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PMOD R2R resistor values


jmh

Question

Greetings,

We built an R2R DAC. Later I found the schematic at https://www.digilentinc.com/Data/Products/PMOD-R2R/PmodR2R_sch.pdf. My question is: Should the 200 Ohm internal resistors going in to the PMODs figure in the R2R values. We used 1.8k and 1k resistors, figuring on including the 200 Ohm resistors to make 2k. The pdf schematic above uses 10k and 20k resistors.

Any comments? Opinions?

 

Thanks,

John

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200 ohm on top of a 20k resistor is about 1% - so not a lot. It will work pretty much the same. Worst case or a 8-bit DAC it might be off by equivalent to a few counts when switching from "10000000" to "01111111". 

An extra 200 ohm on top of a 2k resistor is 10% is a lot. In this case going with 1.8k is a much better idea.

That is most probably why such usually high R/2R values are used - to minimise the effect of any source impedance.

At a guess the output impedance of the 10k/20k DAC is about 15k, which is easy to load down, so it should really be buffered...

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