Attached below is the transfer function of the Sallen-Key stage of the heater driver with the network elements as specified in the previous post (PLS:1749). Data is also zipped and attached. The first stage of the circuit is specified for factor 10 gain.

The cutoff frequency (-6 dB in power terms) is 0.154 Hz, which is in line with the design parameters.

Quote:

I have built a basic shield heater driver based on Rana’s initial suggestions using a buffering diff input, a Sallen-key active LP filter (0.15 Hz, 2nd-order) with a BUF634 in the loop at the output to provide some beef. This is presently just laid out on solderless breadboard, we can transfer easily to protoboard which is the next step.   For the LP filter I chose R1=R2=100kΩ and C1=C2=10µF.  Full circuit is illustrated below. I modified this from what was linked in PSL:1747 as that was not technically a Sallen-Key.     I took a Transfer function of the circuit with the opamp loop closed before the BUF634. However, I have left this on a floppy disk and will need to recover it Monday. It has the expected poll at 0.159 Hz with a 1/f^2 roll off. I will attach this in a seperate post.