Help using a DAC as a current sink

[ifrythings]

I’m not sure what exactly the circuit I want is called or even if I have the right circuit for my project.

A little info about my project, I did a steering wheel swap to a newer steering wheel for my truck, the old steering wheel uses resistive buttons and the new one is CAN bus data. I have the CAN bus part figured out and my idea was to use a DAC to replace the resistive buttons, first problem is the DAC can’t sink up to 16mA, second I tried was using an opamp but the few I had couldn’t sink that current either.

Found an app note from TI about a programmable low side current source, that appeared to be close to what I needed but I couldn’t get it to work in the range I needed. So I came up with this circuit, it looks like it will work in the basic simulator, I’m not sure if there’s a better circuit out there or if this one will work what else I may need to keep it stable. I’m not well versed with opamp issues and compensation to prevent any instabilities if needed.

Any suggestions and input is welcomed.
Hopefully the pictures show up.

[Terry Bites]

Its the volts that matter here.
Controlling the voltage at the PCM node can be done with a voltage output ADC and a buffer.
The buffer can be a single supply opamp who's output will swing to near ground at the max current. eg an OPA310.

The TI circuit cannot pull its drain down to ground and that will put it out of range.  You can make a current sink to suit your requirements (with a negative supply) but it's probably not unecessary.  If you want to go the switch route replace each switch with a cheap mosfet eg 2N7002 (dirt cheap). With the resistors <$1!

[ifrythings]

I tried using a TS27L4CPT quad op-amp but it can’t sink enough current, I will get some OPA4310 on order and try those out as they definitely have a higher source/sink current.

I thought about using individual resistors and logic level FETs but I have 22 buttons to do and it’s a lot easier if I need to tweak the circuit in software then changing resistor values.

[Terry Bites]

If you have a lot of keys its worth thinking about a matrix scanner ic, eg TCA8418. @$2