motor controllers. However the controllers output full-scale as soon as they are connectted to to the DAC channels no matter what the channel output is. If I use a standard variable power supply I get a variable output from the controllers. What is wrong?Well, I suppose two things could be happening. First, when there's no connection to the DAC, the motor controller input actually already HAS voltage on it, with sufficient current behind it that it actually is more powerful than the DAC, so the DAC can't lower it to the proper voltage. The reason your variable power supply would work is because it has a much lower impedance output than the DAC, allowing it to overdrive the motor controller input.
Either that, or there is a current coming from the motor controller input, and it is affecting the DAC because it's output impedance is not that low, and thus cannot "drive" the circuit hard enough, where your power supply can.
If you can find any information on your motor
controller, maybe you can see how much current it actually takes to drive it.
Lastly, I have had a similar situation before. Since you have a power supply that WILL work, can it accept a voltage input, and track it with its output? That's what I did. In essence, I just had a voltage "follower", and the power supply could output much more current, which was what I needed--it just gave me computer control of it.
Mark