My solution to this was always to use 3 resistors for each leg of the ladder (either 2 in series, or 2 in parallel), this leaves you free to choose values. Buy your resistors by the yard on a bandolier and you'd be surprised just how tight the match between pairs of resistors is.plugwash wrote:The trouble with the "measure a load till you find one that matches" technique is that resistors form the same batch are likely to have very similar values. Plus you will need a very good meter.
8 bit means 256 steps so at minimum you would want resistors accurate to 1 part in 256 (0.4%) of the ideal value. This deviation needs to consider both selection innacuracy and tolerance inaccuracy.
Now an 8 bit resistive DAC isn't entirely unreasonable. 0.1% resistors are not terribly expensive nowadays. and if you go for a R-2R ladder design you only need two different values (and there are standard values that differ by a factor of exactly 2).
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