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Suggestions for analog switch to select feedback resistor in TIA

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
 
J

Joerg

Jan 1, 1970
0
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
 
P

Pooh Bear

Jan 1, 1970
0
Kevin said:
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.

I use discrete jfets for audio switching. J174... and J111... family. Also
in SMD - use SST prefix instead. Vishay Siliconix make them and I think
they're second sourced.

Graham
 
K

Kevin

Jan 1, 1970
0
Joerg said:
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
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
 
J

Joerg

Jan 1, 1970
0
Hello Kevin,
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.

That sounds very feasible. Think about a digital solution as well. To
obtain a true 60dB dynamic range look at the effective number of bits
(ENOB) versus frequency and subtract one or so to compensate for
possible marketing optimism ;-)
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 was in a similar pickle in the 90's and had to cascade two logamps
because I needed at least 70dB at a good bandwidth. I believe it was the
AD640. Getting the whole thing stable is no small feat and you need to
master RF layout pretty well for that. But performance was stellar. The
only tear in the beer is that these chips now cost about $30 a pop.
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).

The SD5400 contains four switches on one die, meaning their capacitances
match very well. The trick is to use one switch as a "dummy" just to
compensate for charge injection.

Another switch method that I have used several times is a quad-diode
set, typically very fast Schottkys. I usually transformer-couple the
drive signal but that isn't a big deal. If you use a matched quad or one
of those "quad diode pills" the charge injection almost becomes extinct.

Regards, Joerg
 
J

James Meyer

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.

A "T" type feedback topology might help. Switching the lower value
element in the vertical leg of the "T" should allow for much higher parasitic
capacitance than trying to switch a classical single feedback resistor.

Jim
 
F

Fred Bartoli

Jan 1, 1970
0
Joerg said:
Hello Kevin,


The SD5400 contains four switches on one die, meaning their capacitances
match very well. The trick is to use one switch as a "dummy" just to
compensate for charge injection.

Another switch method that I have used several times is a quad-diode
set, typically very fast Schottkys. I usually transformer-couple the
drive signal but that isn't a big deal. If you use a matched quad or one
of those "quad diode pills" the charge injection almost becomes extinct.

I once used a good trick for ultra low charge injection. You can notice that
for some switches the charge injection sign reverses for some value of VDD -
VEE.
Then you have some charge calibration phase in your process and servo the
switch power supply accordingnly.
A bit heavy on the stuff but ultra low charge injection guaranteed, no aging
and no temperature dependance.
 
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