exactly at the negative input VSUM with your DVM, as the impedance of this node is much lower (16kOhms nominally). This node should show the same voltage level as your INPT node (apart from a few millivolts of input offset voltage).
Cheers, erikl