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Suggestions for analog switch to select feedback resistor in TIA

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

Reply to
Kevin
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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

Reply to
Kevin

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

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Reply to
Joerg

Hello Kevin,

Ok, maybe Vishay doesn't carry the SD5400 anymore. But here goes:

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Regards, Joerg

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Reply to
Joerg

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

Reply to
Pooh Bear

Hello Kevin,

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 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.

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

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Reply to
Joerg

On 25 Jun 2005 11:01:31 -0700, "Kevin" wroth:

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

Reply to
James Meyer

"Joerg" a écrit dans le message de news:MBFve.129$ snipped-for-privacy@newssvr13.news.prodigy.com...

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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Thanks,
Fred.
Reply to
Fred Bartoli

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