Relay or Analog Switches for Quality Signal Multiplexing ?

Hi Shyam,

I don't have much experience with analog switches (I assume you mean ICs with that function), but I have a lot of experience with relays. Relays are great when they are new, but get intermittent. If you can live with the leakage if the ICs then that's what I recommend. Intermittents can be very troublesome. Also, the IC would probably take up less space, so your unit could be smaller.

-S

Dear StandardsGuy

Intermittent failure is a serious problem. PLCs use Relays and analog switches (ICs) both. Relays have bounce problem and operating life also. Contact may break for ever or may get permanently welded are another two sets of problems in relays. Now this intermittent switching problem is also to be watched out. Good point that otherwise I missed.

Hi Shyam,

Good to see you are still about

I've used the Idec RJ & RQ series for the ins & outs on PLC's: http://www.idec.com/language/english/catalog/Relays/RelaySocketFamily.pdf

Mostly low power 500ma 120vac a duty cycle of 20/min, would start to fail at around 2,000,000 cycles & can be changed in seconds. The contacts would oxidedize. On a 12vdc 50ma application that was switched without a load the duty cycle was more like 4,000,000...

Dear Garthh,

The relays you have referred to are Industrial power Relays and they are supposed to be operated at low repeat rate and 10 million operations life with no power and <1 million operations with full load may be OK for power switching in minutes.

For measurement of analog signals, such as reading temperature pressure etc in industrial switching rate is rather fast and almost less than 1s switching time is required. Current is often very little or negligible. Switching every second means 30 million operations per year.

If I select a 1000/500 million operations life type relay then perhaps 100 million operations can be trusted or can be used for three plus years. As these numbers are statistical and there is no surety or guarantee that operating life will not have failure, these relays to be watched out or failure and intermittent failure also.

Cotorelay are often used for high operating life. Perhaps Mercury wetted relay of long life with zero bounce will be ideal.

Hi Shyam. StandardsGuy have already given you the main points.

Relays are nice if you want a high quality analog signal, essentially undistorted. As an example I use them in a high quality preamplifier -that I have designed and built for my home- for changing between the several input signals (i.e. signal sources like CD, phono, mic e.t.c.). In this case, these changes between the several signals are rather rare (and the bounces are not of great concern). But there are other applications (like in telecom) that there is a need for very often and fast interchanges between several analog channels (without bounces too). So, our choice should be -inevitably- electronic switches (mosfet, ICs) due to their durability and reliability. They don't suffer from aging and electromechanical failures (like relays do). They are fast, free of bounces and small in size. They can "open and close" essentially infinite times without any degradation of their performance. The disadvantage is that they may produce a tiny distortion of the signal and they add some noise due to their -very small- resistance.

Relays gives you, also, isolation between the input (coil) and the output (contacts). There are some applications that demand such an isolation (i.e. dry contacts). But even in this case an optocoupler (e.g. an optomosfet) can offer you this isolation (instead of a relay). Again you have better reliability and smaller size.

Hi G.K.

While home audio with 40db to 60db may sound great, and such isolation can be easily achieved by analog switch ICs. However, I am working with signals that require absolute value accuracy better than 0.01% or 100ppm and this means we are looking for 90db signal isolation from noise injection. In analog switches if applied properly one can get <0.01% cross talk but that is not easy to achieve. However it may be worth to get that for speed one can get.

Other alternative was parallel processing to do all processing on each channel separately and then process the signal digitally in calm manner. This is increasing cost and I am getting greater speed also due to parallel processing. I like this one.

Cost of the 18-bit dual slope ADC is about $10-$20 range. Cost of relay and support circuit also $10-$20 range. I want to give up relays if that can help. However, signal multiplexing with relays if can't be avoided then will like to explore relays that will not fail easily.

Hi Shyam,

have you tried solid state relays? or even opto couplers?

Dr.D.Ramakrishna Naidu

Managing Director

Artec Sriman Controlls private Limited

Chennai-600017

www.artecrobotics.com

Solid state generally referred to power switches and analog MOSFET switches with some optical isolation. MOSFET analog switches are no better than other types in leakage current and are not very much suitable for signal multiplexing.