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Originally posted by Groth
Mere mortals such as I are limited to hand matching out resistors - dare I ask how 'cheaply' you get laser-trimmed resistors arrays?
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$2.95 for 0.01% ratio accuracy, and $2.43 for 0.02% ratio accuracy. The parts I can get aren't 10K+10K like I thought though, they are 20K+20K. (A bit higher error due to bias current with a worst case OPA227... +/-0.16% error due to bias current. Some hardware I've worked on lately autocals itself and measures the bias current of OPA227's. I've never seen one exceed 20% of the max bias current spec, so it's reasonable to expect less than a 0.033% contribution to error from using 20K networks instead of 10K.
Quote:
Originally posted by Groth
There is much to said for the ease of error checking with the first stage unity gain. But with the pain I went through trying to stamp out the tiniest input bias current errors and input voltage offset errors before they got amplified, I'm ready to try some early gain. I had more consistent luck with tweaking the bits-to-amps equations than I did tweaking the pots.
I'm not planning to keep the high frequencies for long, and I don't want to challenge Nyquist to a wrestling match.
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My boss is a physicist. He wrestled Nyquist for a while. Very painful to watch.
Anyway, I can get a few select values of 0.1% resistors for about $0.10 each. More error than I'd want to give away at the first stage amp, but perhaps more practical. You do have a good point about summing the shunt signals together and then filtering. Less to filter that way. Might as well take advantage of one of the benefits of a multiphase switcher.
Let me know what gains you are trying to hit, and I'll see what combinations of 0.1% resistors I can come up with.