Running SCC-TC02 module at high frequency?

[JG001]

My project is a mess! Can someone please help me? Here's the situation:

I have a SCC-TC02 module that I'm trying to use to measure temperature from 16 K-type 0.005" diameter thermocouples (TC) using ADG406 multiplexers (MUX). The measurement frequency needs to be in the hundreds of Hz. My project also requires me to periodically cycle the MUXs ON/OFF. I've done the following tests and am kind of baffled:

1)      I created a temperature measuring task that acquired samples at 1 KHz. I kept the MUXs ON and then just cycled through the TC channels (0 to 15). Even at this high sample rate, the temp reading was about 24 degrees C for all channels (great!).

2)      Then I created a little VI that had a button to turn the MUXs ON/OFF. I set the sampling frequency to 1 KHz and started to acquire temperature signals. When I turned the MUXs OFF, temp jumped to something like 1350 degrees C (SCC-TC02 was saturated). This was expected. Great so far. But when I turned the MUXs ON, it took about 350ms for the temperature to come back down to 24 deg C.

I then went back to the spec sheet for the SCC-TC02 where it says that it has a 2Hz bandwidth LPF. Now, I have the following questions:

1)      Is the LPF the reason why I'm seeing a 350ms settling time?

2)      Why wasn't this delay present when I kept the MUXs ON and just changed the TC channels?

3)      What can I do to measure my 16 TCs at a decent scan rate?

Thanks for any comments/suggestions.

[enderWiggin10]

Hi JG001,

I will echo what I wrote to you via email before:

What MUX are you using? Is the MUX from National Instruments? When you turn the MUX off, is it still physically connected to the TC-02? If it is not connected, the signal will float until it reaches the rail as you have noticed. That voltage is built up on the line, and when the MUX is connected again, the signal has to be dissipated, through the wires it is connected to; in this case, the thermocouples have to dissipate the high voltage signal. This is observed with any multiplexing card as well, and is referred to as a "ghosting" effect. If a high voltage signal is on channel 1 and then a low voltage signal is on the following channel in the multiplexing sequence, the first voltage has a chance of artifacting on the low-voltage line. With this said, a good solution to the issue would be to ground the TC02 when not connected to the MUX.

Best,

[JG001]

Adam,

My apologies for not checking the forum first, because I just sent a reply to your email. But just for other forum participants, let me copy the content of my email:

I tried grounding a spare channel on my SC-2345 module and inserting it into my scan chain. The method didn't work. I placed the grounded channel both before and after my temperature channel. No change.

Then I did a test - I kept the TC+ and TC- terminals of my SCC-TC02 open (no TC connected to it) and acquired voltage at this channel. Naturally, the voltage was pegged at just over 100mV (TC02's limit). Then I shorted the two terminals which caused the voltage to be zero (essentially grounded the module). But unfortunately, I did notice a rather large RC time constant. So, I think I'm running into the fundamental limitation of this module (SCC-TC02). A PDF is attached showing my data - if anyone is interested.

What I found is that there is about a 810 ms settling time for this module when going from rail to GND. I'm using K-type TCs which have a coefficient of about 42uV/ deg C. My temperature measurement limits are 0 - 485 deg C, which means that my TC02 voltages will be between 0 and 20.4mV. Even if I use some kind of grounding logic while the TCs are disconnected, a 0 to 20 mV transition still takes about 687ms (see attached PDF). So, there will be a slight improvement by grounding the TCs, but not much.

So ... any suggestions for improving the temperature acquisition frequency? Thanks.

[enderWiggin10]

Thank you for the update and detailed explanation.  Looking more into the problem at hand, this makes sense considering the low-pass 2Hz filtering circuit built into the front end of the module along with the amplifier.  I am not certain of the exact R, C, or L components, but a time constant is very appropriate in this circuit.  This typically is not an issue because the time constant of the circuit is shorter than the time constant of the thermocouples themselves in normal use.  In your situation, the thermocouples are already at steady state, but the circuit still has to respond after the switch is connected again.
 
What SC box do you have the TC02 connected in?  What DAQ card do you have tethered to the SC Box?  I am able to sample at 1kHz in continuous mode with the TC02 on my machine without any issues.  I would suggest using a continuous acquisition for a faster rate.  Let me know if I have missed anything, or if I am overlooking anything 🙂

Obviously, if there is a step input to the TC02, there will be a settling time/time constant delay, but you can achieve a sampling rate as discussed above greater than 800Hz, hardware timed.

Best,