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How to generate a fixed differential voltage dependant on a voltage relative to ground? Answered

What methods can I use to generate an accurate & precise variable differential voltage that's dependant on the output of a +-10V DAC?

I tried to use a resistor with a small variable current sink using the classic opamp+NPN design, but no design I made would sink current below 1V. I need it to continue sinking current down to 0V, I do have a negative supply rail avalible.

(I think my last question was a bit TL;DR for some, so this is a shortened version of that.)

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steveastrouk
steveastrouk

5 years ago

Show us a schematic of your opamp scheme. Wide Compliance ranges of current sources is very difficult ti achueve

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steveastrouk
steveastrouk

Answer 5 years ago

You could wire an opamp as a voltage to current converter, wire the load from the output to the negative rail, and lift the resistor up on a zener

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-max-
-max-

Answer 5 years ago

See the thing is though is that resistor for which there must be a constant voltage drop across, one end of it goes to the output of my pass transistor, which is basically the regulated Vcc voltage rail. That voltage is controllable from 0-15V. So there may be 15V there, 12V, 8.21V, anything down to 0v. However when I get down below 2V or so (depending on the specific topology used) it stops controlling current as the voltage drop across a small resistor between the NPN transistor and ground can no longer be maintained.

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Answer 5 years ago

I think I realized something far more simple though. The DAC71 (of which i have more than a handful) is a "current" model, meaning that I can both sink and source a few mA of current very precisely! It is almost perfect! I am trying to interface it to a Arduino and learn more about it as I examine the datasheet.

My original plans was to use place a resistor between that output and ground, and use a buffer amp to drive a constant current source for where it was needed. However that's like powering a 12V lamp with a wall wart plugged into a 12V inverter, just nonsense!

Since I plan to use a ATmega328p, I do not have enough pins avalible for the parallel interface, so I have like tons of these 74C164 shift registers so that one serial interface can interface all the pins on that DAC. (I wish those shift registers had a blanking pin though. That way the DAC isn't outputting garbage while 16 bits of data is being shifted into it.)

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iceng
iceng

Answer 5 years ago

+1

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Answer 5 years ago

My original and MUCH more detailed question is here. I have a few pictures and a schematic in there that was made in LTspice. On that picture, you will see some current sources, which need to be replaced with real-world counterparts.

https://www.instructables.com/answers/How-to-make-a...