K
Kevin
- Jan 1, 1970
- 0
Hi,
I need to sense the output from an InGaAs photodetector with a BW of
about 1MHz and a dynamic range of almost 60dB optical (~500pA to 250uA
photo current). In the past for low bandwidth applications I have used
a log amp approach (discrete or more recently monolithic solutions)
very effectively.
The best approach I can think of is to use a linear Trans-impedance
amplifier with switched feedback components to give 2 or maybe three
ranges.
One problem I am having is that the analog switches I have found all
have very high capacitances in the off state both between in and out
and to ground. The capacitance to ground is especially bad for the
noise performance as it appears in parallel with the photo-detector.
The photodetector has 5-10pF capacitance (5pF with 5V bias, 10pF with
0V) plus the input capacitance of an opamp such as the OPA657 and
strays gives a total of around 20pF.
For analog switches the old 4053 series looks to be better than most of
the new ones and even it has >8pF capacitance to ground as far as I can
determine from the data sheet.
Another alternative would be to use a discrete JFET or MOSFET although
I haven't been able to find the various capacitances when used as a
switch.
Does anybody have any suggestions for analog switches or other ways to
solve the problem?
I expect that I will have to make a compromise on the low end because
of noise - the goal is to have similar performance to the existing
system that only requires low bandwidth sensing.
thanks
kevin
I need to sense the output from an InGaAs photodetector with a BW of
about 1MHz and a dynamic range of almost 60dB optical (~500pA to 250uA
photo current). In the past for low bandwidth applications I have used
a log amp approach (discrete or more recently monolithic solutions)
very effectively.
The best approach I can think of is to use a linear Trans-impedance
amplifier with switched feedback components to give 2 or maybe three
ranges.
One problem I am having is that the analog switches I have found all
have very high capacitances in the off state both between in and out
and to ground. The capacitance to ground is especially bad for the
noise performance as it appears in parallel with the photo-detector.
The photodetector has 5-10pF capacitance (5pF with 5V bias, 10pF with
0V) plus the input capacitance of an opamp such as the OPA657 and
strays gives a total of around 20pF.
For analog switches the old 4053 series looks to be better than most of
the new ones and even it has >8pF capacitance to ground as far as I can
determine from the data sheet.
Another alternative would be to use a discrete JFET or MOSFET although
I haven't been able to find the various capacitances when used as a
switch.
Does anybody have any suggestions for analog switches or other ways to
solve the problem?
I expect that I will have to make a compromise on the low end because
of noise - the goal is to have similar performance to the existing
system that only requires low bandwidth sensing.
thanks
kevin