How to get -5 to 5V with AD5668

Question

I'm more programmer than electrical engineer. I have connected Arduino with an AD5668 DAC. I want to control analog synth such as Korg MS-20M. For the filter, it takes -5 to 5V.

Right now I can generate only 0-5V with digital values 0-65535.

I want to generate -5V to 5V straight out of Arduino with digital values 0-65535.

AD5668 takes a reference voltage. Right now I supply it 5V from Arduino. Could this be a solution? To supply -5V reference voltage? I would like to make some simple DIY solution hopefully without any new parts in the circuit. I was thinking that AD5668 has all the required in itself like OP-AMP, but I guess it does not generate smooth -5 to 5V voltage.

Answer 1

The AD5668 can only supply 0-5V at its outputs (or 2.5V if you use the -1 version with internal reference). The reference voltage has to be essentially within the supply rails. Applying -5V will destroy the chip.

If you need -5 to +5 you will need to add an amplifier/level shifter at a given output- to multiply by 2 and add a suitable offset. That will require a negative supply of at least -5V (you won't quite get to +5 or -5 with a +/-5V supply).

Here is how I would do it with this particular chip (-2 version with 0-5V out):

^{simulate this circuit – Schematic created using CircuitLab}

You only need one reference buffer (OA2) but you do need one amplifier per output. The output op-amps require power supplies of at least +/-5V. If you use closer to +/-5V you will need rail-to-rail output and input common mode range must include 3.33V. eg. OPA2188. You do need the reference buffer because the output impedance of the internal reference is 7.5K ohms.

The stability will only be as good as the reference, so using the internal precision reference makes a lot of sense. If you were to use the power supply you would add a lot of noise and imprecision into the output.

If you want high accuracy commensurate with the 16-bit DAC you can use a single precision resistor array for the two pairs of resistors such as a Vishay ACASA1002S2002P1AT.

Answer 2

The AD5668 takes a single ground referenced 5v supply, and produces several ground-referenced 0 to 5v output voltages. It interfaces directly to a ground referenced Arduino. If you are going to ride the horse in the direction it's going, you keep those components ground referenced, and supplied with a single 5v.

The AD5668 only produces a 5v swing, and you want a 10v swing. Even if you could shift it below ground, you won't get the swing you want without further components.

The simplest way to build on what you have is to follow the 5668 with an op amp supplied with at least +/- 5v rails (if you use a rail to rail type) or higher than that (if you use an older style opamp).

A gain of 2, with a shift, is quite easy to come by. This is just one solution, which inverts the sense of the output.

^{simulate this circuit – Schematic created using CircuitLab}

Ideally the 5v offset reference and the DAC reference will track, to minimise shifts with temperature or power supply voltage variations, so have a 5v reference which also supplies the DAC reference.

If you still want to use the DAC internal reference (I always do, they're usually good enough for all but the very highest stability), you could dedicate one output channel to providing the reference for the other channels, or indeed use the 2.5v refout feature. Where you use one channel, it would be better to eliminate all the R1, R2 components completely and provide a programmed 1.667v reference directly to all the +ve amplifier inputs. That would give you 7 channels of -5 to +5 control, with just two quad amplifier packages. Where you use the 2.5v refout, divided it down to 1.667v by using R1=10k, R2=20k.

Answer 3

To convert a unipolar DAC output to a bipolar DAC output requires an op-amp interconnected with the REF pin like this: -

The example above is for a different DAC but the technique is standard for unipolar DACs. I would use supplies slightly bigger than +/- 5V for the op-amp just to ensure full coverage and avoid the need for rail-to-rail op-amps.

Even if you are using the internal reference the data sheet says it is available: -

The AD5628/AD5648/AD5668 have a common pin for reference input and reference output. When using the internal reference, this is the reference output pin. When using an external reference, this is the reference input pin. The default for this pin is as a reference input.

Note that the reference out is only 2.5 volts and here's ADI's take on getting a bipolar +/- 2.5 volt output using an op-amp if you used an external 2.5 volt reference (or the internal 2.5 volt reference): -

Note that the circuit to the left generates a bipolar supply from 12 volts. Circuit taken from ADI reference design CN0183