Dear Prahlad,

Yes it does look odd that when you already have a MCU with 80% of its code flash still empty, you go for an analog sub system !

There were a few reasons why I did, what I did.

$ Maths. I had located some transfer functions to calculate the error terms , but it did look that they are going to take up quite a bit of the processor time. 2nd order polynomials?

$ The aspect of error sampling frequency. This is one area where the analog system shines bright. You just have to contend with three gain settings of P,I,D. With a digital approach, sampling freq. is one more variable that has to be tackled.

$ The analog front end with a digital PID. Plenty of circus is involved to get atleast 10 to 12 bit accuracy, repeatably. When I go with the analog PID, I am no longer worried about the quantization error and other non-linearties of the ADC front end.

With all this, I still have my fascination for the digital PID loop controller. It was just that in this case I was looking for a quick-fix, for the problem on hand and the analog system appealed.

All said and done I believe that a good digital PID loop with reliable Auto-Tune capabilities is really a DSP job. Not sure if they are done with 8 bitters like 8051.

Regards

Raghu