# Looking for help with a +/- DAC application

Discussion in 'General Electronics Chat' started by AssaultAndBattery, May 22, 2014.

1. ### AssaultAndBattery Thread Starter New Member

Mar 11, 2014
4
0
Hello,
I'm currently using a MCP4725 DAC to set voltages via I2C and a python program running on a raspberry Pi. As far as I know the MCP4725 will only output positive voltages. I am now in need of a DAC that can put out positive and negative voltages (ideally +/- 10 V). I used an op amp along with the MCP4725 to design a bipolar DAC as per the datasheet (https://www.sparkfun.com/datasheets/BreakoutBoards/MCP4725.pdf).
This works, but I'm having trouble getting the exact right voltage at the output of the op-amp (need ~ mV resolution).
I'm not sure where to go next.
Should I try to find a DAC that goes to +/- voltages (I can't seem to find one that I can fit onto a breadboard the does I2C).
Or should I work on making the DAC Vdd voltage more stable (am using a
LM2596 DC-DC Buck converter to supply Vdd)?
Or should I change the op amp (am using a high precision TLE2022IP)?
Or try to balance the resistors better (0.1% instead of 1%)?
Thanks in advance.

AssaultAndBattery

2. ### MrChips Moderator

Oct 2, 2009
12,624
3,451
In order to generate +/- 10V output you will need a bipolar power supply that provides +/- 12 or 15V.

A 12-bit DAC has 4096 steps. Hence for a 20V range, one step = 20V/4096 = 5mV approx.

You can achieve +/- 10V with your MCP4725 by using an op-amp powered with a bipolar supply and offsetting the input of the op-amp. You can do this by adding a -ve voltage or subtracting a +ve voltage.

Look at the MCP4725 data sheet section 8.5.4 (page 37) for an example on bipolar operation.

Use trimpots to allow you to adjust the gain and offset of the op-amp.

3. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,222
What is your accuracy requirement and why. It may be that you will be better off with a true bipolar DAC.

4. ### AssaultAndBattery Thread Starter New Member

Mar 11, 2014
4
0
Thanks for the replies. To Mr Chips that's what I'm doing now. I have the MCP4725 in bipolar operation using an op amp powered with +/- 12 V. It does work but I'm not getting the output I want. For example I expect to get 1 V as the output but I'm getting 0.9 V, and I'm not sure why (lets assume I set everything up correctly).
To t06afre I'd be fine with a true bipolar DAC, I just don't know where to find one. Right now I only have bread boards and I like using the i2c protocol. Do you know any DAC's that will do +/- 12 V, > 16 bit, i2c compatible and will fit on a bread board?
Thanks again.

5. ### crutschow Expert

Mar 14, 2008
13,482
3,369
Hard to make suggestions without a circuit. Post your diagram.

Here's a search showing a few 16-bit I2C DACs.