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I'm trying to pick the correct amplifier for the following circuit:

enter image description here

Rref is 1 ohm, Vref is ground, and RL is anywhere between 0-30 ohm which voult mean that Vout would have to be around 30ohm *250mA at max. The D/A converter will input 0-250 mV. The DAC I am using is the MCP4921 and it works really well.

However, I tried using the LT1206 as an amplifier, and it didn't work so well. I have a stable 12V supply to the amplifier but I am unable to get the amplifier to work.

Any suggestions or other recommendations for amplifiers?

Using this circuit, I am unable to get V+ = V-. I've tried to implement a voltage follower circuit as well to test the amplifier, setting V+ to 5V, and I would achieve a 4.4V at V- when the supply is turned off and a 10.6 V when they supply is turned on. Again, doesn't make sense to me.

The goal of the circuit is to control the current accross the load to vary between 0-250 mA by using the DAC. Also, The output of the DAC is a DC voltage.

Datasheet for LT1206: https://www.analog.com/media/en/technical-documentation/data-sheets/1206fb.pdf

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    \$\begingroup\$ "It didn't work so well". Did it explode? Did it work but was too slow? No output? Too inaccurate? Was it distorted? Clipped? Unstable? We cannot diagnose without a description of real symptoms. And why did you use the LT1206 anyways? It's a current feedback opamp which does not work like the much more common voltage feedback opamp. Try any other opamp. \$\endgroup\$ – DKNguyen Jan 25 at 20:23
  • \$\begingroup\$ I chose the LT1206 because of it's current capabilities. All though to be honest I don't know the difference between a current feedback opamp and the voltage feedback opamp. I'm trying to control the current through the load by setting the current at Rref. \$\endgroup\$ – George Jan 25 at 20:31
  • \$\begingroup\$ That is a high drive current it has. Regardless, go with a voltage feedback amp. \$\endgroup\$ – DKNguyen Jan 25 at 20:39
  • \$\begingroup\$ Can you edit your question to explain what the purpose of this circuit is? "Vref is ground"? No it isn't. It's connected to ground and Vref could be adjusted to zero so it has ground potential but it's not actually ground. Add in a link to the datasheet. \$\endgroup\$ – Transistor Jan 25 at 20:39
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    \$\begingroup\$ Welcome to the real world where nothing is as it seems and everything is out to get you. \$\endgroup\$ – DKNguyen Jan 25 at 20:48
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Most op amps are not rail to rail and R2R op amps are expensive. you can work around it by picking the supply voltage a bit higher than your goal. for example if the maximum positive rail is 12 V you should choose 14 V. and also for negative rail do the same, -2 V for example.

For sensing current, Instrumentation amplifiers are ideal (such as AD620, AD623 etc). but for a simple project you can use good old but precise OP07 op amp.

With OP07 you can build your own in-amp:


Edit:

To control the load current you can use LM317, here's an example to give you an idea:

schematic

simulate this circuit – Schematic created using CircuitLab

When voltage drop on Rsense is higher than the referense voltage, the op amp which configured as an comparator will pull the adjust pin of LM317 to ground. it will oscillate and will keep the current at a constant value.

Do the shunt and reference voltage calculation, good luck.

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  • \$\begingroup\$ Thank you for your reply, all though I'm abit confused as to how I can use this to replace my amplifier. I can connect v1 to my DAC and v2 to ground. But all my resistors would be static, wouldn't that be problematic? I don't want a constant gain. Unless I don't understand the circuit properly \$\endgroup\$ – George Jan 25 at 21:19
  • \$\begingroup\$ @George Are you making a dummy load? \$\endgroup\$ – ElectronSurf Jan 25 at 21:21
  • \$\begingroup\$ No it's not a dummy load. The load can be anywhere between 0-30 ohms, and I need to be able to supply 0-250 mA to it. \$\endgroup\$ – George Jan 25 at 21:23
  • \$\begingroup\$ @George See the edit. \$\endgroup\$ – ElectronSurf Jan 25 at 21:42
  • \$\begingroup\$ I'll give it a go. thank you \$\endgroup\$ – George Jan 25 at 21:56

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