Re: Suggestions for analog switch to select feedback resistor in TIA

Joerg wrote:
> Hello Kevin,
>
> Not sure what you are trying to do but AD has lots of nice logamps. That
> and a low noise current to voltage converter might do.
>
> Anyway, if you are looking for a very low capacitance switch there is a
> nice quad array: SD5400. But it ain't cheap, several Dollars. For some
> reason the Vishay site doesn't seem to find it today but that site seems
> to have problems at times.
>
> Regards, Joerg
>
> http://www.analogconsultants.com
Joerg,

Thanks for the pointer to the SD5400, that looks promising.

We are using a scanning mirror to sequentially reflect a number of
light beams over a single photo-detector to measure the optical power
in the beams. From the scanning rate we want to use the bandwidth
required is in the region of 1-2MHz, and we are looking for a 60dB
dynamic range.

I have used the Analog Devices AD8305 log amp on a similar product
where there was a photodetector per channel (no scanning) and they
worked well but their bandwidth at 10nA photocurrent is less than
100KHz. In general a log amp has trouble maintaining a wide bandwidth
over a large dynamic range as the compensation has to be selected to
get stability at high currents which results in over compensation at
low currents. I haven't yet looked into the more complex log-amp
comfigurations that can avoid some of this problem.

I may also be able to use an integrating approach to measure the energy
in the light beam but even there I have had trouble in that the charge
injection from the best analog switches tends to be in the 1pC range
and I am trying to measure something a couple of orders of magnitude
less (10nA for 1us).

thanks

kevin

.

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