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Electronics => Projects, Designs, and Technical Stuff => Topic started by: cozdas on December 26, 2019, 05:16:17 am

Title: 20-channel DIY Scanner card for Keithley DMMs and DAQs
Post by: cozdas on December 26, 2019, 05:16:17 am
[attach=1]

I implemented this some time ago and mentioned that in the DMM6500 topic (https://www.eevblog.com/forum/testgear/new-keithley-dmm6500/msg2401032/#msg2401032)

Few people showed interest in this project thus I wanted to opensource it. You can find the details along with project files and sources in this github project page (https://github.com/cozdas/CozScan2020).

Feel free to ask questions and post comments.

Happy holidays.
Title: Re: 20-channel DIY Scanner card for Keithley DMMs and DAQs
Post by: Alexcnc on December 26, 2019, 12:16:12 pm
Thank you for bringing the project open source!
Title: Re: 20-channel DIY Scanner card for Keithley DMMs and DAQs
Post by: thm_w on December 27, 2019, 01:51:29 am
Thanks for the notification, this is really good documentation  :-+. Hope to build it.

Some photoMOS, first is one OP used:
Code: [Select]
    PN     Voltage  Resistance   Leakage
TLP3558A -  160V   2R    1uA    $1.5-2.50
AQY212EH -  60V    2.5R  1uA    $0.25+ 
AQY210EH -  350V   25R  1uA     $0.25+ 
AQV253A  -  250V   8R   1uA     $0.8
LH1540   -  350V   22R  1uA     $0.8
PVA1354N -  100V   5R   1nA?    $2+  (limited life)
PVA3324   - 300V   24R  1nA?    $2

- Equivalents for AQY: TLP227G, TLP222G, G3VM-351D, LCA110L (6pin, lower R typ) $0.5-3

Reed relay
- D1A050000 - 200V 0.1R 1nA - $0.60

Not sure how much resistance matters? Shame if 25R is too high as there are a ton of surplus ones, probably from telecom gear.
Voltage will depend on what circuits you plan to measure. Original spec was ~200V but I'm sure the relays were rated for slightly higher.
Title: Re: 20-channel DIY Scanner card for Keithley DMMs and DAQs
Post by: cozdas on December 27, 2019, 04:04:14 am
Quote from: thm_w on December 27, 2019, 01:51:29 am
Thanks for the notification, this is really good documentation  :-+. Hope to build it.

Some photoMOS, first is one OP used:
- TLP3558A - 160 V 2 R - $1.5-2.50
- AQY212EH - 60V 2.5R - $0.25+
- AQY210EH - 350 V 25 R - $0.25+
- Equivalents: TLP227G, TLP222G, G3VM-351D, LCA110L (6pin, lower R typ) $0.5-3

Not sure how much resistance matters? Shame if 25R is too high as there are a ton of surplus ones, probably from telecom gear.
Voltage will depend on what circuits you plan to measure. Original spec was ~200V but I'm sure the relays were rated for slightly higher.

If you implement it let me know the outcome.

My thinking regarding the on-resistance was to keep the loading error less than 1 digit (1ppm). With 10MOhm input impedence, TLP3558A results in ~0.2ppm which is 3.2 LSB of the 24bit ADC.
Title: Re: 20-channel DIY Scanner card for Keithley DMMs and DAQs
Post by: Gregg on December 27, 2019, 06:04:13 pm
Thank you codzas for your hard work.  Now I have another reason to want a DMM6500.   :-+   :-+
Title: Re: 20-channel DIY Scanner card for Keithley DMMs and DAQs
Post by: Kleinstein on December 27, 2019, 06:16:29 pm
I would expect the 25 Ohms or even 100 Ohms to be still OK. It may effect the 10 MOhms input impedance ranges a little, but this would be higher than 10 V and usually the lower accuracy ranges anyway.

I would be more worried with leakage between the channels. So it may be a good idea to not use the very low resistance ones. At least unused channels should not contribute much to leakage.  So the scanner may not be a good choice for high impedance source.
Title: Re: 20-channel DIY Scanner card for Keithley DMMs and DAQs
Post by: thm_w on December 27, 2019, 11:53:27 pm
Quote from: Kleinstein on December 27, 2019, 06:16:29 pm
I would expect the 25 Ohms or even 100 Ohms to be still OK. It may effect the 10 MOhms input impedance ranges a little, but this would be higher than 10 V and usually the lower accuracy ranges anyway.

I would be more worried with leakage between the channels. So it may be a good idea to not use the very low resistance ones. At least unused channels should not contribute much to leakage.  So the scanner may not be a good choice for high impedance source.

I added leakage current spec to my post. Seems the mos ones above at least are similar, for their max spec.

LCSC has some reed relays for ~$0.60, with better leakage (nA). But of course life is limited (10^8 cycles, about 3 years switching every second). There are also normal relays, and spec for the cheapies is really bad (~5uA, 10^5 life only!). Maybe some value in having a few reed and few photomos or similar.

One thing I'm not sure of, what is the purpose of channel 10 on the original  (https://github.com/cozdas/CozScan2020/blob/master/Documents/KeithleySchematic.png)scan card? It uses K111 relay and U105 3301 photomos in series, with some 100R resistors. The other channels use a single relay.
Title: Re: 20-channel DIY Scanner card for Keithley DMMs and DAQs
Post by: cozdas on December 29, 2019, 04:28:20 am
Quote from: thm_w on December 27, 2019, 11:53:27 pm
One thing I'm not sure of, what is the purpose of channel 10 on the original  (https://github.com/cozdas/CozScan2020/blob/master/Documents/KeithleySchematic.png)scan card? It uses K111 relay and U105 3301 photomos in series, with some 100R resistors. The other channels use a single relay.

SCAN-2000 card that I bought from eBay (probably an older revision) doesn't have solid-state switches, just mechanical ones. Although the SCAN-2000 manual doesn't explain the why there are Solid-State Relays (apparently in the newer revisions), SCAN-2001 manual does explain it. I think with a special mode enabled in the DMM, the mechanical relays of both channel 5 and 10 are closed and the fast switching between ch5 & ch10 is achieved by the photo-mosfets.

[attachimg=1]

Title: Re: 20-channel DIY Scanner card for Keithley DMMs and DAQs
Post by: thm_w on March 15, 2020, 07:11:05 pm
I went to test the card and it looks like I've got the pinout back to front  |O
Time for Rev 2

[attachimg=1]

[attachimg=2]

Title: Re: 20-channel DIY Scanner card for Keithley DMMs and DAQs
Post by: voltsandjolts on March 15, 2020, 08:22:38 pm
I have a K2000 DMM (2006 vintage, latest firmware) which probably won't support a 20 channel scan card.
I say that because there is a specific version (K2000-20) which does support a 20 channel card.
Does anyone know what the differences are between K2000 and K2000-20?
Is it just firmware?

Cross-reference to 10-channel card which works with K2000 DMM:
https://www.eevblog.com/forum/circuit-studio/example-project-relay-scan-card-for-k2000-dmm/ (https://www.eevblog.com/forum/circuit-studio/example-project-relay-scan-card-for-k2000-dmm/)
Title: Re: 20-channel DIY Scanner card for Keithley DMMs and DAQs
Post by: voltsandjolts on June 22, 2020, 02:20:36 pm
Another option for 20 channel SSR scan card here:
https://www.eevblog.com/forum/circuit-studio/example-project-20-channel-solid-state-scan-card-for-k2000-dmm/ (https://www.eevblog.com/forum/circuit-studio/example-project-20-channel-solid-state-scan-card-for-k2000-dmm/)

Title: Re: 20-channel DIY Scanner card for Keithley DMMs and DAQs
Post by: Anders Petersson on March 16, 2021, 03:49:36 pm
I built one (slightly modified). It works with Keithley DMM6500 but is not recognized by Keithley 2010.
My original 2000-SCAN card is recognized by Keithley 2010, so it's likely the 2010 doesn't support 20-ch cards, only 10-ch. (Neither the manual of DMM6500 nor 2010 mentions support for 20-ch cards.)

EDIT:
Looking at voltsandjolts version (https://www.eevblog.com/forum/circuit-studio/example-project-20-channel-solid-state-scan-card-for-k2000-dmm/ (https://www.eevblog.com/forum/circuit-studio/example-project-20-channel-solid-state-scan-card-for-k2000-dmm/)), I see a suspicious section in the source code:

Quote
    //The SPI clock line from K2000 idles high, whilst from DMM6500 it idles low.
    //Determine idle state of SPI clock by waiting for 50 samples of the same value

Possibly the Arduino code just needs to do something similar to support K2010.
Title: Re: 20-channel DIY Scanner card for Keithley DMMs and DAQs
Post by: Neuromodulator on May 06, 2021, 08:22:56 am
It would be interesting to fabricate the piece that keeps the PCB in place.