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Signal relays vs LC2MOS at low levels

Hi, all:-

I'm in the process of designing relatively high-precision signal conditioning and data acquisition system front end.

It's desirable to have some kind of gain/range switching to maximize the dynamic range over different operating modes.

Available options for switching are ADGxxx type analog multiplexers (voltages are relatively high.. say +/-15V so HCxx are out) or "2A" telecom style signal relays as are quite commonly used in bench-top instruments.

All the relay data sheets that I've looked at have disturbingly high recommended minimum voltages/currents. Say 10uA or 100uV-10mV (usually the latter). I'd prefer to have just 10-20pA or so at < 1uV. With some compromise I could arrange to have 10uA flowing through the contacts, but that would result in a (possibly variable) ~1.5uV voltage drop. If it varied with vibration that would be Bad. There's a reference to some with special Silver-Palladium contacts for low-level loads, rated at 10mA max, but I see no availability.

Any actual experience with these things? It might be possible to "exercise" the contacts occasionally, or just before a measurement.

Also, on the ADGs, the typical leakage on the best ones is in the 10's of pA (according to the data sheets, I have not yet made any measurements). Any experience on the effects of long term radiation? Usually things get worse, but how much worse?

Also a bit worried about the "sealed" relays in low pressure.

Thanks for any info,

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward" 
speff@interlog.com             Info for manufacturers: http://www.trexon.com 
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany
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For relays, for very small signals and distortions, you might see if you can find mercury-wetted types. They're still around, if a bit pricey.

I know normal relays are regularly used for small signals, but I don't think I've ever seen any sort of guarantee applied to that use.

Tim

-- Deep Friar: a very philosophical monk. Website:

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Reply to
Tim Williams

We use

FUJITSU FTR-B3GB4.5Z-B10 OMRON G6KU-2F-Y-DC5 NEC ELEC UB2-5S

(all direct replacements)

about 16,000 so far. No problems. These are sealed, latching, DPDT relays. We use latching to keep coil power hence thermal EMFs down. We use these in low-level, thermocouple and such circuits. No semiconductor can come close to their on resistance, off capacitance and leakage, bandwidth, or voltage capability.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators
Reply to
John Larkin

The 20-bit A/D's are supposed to be good for eliminating this kind of headache.

Reply to
bloggs.fredbloggs.fred

Haven't you seen fretting or low drive current oxidation of the relay contacts causing failures?

Cheers

Klaus

Reply to
Klaus Kragelund

On a sunny day (Sun, 15 Sep 2013 13:19:46 -0400) it happened Spehro Pefhany wrote in :

Generally avoid relays for small signal altogether. I have seen those used in video / audio cross bars in TV studio, and replacing relays was a frequent occurrence. Indeed all relays (also gold contact) need a minimum 'wetting' current. In the uA range that condition would not be met.

And on the high current side, I remember one of my first assignments was in the city where we had all those film editing tables at different locations, go there with a box of relay contacts and ask politely if I could interrupt their work and replace those contacts as preventive maintenance, learn to know the people and places...

In short, relays are not reliable, at any level. Just as an idea, is it not possible to amplify your signals to a more usable level before doing the switching? Or are you also a TeraOhm collector?

Relays trimpots batteries electrolytics weather etc etc connectors all unpredictable.

Reply to
Jan Panteltje

I wouldn't agree that electrolytics are unpredictable. If you know the working conditions, it will be determistic

Cheers

Klaus

Reply to
Klaus Kragelund

On a sunny day (Mon, 16 Sep 2013 02:09:49 -0700 (PDT)) it happened Klaus Kragelund wrote in :

I have had a radio-tv repair shop for many years. Electrolytics will just stop working whenever they feel like it, in the same equipment and circuit where other ones will last for 30 years.

Electrolytics, tubes, high value resistors, solder connections too. Even some foil capacitors. HV transformers, HV rectifiers, CRTs, all common stuff to replace.

As to elcos there also was this series of those in PC mobos that went all bad after a short while ... But even the good ones, sometimes go bad.

Elcos are most certainly a life time limiting factor in electronic equipment, more than anything else. Come to think of it I replaced the caps in the power supply of this Samsung monitor too some years back, reported here, else it would have been end of life already.

Reply to
Jan Panteltje

The level is pretty high (volts), but I'm looking for uV. Similarly, the resistances are not very high (1K ~

Reply to
Spehro Pefhany

Thanks for the part numbers, John.

Just to be clear, do you use them with essentially zero contact current?

The Omron datasheet, for example:

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$file/G6K_0313.pdf

(front page)

--
Min. permissible load (See note) 10 uA at 10 mVDC 

Note: This value was measured at a switching frequency of 120 
operations/min and the criterion of contact resistance is 50 Ohms. 
This value may vary depending on the switching frequency and operating 
environment. Always double-check relay suitability under actual 
operating conditions
Reply to
Spehro Pefhany

I wonder - is this why I see so many failures of relays in scope attenuators? They usually fail with high contact resistance anywhere from .5 Ohms up to 20 ohms or so.

Reply to
JW

Yes, but that was a case of planned obsolescence (they lost the law-suit over this) and the other cases are caps used in designs without doing the math.

If you contact the major manufactors of electrolytic capacitors, you get quite good and reliable lifetime data (mostly IRMS and ambient temperature related). I have even gotten special capacitors with burried thermocouples for testing.

Cheers

Klaus

Reply to
Klaus Kragelund

On a sunny day (Mon, 16 Sep 2013 05:16:57 -0700 (PDT)) it happened Klaus Kragelund wrote in :

Yes, but the best feedback is always from the service departments... Comebacks are expensive, very expensive under guarantee for manufacturers of consumer stuff. Manufacturers will always present their data so you really want to buy, that goes for datasheets too :-)

Reply to
Jan Panteltje

I have very little doubt about the reliability of most aluminum electrolytics. Even off-brand parts are pretty good if they're not pushed too hard ripple-current wise. They are like power relays- lifetime is known fairly well and will be less than semiconductors, but good designs can be created using them.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward" 
speff@interlog.com             Info for manufacturers: http://www.trexon.com 
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

On a sunny day (Mon, 16 Sep 2013 09:43:14 -0400) it happened Spehro Pefhany wrote in :

Some time ago somebody posted a picture here of a Chinese capacitor with a high uF marking, opened with in it a small elco :-) So much for 'off-brand', or even 'brand', you would have to order directly from the manufacturer. One of the major distributors was hit by fake stuff too. I had some fake MAX232s.

Reply to
Jan Panteltje

I'll check with testing, but I think we've had zero failures. We are working at microvolts and picoamps, high impedance thermocouple signals and similar stuff. We're switching ranges and switching channels to a test bus, so we don't switch often. I'd expect some applications to switch once a month or so.

I think we mostly buy the Fujitsu parts. We use non-latching versions, too.

I have some timing data around here somewhere. They will go on/off in about a millisecond, something like that.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators
Reply to
John Larkin

no problems with Fujitsu parts, low level switching

no problems Bourns 3314 series. Trimpots don't need code!

Panasonic lithium coin cells seem to last forever. We've discharged new ones and ones back from the field for

10 years, same capacity. Rechargables suck.

Seem pretty good. Tantalums explode if you mis-apply them.

Through-hole or surface mount?

Usually no problems.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators
Reply to
John Larkin

Hi Spehro, I wish I could help. For my own edification, do you have any idea what the problem is at low currents? (and how that might show up in the circuit.) Do mechanical switches have the same 'issues'. Or does the sliding /wiping of the contacts help keep them clean?

George H.

Reply to
George Herold

Might have been me.

Or from an authorized distributor. Just no ebay and be careful with "market" items. I did get some caps directly from the manufacturer (not China, this time) that were completely missing the rubber bung. Something like 47uF/16V 0.002CV. Naturally, their capacitance measured a bit on the low side by the time we got them.

Were they good enough? Or just re-marked garbage?

Reply to
Spehro Pefhany

There are electrolytic caps in just about every piece of electronic mains-powered consumer goods out there.. light dimmers being a rare exception. CF bulbs, AC adapters etc. etc. are all dependent on the cap working well enough, and many applications are really sub-optimal being a hot environment. A lot of stuff the service department will never see if it lasts much past the warranty period. I've seen repairs come back on stuff that was 15+ years old in the industrial world. Re-capping the PS caps only (not the signal ones) was a frequent repair. That's why I use better rated caps in those locations.

Reply to
Spehro Pefhany

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