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What about switching a resistor to ground previous to making the measurement. Or as part of a self test sending a current through all of the relays.
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Hello,
The thermal EMF spec is for the TXQ is 5uV and .3uV for the TX-S but those are for the non-latching parts. Are you using latching versions and if so have you measured the thermal EMFs (or tried - I guess it's quite tricky to do)?
I use the TQ2-L2-5V version (the latching type). Otherwise a 1-2 week operation would not be possible from a 9V block.
picture is here: https://www.eevblog.com/forum/projects/lm399-based-10-v-reference/msg567203/#msg567203
I am not able to measure thermal EMFs reliable since my 24-Bit ADC is quite noisy. (1uVpp with a integration time of 1 minute).
A partly compensation of the thermal EMFs is done in my setup since I am doing a offset measurement through the relays.
I can actually switch the +/- signal of the output to every + pin of the references or to the common ground. So its possible to do difference measurements between 2 references also.
Some cleaning of the contacts is also done by daily switching of the relays.
With best regards
Andreas
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finally received the Caddock MG745 40MOhm 0.1% resistors.
In my first checks they are well within spec and spread positive and negative in value.
That should help in combining them to a my specific values.
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Wow, thanks so much for this thread! I'm working on a scanner myself and it never occurred to me I could do it self powered!
Sent from my Tablet -
By the way, these might be a nice stand-in for those Omron relays: http://www.mouser.com/ProductDetail/TE-Connectivity-Axicom/V23079B1201B301/
$3/ea, 5V, DPDT 2-coil bistable in through hole. According to the datasheet thermal emf is <10uV, which is as good as the baseline version of the Coto Low Thermal relays (much better I imagine though as they are latching so no internal heat generation).
Sent from my Tablet -
It would be prudent to stock up on Mercury wetted bistable reed relays for this kind of use while they are still available on ebay / surplus market.
They're the only contact technology capable of truly dry switching, and with stable contact resistance and a cycle life long enough to never have to worry about.
Edit: ...and of course zero bounce. -
The scanner of the Prema 5017 SC uses Omron G6AK-234P, just for the info. The scanner is specified with thermo forces typ. ±1µV, max. ±2 µV after 1,5 h warm-up. The Prema 2080 scanner, that I had, is specified the same way using the same relays.
http://ohh.de/5017.htm
http://ohh.de/2080.htm -
Prema has a nice and bulky series 2000 of precision scanners. The top MUX 2010, that I had, have a max 50 nV of thermo forces.
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Prema has a nice and bulky series 2000 of precision scanners. The top MUX 2010, that I had, have a max 50 nV of thermo forces.
Never heard of that model, the only info I found is the manuel:
http://www.prema.com/index.php/de/service/messgeraetewartung/item/download/116_124e71e4309f5edffbaa2bfc370ef7b5
Smells like a teardown and a lot more photos of your unit, cause less then 50nV (measured on the plugs) is very interesting for the volt nuts here
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Smells like a teardown and a lot more photos of your unit, cause less then 50nV (measured on the plugs) is very interesting for the volt nuts here
And another saved eBay search! -
Inspired by Zlymex, I started to design my own scanner based on the same idea.
The goal is to make some improvements:
1. indicate active channel's number (1 of 16 LEDs). The measurements are made very slowly.
If there is some bad connection etc. it will be easy to identify.
I tested green LEDs that work satisfactory biased with 50uA of current.
2. provide Manual control of the channel
3. provide power-on reset (quite tricky part...)
4. use RJ45 connectors, so the user just cuts off-the-shelf CAT6 cable in two pieces and strips wires only on one side.
This also makes testing of relays easy - just disconnect RJ45 cables and plug in RJ45 plugs with pairs shorted by loop of wire.
The RJ45 connectors will be hidden inside metal enclosure to ensure thermal equilibrium.
5. use SMD components (prepared to reflow soldering, better EMF performance expected)
6. the board should fit standard TUFxx Fischer Elektronik enclosure
7. It's nice to have a spare relay on board (tribute to Data Proof scaners)
8. the design should be expandable to 2 wire 32 channels; 4 wire 16 channels; 4 wire 32 channels by using the same boards (with some PCB jumpers, desoldering of some components but basically same design)
9. last but not least: the scanner shall provide break-before-make action not to disturb expensive gear and save the relays.The quiz is: Open the schematic pdf file and find as many bugs as possible.
If anyone is interested in factory made PCB for this project let me know ->PM.
Zlymex has a free PCB if he wants
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Interested! Weighing up another 6 1/2 - 7 1/2 Meter or a 34970A and boards.
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First bug found: net NE0 added to J5 connector.
Without it the slave board would not break-before-make.
Schematic upgraded to rev 0.2
Edit:
SW1 pin 6 connected to wrong node.
Schematic upgraded to rev 0.3
Edit: Switched from 74HC4538 to 74HC123 for two reasons:
4538 has built-in power-on reset circuit which would disable generation of a pulse after turning scanner on. This is not wanted in this circuit.
4538 draws much more idle current.
Schematic upgraded to rev 0.4
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Now Interested for interests sake. Santa is bringing me an Agilent Box
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Heyo,
I've been thinking about scanner switching arrangements.
What I'd like:- Vrefs are switched to (floating) battery supply while being measured
- DMM can measure a single Vref or the difference between two Vrefs
Here's what I've come up with (attached).
You can't compare any Vref against any other Vref: they are separated into two groups, and any Vref from group "A" can be compared to one from group "B".
With this restriction, you can get away with only two DPDT relays per Vref, which is pretty cheap.
Here, four Vrefs are shown, but the idea is that any number of Vrefs is supported.
The DMM also uses two SPDT relays and can do the following:- Measure CH A to CH GND (i.e. measure a single Vref from group "A")
- Measure CH B to CH GND (i.e. measure a single Vref from group "B")
- Measure CH A to CH B (i.e. measure the difference between a Vref from group "A" and group "B")
Just to walk through some examples:
To measure Vref 1:- Vref 1 input is switched from SUPPLY to BATTERY A
- Vref 1 output is enabled, placing it across CH A - CH GND
- DMM inputs are switched to CH A and CH GND
To measure Vref 2:- Vref 2 input is switched from SUPPLY to BATTERY B
- Vref 2 output is enabled, placing it across CH B - CH GND
- DMM inputs are switched to CH B and CH GND
To measure Vref 1 against Vref 2:- Vref 1 input is switched from SUPPLY to BATTERY A
- Vref 2 input is switched from SUPPLY to BATTERY B
- Vref 1 output is enabled, placing it across CH A - CH GND
- Vref 2 output is enabled, placing it across CH B - CH GND
- DMM inputs are switched to CH A and CH B
Edit: the caps on each Vref are large enough that they can be switched from SUPPLY to BATTERY A/B without losing power. This is a somewhat simplified schematic, as there would probably a regulator after each capacitor, keeping a stable voltage supplied to each Vref.
Edit 2: to elaborate, Vrefs 1, 3, ... are in group "A" and Vrefs 2, 4, ... are in group "B". So you can measure Vref 1 against 2 or 4, but not against 3. To get the difference between 1 and 3, you'd have to do it indirectly by measuring 1-2 and 3-2, then calculate 2-3. With a large number of Vrefs, many cross-checks are possible, so errors in the system should become apparent.
Edit 3: the batteries can be charged, but all Vrefs must be on SUPPLY, so no (floating) measurements can take place while the batteries are being charged.
Edit 4: I wonder if this thread should be moved to metrology?
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doktor pyta, I no longer see the pdf of your schematic, was it removed from your post?
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Hi cellularmitosis,
it was weird situation when only I was attending my own quiz of searching for bugs
Besides the thread was filled with schematics which all contained bugs.
You will find rev1.0 schematic in the attachment.
Take a look on the list of operation modes.
*I don't know if anyone ever will use 8 wire mode but it is also available.
EDIT: new file loaded: operation mode table improved for clarity.
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Dont tell me that scanner-mainframe you just bought with the load of 20-relay-cards dont suffice cellularmitosis
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Dont tell me that scanner-mainframe you just bought with the load of 20-relay-cards dont suffice cellularmitosis
The scanner gets me started immediately with something that works, but does no good for the community! Ideally I can produce some smaller scanner kits, benchmark them against the Keithley cards with the COTO relays, and publish an open source design to help out fellow volt nuts
Buying old gear is nice, but reproducing equivalent results in a DIY project is a lot more satisfying
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Hmm, since my old Keithley 7064 card produces unstable readings in some configurations i would be interested in some new cards. The easiest solution for me would be designing keithley 705-compatible cards (defined lengths and a 15pin-card-edge-connector), but then i have the problem that they were build to drive non-latching relays and i would have to add drive logic on the cards to use latching-relays. Meh.
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Just whipped up a tiny board for a single resistor which uses machine-pin headers for a pluggable 4-wire connection. It has multiple footprints, so it will accept a vishay foil, a wirewound, etc.
Now, to design a scanning motherboard to plug them into...
https://github.com/pepaslabs/resistor-plug
https://oshpark.com/shared_projects/hp93BAlQ
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Just whipped up a tiny board for a single resistor which uses machine-pin headers for a pluggable 4-wire connection. It has multiple footprints, so it will accept a vishay foil, a wirewound, etc.
It would be better to have the sense connections next to each other to minimise any temperature difference and thermal EMFs. -
Dont tell me that scanner-mainframe you just bought with the load of 20-relay-cards dont suffice cellularmitosis
The scanner gets me started immediately with something that works, but does no good for the community! Ideally I can produce some smaller scanner kits, benchmark them against the Keithley cards with the COTO relays, and publish an open source design to help out fellow volt nuts
Buying old gear is nice, but reproducing equivalent results in a DIY project is a lot more satisfying
An open low thermal scanner would be a great community asset.
I'll offer my Data Proof 160 (still unproven, but works) for benchmark as well. -
Update. Try to find a typo (engraving company fault
)
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Update. Try to find a typo (engraving company fault
)
C'mon, what's a two CH16 between friends... ?