# Generating negative BIAS voltage

I am trying to generate a programmable Bias voltage from 0 to -2V with <0.5mV error.

I was able to generate a programmable positive bias voltage of similar range using DAC. based on this resource .

http://www.ianjohnston.com/index.php/onlineshop/handheld-precision-digital-voltage-source-detail

My idea is to have a negative 5 Voltage supply on the board and connect 0V to 5V, -5V to the ground, So everything is inverted, it will just give me the negative bias voltage with the same accuracy.

However, My problems is that The DAC requires SPI communication from uC which is 3.3 V. I could not talk to the negative section of the board since They are at completely different voltage level.

Is there anyway for me to get around this? Do i have to get another uC on the board just to talk to that DAC?

• Use the DAC on +3.3 or +5V, feeding an inverting amplifier using an opamp from +5 and -5V. (You'll need 2 identical precision resistors for the opamp) – Brian Drummond Sep 18 '16 at 22:41
• @BrianDrummond, your comment should be posted as an answer. – The Photon Sep 18 '16 at 23:19
• Thanks for the reply, For the positive supply of the opamp, What could be the problem if i tie it to the ground? – Eddy zhang Sep 19 '16 at 23:49

## 1 Answer

There are some DACs around which are able to produce a negative voltage directly. Most of them are older designs and often enough they are not recommended for new designs. You can find them easily if you filter them on Digikey (or anywhere else) based on the supply voltage and select only those who accept a negative supply voltage.

A different, more universal way is the one Brian Drummond suggested in a comment.

You use a positive voltage output DAC, and connect an inverting opamp in series and use that as output.

simulate this circuit – Schematic created using CircuitLab

There are some considerations you have to make. The output impedance of the DAC will lead to a gain error. This can be countered by buffering the DAC signal before feeding it into the inverter (some DACs have a buffered output). The resistors are critical, so they should be high quality low temperature drift ones, maybe using a matched resistor network makes sense here (the absolute value is not as important as the matching of both resistors).

For stability using a chopper opamp might be a good idea as well, they also come with a very low offset voltage, so you don't have to trim the devices.

A different approach is depicted in this application note by Texas Instruments. I have just looked at it briefly (to see it's different), so I can't comment on the benefit of their approach.

• For the resistor value selection in your schematic. I am thinking i should make the resistance value very large (>100K). In this way, It will minimize the gain error, as well as the error from trace resistance.(Also, The DAC has a bufferer output). Does this make sense? – Eddy zhang Sep 19 '16 at 23:46
• @Eddyzhang you shouldn't make it too large as that will increase noise, I'd guess that around 100k to 250k is an okay value. But maybe you can get some different values and try which performs best. – Arsenal Sep 20 '16 at 7:59