# How do I amplify voltage from a DAC?

How do I do I output an analog signal from 0V - 10.67V using a DAC and provide a Vcc of 5Vdc that can supply at least 3.5 Amps to the rest of my circuit?

I am trying to make a circuit that outputs a specific voltage in the range of 0V - 10.67V . This circuit is also part of a larger circuit with a current draw of 3.5 Amps. I'm using an Atmega328P and an MCP4922 12-bit DAC powered by a 5V AC/DC wall adapter. My goal is to be able to send the Atmega328P a number and have the MCP4922 output the corresponding voltage which I am able to do but I cannot make the MCP4922 output a voltage higher than (approaching but not equal to) Vcc. I have a couple ideas on how to get to the 10.67V so far but I am not sure which would be best or if I am even on the right track.

MCP4922 datasheet: here

Idea #1:

Keep the circuit and power supply "as is". Except at the Vout of the DAC, add on an op amp configured for single side supply operation in a non-inverting gain circuit like shown in this example from microchip.com.

Then I would set R2 and R1 so that the Vout = Vin * Gain. Where Gain = 10.67 / (Vref of the DAC). I would reduce my calculations for the DAC in half. Where previously, if I wanted to output 2 Volts, I would tell the DAC to output 2V. Now, if I want to output 2V, I tell the DAC to output 1V and the opamp will double it to 2V. By the same logic, if the DAC outputs 5V, then the op amp will convert it to 10V.

Is this doable? Good idea?

Idea #2:

Get rid of the AC/DC wall adapter and switch to a 12Vac transformer. Take the 12Vac output and split it in two +/-12V lines. Take one line and use a (full wave?) bridge rectifier with a smoothing capacitor to convert the 12Vac to 12Vdc. Take the other line and use the 12Vac with an op amp in a non-inverting amplifier configuration. Where the Vout of the DAC would go to the (V+) of the op amp. And where the desired gain would be = 10.67 / (Vref of the DAC).

*My apologies. I made an edit and removed the second part asking about how to step down 12V to 5V to make this topic more specific.

• Idea #1 isn't doable, The op amp can't output a signal higher than it's own power supply rail. – Mike Aug 9 '14 at 22:51

To practically reduce 12V to 5 you really need a switch mode converter. There is quite a lot of these chips available, but designing with them may get hairy pretty quickly. But there's no other way really, since wasting $7*3.5 = 24.5W$ just on regulation should sound like a really bad idea.

since you have two apparently-unrelated questions (amplify DAC output, and drop 12 volts to 5 volts), you should have posted them as separate questions.

The op-amp should work to amplify the DAC output to the 0 - 10.7 volt range - but the positive power supply to the op-amp must be at least 12 volts.

There are switch-mode DC-DC converter modules readily available on eBay that will convert the +12 volts to 5 volts.

Your suggestion of a resistor voltage divider network is a Very Bad Idea - it will waste a large amount of power (much more than a 7805-like linear regulator) and the output voltage will vary with the current drawn.

• Ah! It's a buck converter! – EEstudent Aug 10 '14 at 1:04

assuming you will use vref=5 volt, 10.67/5= 2.134

Gain of the circuit= 1+Rf/Rg... Rf= 2.2k ohm. Rg= 2k ohm, Gain=2.1

A common single supply opamp is the LM358 and it will work fine at a low sampling frequencies (less than 1khz), power it with 12 volts. Also you should implement a lowpass filter at the output to obtain a proper analog output signal.