Can FPGAs inputs detect low currents?


I'm new to FPGAs.

I need some kind of 30-40 channel input extension beeing able to detect

0.01mA currents. (The current should flow through your fingrs .. ;) I already know that implementing some kind of multiplexer is no problem. But can I use a FPGA as well to detect these low currents? This could be done by a analog comparator or by amplifying using two transistors.

Of course this could be done using asics. But I don't know wether I need that a high volume to pay off.

Marc Weber

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Marc Weber
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If you read the data sheet, you find a 10 microamp max leakage current specification. That is the same magnitude as your sense current. Looks like a problem... Let me tell you a secret: Th FPGA input leakage current is in reality much lower. We specify 10 microamp because that is easy to test, and the fall-out is zero.

My suggesti> Hello

Reply to
Peter Alfke

For current that flows through one's fingers, is there enough ESD protection for your needs? FPGAs are more robust than some circuits but give it a zap with some fingers fresh off the dry, winter carpet and TSZAAAP!!!! No more FPGA.

Is finger current sensing what you need or simply what you think may work well? There are capacitive touch sensor techniques being promoted by the likes of Analog Devices (REAL nice capacitive sensor) and Cypress with their CapSense technology. You can keep all your sensitive silicon insulated from the vicious outside world.

- John_H

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In every IOB, in every Virtex E, Virtex 2, Virtex 2P, Virtex 4, Virtex

5, and every Spartan 3, 3E, 3A, 3AN there is:

a CMOS differential input comparator.

Unfortunately the leakage current specification for any IO pin is +/- 10uA.

Your signal, .01mA IS 10 uA! so we can not be used The signal you need to detect is the same size as the leakage variations).

Leakage below 10uA takes a very high quality comparator (expensive), so I doubt there are any 'simple' solutions (like you claim).

"Any ASIC" would also require less than 1uA pin leakage, which is non-trivial. Low leakage means the pin has very little ESD protection (ESD protection results in more potential for leakage).

Good luck. Perhaps there is a low cost, low leakage quad comparator which could be used for detection before the FPGA?

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(15nA leakage)


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You should give more info on what you are trying to do, and is this a Lab situation, or something you expect to have in volume production.

If you want to sense 10uA to 1% precision, then no, Digital devices will not work for you.

If you just want to sense if someone has touched a terminal, than you might be OK. Leakage currents are highly temperatute dependant.

Rather than run 40 wires to the FPGA, you could use low cost logic devices like HC165, or HC597 to chain all your detect points into a serial data stream, or if you need lower voltage, use a CPLD like Atmel ATF1502BE/ATF1504BE - that specs 1uA MAX leakage, and has good schmitt buffers.

You may not need a FPGA at all, if you just want to sense & act on a few lines - CPLDs can do that very easily.


Reply to
Jim Granville

Thank you all for your replies.

Can you/ would you use an fpga to replace a microcontroller already containing already USB support? (1000+ < $1,70).

I hope this is not to much off topic. I want to design a new computer interface. A mouse keyboard hybrid. Similar to the one described on

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(US patent only) but the two moveable parts should be more like a mouse:

= = =. x |_,_,_, ( x = optical sensor, , = keys, ~ = USB cable ) =============~~~~

Because they should be moveable I can't imagine using some kind of fixed grid to reduce inputs. I want to have have between 30 and 40 keys The Fxx keys could be accessed using a special shift key. I'd like to have two mouse pointers as well. Some drivers already have been written for WinXP .. (cpnmouse on sf) There are different ways to implement the keys: a) standart keys you have on most keyboards:

- to big. Why do you need to press a key down anyway ? All you need is feedback. b) using your finger as resistor. (This has caused the original question) (-) ESD problems 30-40 channel using multiplexers (approx 4EUR) : (eg 5x8 channel : DG408DYZ ¿ INTERSIL ¿ IC, SM 8-KANAL ANALOG MUX) quantitios: quantitiy 100+ 5*0,79 EUR (de.farnell)) + one transistor + comparator or two transistors two amplify current I've tried this using a atmel avr microcontroller and it did work well using IO extensions would mean overkill when adding 30 to 40 amplifiers ? using FGPA: nice: everything can be integrated wihtin a chip, even the microcontroller ? eg "The LatticeMico32" is a highly configurable 32-bit Harvard architecture "soft" microprocessor core for Lattice FPGA devices" [1] Don't know how much this component would cost? Parhaps you can give me an estimate (FPGA/CPLD to just collect inputs, FPGA conctaining the microcontroller as well?) c) capacitive sensors (-) you need two wires for each "key" (-) I think they are more expensive compared to a conductor (But I don't know exactly because I don't have much experience in calculating final production costs (lacking experience) I know about: (approx >= 4.05 EUR )

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(3x $1.35 quantity 1000+) c) some kind of touch pad

- you don't feel wether you have hit the key or not because everything is one plane. e) tactile switches ? Do you think there are some beeing an option? Mmmh I just don't know which is the best way to implement this yet.

And I don't know yet which controlling component to use. Atmel AVR. I have a developement board (avr-gcc etc) The chips used by avagotech's SDNK reference kits? (Cypress / freescale)

If you are interested and want to join drop me a mail.

Marc Weber

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Marc Weber

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