Low current switching - Page 1
Forum section: electronics / projects

author: PrzemRS
author_rank: Newbie
timestamp: December 12, 2016, 07:02:11 pm
content: Hi all!This is my first technical post here so I hope this is good forum section for my post.I have some question. How can I switch few (3+ nA) current with high accuracy? I plan to build PC controlled low current generator based on FPGA and some DAC. I will use this generator to test current converters so is need to be as accurate as possible. Basic block diagram in attachment.It must be possible to connect any converter, so in settings will be option to set pinout. How make it in hardware on current signal pins? For now I have sall problems with my nano ammeter so is hard to measure itBest regards.Przemek

author: Kleinstein
author_rank: Super Contributor
timestamp: December 12, 2016, 07:34:42 pm
content: For low leakage one could use relays and a careful layout with guard traces and similar.Also good quality JFETs or MOSFETs can be an option. However for lowest leakage one might have to screen them: typical performance can be a lot better than worst case specs. One may need to test the final circuit anyway as those FETs are also ESD sensitive and the board may show leakage.With FET switches one has to expect leakage in the 10s of pA range.

author: mc172
author_rank: Frequent Contributor
timestamp: December 12, 2016, 07:50:55 pm
content: You will not be able to use relays or any other type of physical contact device to switch such low currents. Relay contacts have a typical wetting current around 1-5 mA (yes, milli) depending on how much you are prepared to pay.

author: Gyro
author_rank: Super Contributor
timestamp: December 12, 2016, 07:52:37 pm
content: Quote from: Kleinstein on December 12, 2016, 07:34:42 pmIt might need to be a Mercury wetted one for reliability at those sort of current levels, there are still a few kicking around on ebay in Europe.

author: PrzemRS
author_rank: Newbie
timestamp: December 12, 2016, 08:13:46 pm
content: Thanks for fast reply. I thought about using a MOSFETS but I'm afraid about internal capacitance between drain and source. It may destoy my signal due to charging capacitor I think

author: Marco
author_rank: Super Contributor
timestamp: December 12, 2016, 08:38:12 pm
content: Why would you use a FPGA? Just for the exercise? Modulated current sources don't tend to achieve very high frequency unless they are right next to the DUT. If you are connecting something with cables, then worrying about a small signal FET's capacitance is a bit silly after all ...

author: PrzemRS
author_rank: Newbie
timestamp: December 12, 2016, 08:54:08 pm
content: This tester is part of my master thesis work, so one of points for this task was to use FPGA onboard. I think maximum frequency still will be in kHz due to nA current so it is not much. My task is to get as best results as I can (lowest current as possible and highest frequency as possible without limitations).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 12, 2016, 09:50:40 pm
content: Quote from: PrzemRS on December 12, 2016, 08:13:46 pmYou didn't specify charge injection, or switching speed or repeat rate, so we are left to assume that anything is suitable.Tim

author: PrzemRS
author_rank: Newbie
timestamp: December 13, 2016, 08:23:20 am
content: Signal parameters will depend from user because output signal should be created by user in PC by bezier curves. So for now is time to make tests to see how it worksthanks for support!

author: RobK_NL
author_rank: Frequent Contributor
timestamp: December 13, 2016, 01:28:36 pm
content: Quote from: mc172 on December 12, 2016, 07:50:55 pmStrange then, how Keithley manages to sell scanner cards that are specified for pA levels. And, yes, those do actually contain physical contact devices.For the nA levels we're talking about here, I would use something like this:https://www.digikey.nl/products/en?keywords=255-4236-5-NDIt has AgPd contacts, especially suited for low level switching.

author: voltsandjolts
author_rank: Unknown
timestamp: December 18, 2016, 03:32:49 pm
content: ^^^Those Panasonic relays with Silver Palladium (AgPd) and Gold (Au) contacts are nice and reasonable price too (GBP4.52)I noticed that my Tektronix TDS3034 front end uses a latching Omron G6KU-2F-Y-4.5 to switch the 50ohm input resistor and latching Fujitsu B3GB4.5Z RF relays to switch AC/DC coupling and a self test signal. The Fujitsu B3GB4.5Z contacts are Silver Nickel (AgNi) with Gold Plating and (with a 10x oscilloscope probe fitted) will be switching currents at the nA level. They are surface mount and available at Mouser for GBP3.90.http://www.mouser.co.uk/ProductDetail/Fujitsu/FTR-B3GB45Z/?qs=sGAEpiMZZMt%2fATQILEa77ZuvPWshkswdId1W%2ffiBOsc%3d

author: PrzemRS
author_rank: Newbie
timestamp: December 18, 2016, 06:13:33 pm
content: Wow this relays are realy nice, and have good price and dimensionsSo they will be good for this project also

author: Benta
author_rank: Super Contributor
timestamp: December 18, 2016, 06:34:32 pm
content: Take a look at Vishay/Siliconix DG611A/612A/613A analog switches. Leakage current is in the sub-nA range.