hung_wai_ming(at)hotmail.com said:
winsonpku
I guess from your description, it comes from the mismatch of the two ratioed resistors, where the centre part does not match with the rest of the resistors. Most of the time, we may increase the small resistor by paralleling them. For example, you may have a resistor of 10k matching with 200k, a ratio of 20, in order to match them well, we will increase the finger number of 10k resistor. Say, each finger has 10k, and originally, you will have 1 finger in the center and 20 fingers surrounds it, but we will pair up the 10k by splitting them into two parallel 20k (parallel sum=10k) so that you have 4 fingers of 10k interdigitated in those 200k resistors to minimize the process profile effect.
I have faced similar problem before as well