yvkrishna wrote on Oct 16th, 2012, 2:55am:Hi BOE,
We can always calculate the modified condition for achieving zero temp coeff in the presence of finite TC for resistors.
my observation so far is that the impact of resistor TC is not so much(ofcourse it depends on process one uses) for achieving ZTC current.
What else is the limitation?
Thanks,
yvkrishna
yvkrishna,
I've done this many time and it works fine. Like you figured out, you have to account for the TC of the resistor, but this is true even if you use a voltage to generate the current (your voltage TC has to be the same as the resistor TC.
The first consideration is that it is difficult to trim the TC with a single measurement. If you were doing a voltage-mode reference you can room temperature trim to a "magic" voltage that will give you zero TC for all errors except opamp offset. After that you would trim out the error due to the sheet resistance in your V to I circuit.
The second consideration is that with a current reference you would be trimming the magnitude and TC of the current simultaneously. While you can measure the current directly and trim it to spec, the resulting temp co will depend on how much of the error is due to sheet resistance variation or opamp offset, mismatch and bipolar IS variation. One way to get around this problem with the circuit you showed is first trimming the voltage across R20 (e.g. by varying the ratio of M1 and M0) to the "magic" voltage, and then trimming the output current by sizing M22.
Hint - when trimming don't actually change M0/M1/M22 sizes, but instead put a current dac in parallel with them that is biased with a smaller current scaled from the main current.