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I was reading the DAC902 datasheet. On page 13, Figure 4: enter image description here

They basically mention that the 26.1 ohm resistor is in parallel with R4 to give an equivalent resistance of 25 ohm, which does makes sense. Now, they also mention that the 28.7 ohm resistor is in parallel with one of the 200 ohm resistors, I'm assuming its R3. How does that work out?

They don't seem like they are in parallel.

Analog
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  • Well 26.1Ω||(200Ω + 402Ω) = 25Ω and 28.7Ω||200Ω = 25Ω – G36 Jun 21 '21 at 16:16
  • That's true, but why is the 28.7 in parallel with the 200 ohm. I'm just not seeing it. – Analog Jun 21 '21 at 16:41
  • Due to the negative feedback action (via R2 ) and large open-loop gain of an op-amp (gain without the negative feedback). Try to study why Rin of inverting amplifier is equal to R1 (virtual short/ground). https://electronics.stackexchange.com/questions/441184/op-amp-virtual-ground-principle-and-other-doubts/441207#441207 – G36 Jun 21 '21 at 16:51
  • Impedance is measured to ground. That's the bottom side of R4 for one leg, and the right side of R1 in the other leg (to the opamp's virtual ground). – td127 Jun 21 '21 at 16:51
  • If the Iout-bar is held at ground, the negative op amp input will be a virtual ground, so that looks like 200 ohms to ground. If Iout is held at ground, Iout-bar looks like 200 ohms in series with 402 ohms. Differential op amp buffer/amps are not symmetrical in terms of input impedance. – Cristobol Polychronopolis Jun 21 '21 at 16:54

1 Answers1

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The OA has a virtual null or diff. input R is null with negative feedback so the diff. load on diff source = R1+R3 and R2,R4 has no effect from negative feedback on loading on current source.

But the single input gain = R2/(R1+RL)* Iout * Req

I’m not sure why they selected two different RL’s that add up to 48.7 except to adjust R4 up to balance CMRR and gain =2

CMRR depends on tolerance stackup error of all resistors.

Tony Stewart EE75
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