Filter design for thermocouples

[mhs100]

Hi all

 I want to design a 2nd order low pass active filter circuit for thermocouples , can anyone has a design of this which you have already tried and works well with thermocouples? What resistor and capacitor values you used?

I have studied the NI tutorials for signal conditioning of thermocouples.It is written that the frequency of thermocouple signal is near about 4  Hz. What is the cutoff frequency that you set ?

 Please send me the circuit or a link of this. Any help will be greatly appreciated.

Regards

mhs100  

[Buechsenschuetz]

There is no distinct cut-off frequency or frequency range when using thermocouples. The freq range of the amplifier should match the system requirements, and you also have to consider the thermal inertia of the thermocouple itself. I have only little experience with thermocouples but I think a cut-off freq of 10Hz or so is a reasonable value. - For a quick solution, you can simply use two first-order filters. A first-order filter can be designed quite easily when using an op-amp with feedback resistor. You should use a low input bias current op-amt (FET input or CMOS) so you can use a higher value of feedback resistor (it will be in the MOhm range). Switch a capacitor in parallel with the resistor and you are done with a first-order filter. The basics are explained here: http://en.wikipedia.org/wiki/Low-pass_filter

[mhs100]

Thanks for the reply .

Actually i know the basics of low pass filter but if anyone has the practically tested design of a filter for thermocouples, that is what i wanted for guidance. What tolerance of resistors and capacitors are acceptable

for thermocouple applications.

Regards

mhs100

Message Edited by mhs100 on 12-11-2009 06:35 AM

[Buechsenschuetz]

I have not used such a circuit for thermocouples myself. If you use two first-order filters in the basic configuration (R/C feedback), resistors with 1% and capacitors with 5% will yield sufficient accuracy. If you use a more complex design (second-order filter with a single op-amp) you should use capacitors with 1% tolerance since most of these circuits require close matching of actual component values. Caps with 1% are available in a limited range of values only, so usually it is better to start the design with a given (and easily available) capacitor value and calculate the resistors accordingly. Also, it is better to add a passive filter (with a much higher corner frequency) BEHIND the active filter to cut off any ringing or oscillations which might occur due to phase errors of the op-amp(s) at higher frequencies. 

[mhs100]

Hi

Which topology of low pass filters do you think is suitable for filtering of thermocouples? either Sallen-Key topology or Multiple feedback topology. Why?