raja.cedt wrote on Feb 13th, 2012, 2:50am:hello Robg,
seems you are an expert in BGR so i will ask some Questions, even if you are busy please don't mind answer after some time.
1.Regarding noise, i got your point. It can be minimized by reducing ratio. About temp stability i need 50ppm, so i would like to know with normal BGR what is the typical ppm and if i want 50PPM what are techniques should i use.
Thanks for the compliments.
ppm will depend on your temperature range. You can expect about 4mV drop from 30C to 125C (or from 30C to -55C) in a perfectly tuned bandgap, plus whatever PTAT error you get from mismatches and process corners.
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2.About the op amp what is the main criterion to design, when i designed i just used simple op amp. Is there basic idea on Bandwidth and gain of the op amp. I know it has to be low Offset, because it will be directly coming at the o/p.
I'd say the biggest mistake people make is underestimating the requirements of the opamp. Your opamp will be configured in a gain of 8 or so, so all of your parameters that depend on loop gain will reduced by 18 dB. Power supply rejection, load rejection, and systematic offset are probably the ones that will hurt the most. And for the same reason, your offset and noise will be gained up by that factor of 8. (Using a high ratio of current densities like we talked about will reduce the amount of gain needed, which is why it is so important.)
There are two schools of thought on bandwidth - way lower than the load bandwidth so the output won't move with transient loads, or way higher so that it will settle quickly. To get a low bandwidth you'll need a huge compensation capacitance, or you'll have to lower your loop gain which will reduce your PSRR, load rejection, etc. I can't think of a case where I've opted for the "low bandwidth" solution unless I had an external capacitance.
Good luck!
Rob