Passive RC Filter Loading?

tangodirt

Yes, I did run the experiment again with the separated resistance and capacitance values. The experimental results showed a second order corner, but was marginally worse than the first order filter. I'm assuming this is because of poor component quality, but perhaps there is something else I am missing.

The theoretical response matched the buffered response almost perfectly, which I find very interesting.

NascentOxygen

"high" as in "won't measurably load the preceding filter stage"? Then that's good!

Should I surmise that your green plots are of two identical stages, cascaded? Looks like you should now be able to come up with something satisfactory for filtering those high frequencies.

tangodirt

The ADC they use has an input impedance of greater than 10 GΩ, so I do not expect filter loading, although I would assume an active filter would entirely eliminate any concerns.

Yes, I suppose I did not explain this very well. The first order filter uses a nominally 0.1uF capacitor and a 3.3kΩ resistor. The second order filters used the same 0.1uF capacitors, but with a 2kΩ resistors. For the decade separation filter, the same 2kΩ resistor was used, but the second stages used a 20kΩ resistor with a 0.01uF capacitor. The models used the measured values for resistance and capacitance.

Now that you mention it, there is probably a nice IC containing higher order filters, right? Looks like Maxim makes a few. Can anyone recommend any IC's to accomplish this? I'm fine with passive resistors, but if there's a nice filter out there that already exists, why reinvent the wheel.

NascentOxygen

10GΩ is fine. You could easily scale up to be 1:100 as I suggested.
With the first resistor just 3.3kΩ, then you need to consider the impedance of whatever is driving this, because its impedance (hopefully resistance) becomes part of the first R in your filter calculations. So the input to the filter comes directly from the thermocouple? Do you know what typical impedance the manufacturer quotes that to be?