Electronic – Voltage modulation with an LDO

It looks to me like a netlist/layout error:

If you are placing your fixed resistor on the dig pot footprint, it would also be messed up. Looks like you accidentally wired the CO+ and NO2- together and the NO2+ and CO- together so the LDO(Vout-Vadj) is not across a resistance as you believe.

It's really hard to do this with a generated voltage without degrading the voltage accuracy of the regulator.

But if you have a handful of digital outputs that you can dedicate to this, you can use a resistor digital-resistance-converter (DRC?) like this. I've represented the regulator as an op-amp.

^{simulate this circuit – Schematic created using CircuitLab}

The four trim resistors are switched out of the circuit by setting microcontroller pins to inputs and they are switched into the circuit by setting microcontroller pins as digital output lows.

When all four pins are inputs, the voltage divider is R1 and R2. When some pins are set, the divider is R1 and the parallel combination of R2 and the trim resistors.

I would suggest setting the lowest voltage with R1 and R2, the select the trim resistors to pull the voltage over the range that you want. Do not choose all four resistors the same value. Instead you will want something like a 1k,2k,4k,8k sequence so you can get 16 different values out.

For extra credit:

• If your microcontroller has switchable high-side outputs (open-drain) you can do that instead of switching input to output, then
• If your microcontroller has outputs as bits in a PORT register, arrange the resistors with the largest value on the least-significant output bit. Then with careful resistor value choice, you can write 0-15 to the port register to get monotonically increasing voltage.