multiplexor Inputs

One of the muxes I'm looking at is the Intersil 8x2 DG407DY. Using two of those would switch the pos and Neg of 16 channels. Any comments on these? Wow, that AD595 is pretty expensive. Any alternatives that work well?

Thanks

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Richard, I'm afraid you will need one compensation chip for each channel and also the required filter afterwards, and only then mux, because a mux in front will corrupt the signal. The charge injection will probably saturate the amp and the long time constants of the filter will need very long settling times. If you make up some kind of amplification/filtering all connections have to be at the same temperature ( that is why that chip is ceramic and expensive). But depending on your required accuracy you could thow away precision and have an instrumentation amp up front followed by a filter and you could subtract some temp dependent ice-point correction, that you have amplified identically, or probably leave that out as well. Who cares about +/-20°C then? So you save the AD595 as well. Go figure out.

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ciao Ban
Bordighera, Italy

Hi, it's not really feasable to switch the thermocouples directly. A better aproach is to have each thermocouple connected to its own amplifier/cold jinc compensator/filter input stage, then multiplex the output voltages. You dont need any exotic chips, even a 741 will do at a pinch and a diode can be used for cold junc comp.

As others have mentioned, the problem with using a mux upstream of the AD595 is cold junction compensation errors. One way around this, if you don't need extreme accuracy is to use a temperature sensor, such as the AD590 to measure the cold junction temperature (assuming that it is roughly the same for all 16 phusical thermocuople interface points). You can then replace the AD595 with an ordinary OP Amp or Instrumentation amplifier. You can do your own cold junction compensation in software, using the known thermocuple characteristics. I have succesfully used this technique for measuring high temperatures, where the variation of the cold junction temperatures among thermocouples was negligible. One way to ensure that the temperature variation is small is to place a large ground plane under the thermocouple connections. The ground plane will tend to equalize the cold junction temperatures. You can place the cold junction temperature sensor on or above the ground plane.

Ach, ye could hae fooled me. What makes ye think ye cannae do it?

...grounded junctions...

If you don't care a whit about accuracy and are using only the basest of base-metal thermocouples.

Best regards, Spehro Pefhany

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You don't really need one compensation chip per channel - you need to measure the temperature of the cold (local) junctions, so if you can keep them the same within the limits of accuracy you want to work with then one sensor will do. Most small multi input instruments work like this.

It is possible to use passive filtering, protection etc and a multiplexer and this can work well. You can get better performance by using buffer/filter amplifiers before the mpx. If you do it right you can use your buffers to get rid of common mode noise, filters to get rid of serial mode noise and then use a cheap modern mpx running off a 5V supply. Several modern ADCs or micros have built in temperature sensors and you can (with care) use one of these for the cold junction compensation.

Linear technology have the LTC2480 with built in sensor and an app. note showing how to use it for a single channel thermocouple system. (The package is not hand built prototype friendly - I know I've just soldered 5 of them !).

(I'm running a test on this now and the reference thermometer in the dry well calibrator says 70.02C and my PC reading out from the prototype says

69.9C. I'm happy because that was calibrated at 25C. I know from tests on other prototypes that the effect of ambient temperature changes in the range 5 to 40C is less than 0.5C.)

Michael Kellett

I had planned on using Phoenix Contact connectors all in one row and have the muxes and AD595's right next to them pretty close. I would think the temp would be very, very close this way, however I could be more wrong than I think.

The large ground plane sounds like a great idea.

Most of the temps measured would be in the 200 to 1200 Degree F Range if that make a difference. Accuracy of 1 - 2 Degrees F is plenty good for this application.

I have seen on the Net where some state to add 10k resistors to ground for each lead on each pair right before the mux to reduce noise, and also to help reduce the antenna effect from one open channel from affecting other channels. Does this make sense?

Richard

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That can be quite challenging to achieve in practice especially if you don't want to allow a 1 hour warmup or have a housing to contend with. A 10°F difference can arise fairly readily, depending on what else is around.

Not to "reduce noise", but it might be important where you have multiple instrumentation amplifiers- to establish a ground within the common-mode range. Most T/C's are low enough resistance that 10K will only affect the reading by a small amount. Commercial instruments will usually operate within specs for source impedance of 100 or even 1000 ohms maximum, so the effective shunt 20K could add significant error.

Best regards, Spehro Pefhany

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"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

Thanks for all the input. Sure gives me more to think about.

Richard

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