SCAN-2000-LV-20 a low voltage scanner card for Keithley 2000 - Page 1
Forum section: electronics / projects

author: ketil b
author_rank: Regular Contributor
timestamp: November 16, 2020, 10:02:38 am
content: Hi allFirst, off I would like to thankvoltsandjoltsandcozdasfor there excellent work.I have a project coming up where I need to log some low voltages (-/+10v) over a relatively long period of time. So I decided to make a scan card for one of my 2000 based around the DG412 quad SPST analogue switch.I broke out the USART of the stm32 and added the layout for an isolated DC to DC converter so I could use it as a stand-alone board if I wanted.I am just working on the layout at the moment then on to some coding.Thanks allKetil

author: ketil b
author_rank: Regular Contributor
timestamp: November 16, 2020, 10:55:27 pm
content: I finished the layout, I think

author: jbb
author_rank: Super Contributor
timestamp: November 17, 2020, 12:27:50 am
content: I haven’t made a card like this before, but I expect others around here have.I had a peek, and thought of a couple of things:- check V+ and VL supplies are the right way around!- I see there’s an A/B bank switch. Is that for 4 wire operations? It’s the only mechanical relay...- a small series resistance on each input pin might be a good idea (but has accuracy implications) to reduce clamp currents- what happens if quite a lot of current flows into the clamps? Will they pull the rails up?- what happens if input is applied when the card isn’t powered?- there will be some small leakage currents through muxes and diodes. How will the scanner ground be referenced?- if the ground is referenced through the multimeter negative, what happens if someone applies high voltage to a negative terminal (eg measuring across a shunt resistor on +10V rail) and a low voltage (eg -10V) somewhere else?

author: ketil b
author_rank: Regular Contributor
timestamp: November 17, 2020, 12:32:00 pm
content: Quote from: jbb on November 17, 2020, 12:27:50 amWhoops thanks.QuoteYes either 20 2 wire or 10 4 wireQuoteGood idea, 200Ω will limit the current throw the diode to about 120mA with 40V on the input.QuoteYes, they will! but hopefully not that much if the max is about 120mA.QuoteNow, this I hadn't thought about. The diode will forward bias and that current will have to get to ground somehow, either throw the output correct of the DC to DC or the DG412s. I might have to try and destroy some chips to see what happens.QuoteThe leakage should be around 10 to 20 nA per channel and the scanner will be referenced to the ground of the multimeter. I did realise that the ground at the DIN conector is the digital ground and not the Lo input multimeter. I might have to float the scanner and reference it to the Lo output of bus A.QuoteThis I don't know! when one is connected it will be referenced to the Lo input of the multimeter and the other one will be floatingI have to rap my head around that, I feel that bad things might happen.Thanks for this, you have given me lots to think about.Ketil

author: jbb
author_rank: Super Contributor
timestamp: November 18, 2020, 02:12:12 am
content: HmmYou might also consider DG409. It’s got slightly more leakage on the off channels, but way less leakage than 4x DG412 (which it would replace).The DG409 has a little more information about clamp diodes and what happens to inputs with no supply present.Both DG412 and DG409 have a max supply rating of 44V from V+ to V- and 25V from GND to V-. So in principle we could get around this ‘one input pair at +10V and the other at -10V’ scenario by allowing a signal range of up to +-20V (if not to operate, maybe to survive without damage). You’d need to be careful with the clamping because there’s not a lot of room there between 20V and 25V!Oh yes. I forgot: if you want really though overload protection it’s probably good to have some small resistors (maybe 100R?) between the BAV199 clamps and the DG4xx input pins. The BAVs might handle a couple of hundred mA, but they’ll have 1V or more drop at that point. Whereas the DG4xx input clamp will start to conduct around 0.3V. If you want to limit that to 10mA or so (I forget what datasheet says) then you’ll need a resistor to limit the current.