multimeterpassive-networksresistorssimulation
I am trying to simulate a Bourns 4310R-R2R-104 dual terminator 104 circuit resistor network.
According to the datasheet, the circuit looks like this:
When I read the resistance between pin 1 and pin 2 on the physical package I get 200K ohms:
I built an equivalent circuit in Circuit Wizard and measured between the same pins but get a different result:
Why is this?
Your multimeters will have round about 1 or 2 ohms input impedance. They have a small (but not zero) impedance so they can develop a voltage across it which can be measured. In other words they "infer" current by the voltage developed across the small resistor and this small resistor is affecting the current through the LED.
Both meters will have slightly different resistances and hence the different readings.
Well you can certainly get op-amps that do work well from a 5V rail but ignoring that part of the question and concentrating on the two 0V points on the first circuit, if these were at 1V then there would be no problem. However, the outputs from the MCU (shown as switches connected to either 0 volts or positive rail) cannot be disentangled from 0V.
But, if you used a bunch of analoge switches (as you have basically drawn in the diagram) this would work fine. The two triangular shaped ground points could connect to any abitrarily low voltage (such as 2V or 1V) and the range of your DAC would stretch from whatever voltage you choose upon for the "ground" up to whatever voltage you choose for the other supply to the analogue switches.
There are suitable parrts that run from 5V and give very low impedance for "on" and beyod-the-rails capability. I'll try and find one to recommend.
Here's a single device (one-eigth of the way) that does SPDT but I'm sure with a bit of searching a quad device could be found (half way there): -
Best Answer
Wrong datasheet
Mouser links to the wrong datasheet. The part number you have looks like ?????-R2R-??? but the datasheet you link covers parts with 101, 102 and 104 as that second group. If you look at this datasheet instead:
https://www.bourns.com/pdfs/r2r.pdf
You will see that you have a SIP package R2R ladder with 100k resistors. If you look at the bottom left schematic in the group of four, you'll see that it does indeed have a resistance of 200K between pin 1 and pin 2.