Analyzing example I-to-V circuit for a DAC

[Pulse Cloud]

Hello, I need help analyzing the circuit above, taken from page 18 of the AD9740 current-output DAC datasheet.

I know that for a

Vout = R2/R1 * (V1-V2) + Vref, but I'm having trouble figuring out the value of R1.

At first glance I thought R1 = 225 Ohm because the 25 Ohm resistors are there just to convert the current outputs to voltages, but why would AD choose 225 Ohm instead of 250 Ohm? 250 would make the R2/R1 ratio nicer to work with. What's making my head hurt, though, are the 25 Ohm resistors, which added to the 225 ones result in the 250 Ohm I expected.

So is R1 = 225 Ohm or R1 = 225 + 25 Ohm?


Thanks.

[tszaboo]

You have a current source, that is a Norton equivalent. Convert it tho Thevelin, and the 25 Ohm is in series.
http://en.wikipedia.org/wiki/Norton's_theorem

[Pulse Cloud]

You have a current source, that is a Norton equivalent. Convert it tho Thevelin, and the 25 Ohm is in series.
http://en.wikipedia.org/wiki/Norton's_theorem

*gasp* Of course!
(This is kind of embarrassing.)

[dannyf]

Since the 25ohm resistors are so much smaller than the serial resistance from the gain-setting resistors, you can simply view the output from the DAC to have been converted to voltage (by the two 25ohm resistors), and then they are summed via the differential amplifier.