Floating Current Source IC? - Page 1
Forum section: electronics / projects

author: SnoopKatt
author_rank: Contributor
timestamp: April 13, 2015, 04:42:57 am
content: Hi,I'm wondering if there are any IC's out there that can provide constant current on a floating load? I'm looking to provide 200 uA to a floating load without either terminal of the current source needing to be connected to a shared point on the input voltage supply. Due to the fact that the load's resistance and other parameters may change, I need to make sure 200 uA is entering and leaving. While I found some solutions that can pull and push 200 uA from each end, they require at least two IC's, and I would like to simplify this with one. I spent hours searching around online, but I haven't found anything. I looked around for "Voltage Controlled Current Sources" & "Floating Current Sources" but no avail. Do you guys know of any single IC solutions that would work?Thanks!

author: luky315
author_rank: Regular Contributor
timestamp: April 13, 2015, 05:50:50 am
content: Simple, roubst, expensive: LT3092The "improved howland current pump" works fine.Or you find something here:http://www.linear.com/solutions/Current_Source_Solutions

author: SnoopKatt
author_rank: Contributor
timestamp: April 13, 2015, 08:55:41 pm
content: Ah I misread the REF200 sheet. That should do the trick, thanks!

author: SeanB
author_rank: Super Contributor
timestamp: April 13, 2015, 08:58:57 pm
content: What is the supply voltage that you are dealing with. If over 40V both the LM334 and the REF200 are not suitable as is.

author: SnoopKatt
author_rank: Contributor
timestamp: April 15, 2015, 07:00:18 am
content: It's +10 to -10 so I think it should be alright.

author: SnoopKatt
author_rank: Contributor
timestamp: April 16, 2015, 08:54:50 pm
content: Talking to some of my professors, I recently learned that there cannot be a mismatch between the current sources of more than 25 nA! Since it's being applied to the skin for reverse iontophoresis, a larger mismatch will be harmful. Are there any special chips that work similarly to the REF200 but with less margin of error?Thanks!

author: Marco
author_rank: Super Contributor
timestamp: April 16, 2015, 09:29:33 pm
content: Can't you just float your circuit? If it is isolated except for the current connections then the current into the DUT is going to be the current out of the DUT, regardless of any connections the DUT has.If that's not possible, I'd say it's time for a different approach ... take tightly matched quad or 2x dual resistor array (LT5400 for instance). Use one pair of matched resistors attached to the supply rails with a REF200 in between, this will get you a precision matched voltage relative to both supply rails. Use the other pair as current shunts also attached to the rails, with a P-MOSFET attached to the top one and a N-MOSFET to the bottom, drive the MOSFETs with (auto-zero/zero-drift) opamps to get the same voltages across the shunts as across the ones driven by the REF200.You could also use one of Linear's precision switched capacitor ICs to get the matched voltages relative to the supply rails instead ... but it couldn't handle -10V to +10V.PS. if your circuit does have an earth connection and your DUT decides he wants to slide his slippers across the floor a bit you won't be able to match the currents, 10V is going to lose to kV's.PPS. will need some series resistance after the MOSFETs and low leakage TVS's from the drains to the rails.

author: SnoopKatt
author_rank: Contributor
timestamp: April 16, 2015, 11:08:16 pm
content: Quote from: Marco on April 16, 2015, 09:29:33 pmIdeally, that's what I would like to do. Unfortunately, since the body is capacitive, even if it's just (10V)->current source->body->(-10V) (those in series), my professor told me that a harmful charge can be left on the body. Is there a supply that acts something like this, where it can just be specified a current to apply across a load (Autolab PGSTAT101)?http://www.ecochemie.nl/Products/Echem/CompactNonModular/PGSTAT101.htmlQuote from: Marco on April 16, 2015, 09:29:33 pmAh, good thinking. That sounds like that should control the mismatch pretty well, thanks!

author: Marco
author_rank: Super Contributor
timestamp: April 17, 2015, 02:22:15 am
content: Quote from: SnoopKatt on April 16, 2015, 11:08:16 pmThat's simplya single current source, not really different from using a single REF200. It regulates high side voltage with a low side shunt to create the current, but that's an artefact of implementation.When your circuit floats the +10V and -10V only exist within the reference frame of your circuit, the DUT initially sees the current as being caused by balanced voltages around it's own potential. If the DUT is capacitive without enough leakage to offset the 200 uA then eventually you'll charge it to 20V between the electrodes (potentials to earth at each electrode depend on the resistances between the electrodes and the electrodes to earth, not your circuit) after which the current will equal the leakage current.This is almost exactly what will happen with the matched current sources, the only thing the matched current sources can do is prevent part of the currents going through earth if your circuit is mains earthed under some circumstances (it can still happen without yet more circuitry). The floating current source does it better.