DAC8814: Looking for multi-channel precision current DAC

Nik,

These two DACs are multiplying DACs, or MDACs. While in the parametric search tool they are listed as current output DACs, which is accurate, it's not necessarily a proper current source or sink DAC as you would normally think of. Essentially the output is a current because the R-2R MDAC ladder is connected to the voltage reference, but this current isn't buffered in any way - so the exact current through the ladder is still influenced by load conditions (i.e. think of this as more of a programmable resistor than a constant current source or sink).

In a normal application the MDAC acts as a binary weighted current output, driven by the reference voltage. It is followed by a transimpedance amplifier at the output to convert the current into a voltage - often with a ratiometrically matched internal resistor to get this done more precisely as the R-2R imepdance itself could vary 10-20%, depending on the device.

For your purposes do you actually need both source and sink capability or just a current sink?

Hi Kevin,

Thanks for your support! This means that If I connect the Iout to Vdc (say 4V), then my total current would be I = Vdc - Vref / R correct? If so, unfortunately I'd need something that I can sink directly to ground.

I'm essentially looking for something that can mimic an electronic load, a current source to ground, to the rails of a DC regulator. My goal is to characterize the DC regulator with different load conditions (Iload) and I was wondering if any DAC solution can mimic the load to ground, on the regulated output rails.

Maybe below picture will help!

Nik,

Page 16 of the DAC8814 datasheet illustrates a simplified diagram of the internal R-2R ladder. If you were to connect either your 4V or 3.3V sources to the VREFX pin of the DAC and GND to IOUTX you would effectively have your voltage source, a variable resistance, and GND in series. As you change the codes of the DAC, you would change the equivalent input impedance seen by your voltage sources.

So, the exact current value would be dependent on both the voltage source and the equivalent impedance of the DAC ladder, which I do not think is what you want. You need a constant current sink so you can characterize the voltage response of your sources.

We have a circuit cookbook for a programmable low-side current sink which should get you where you want to be. You can view that here: www.ti.com/.../slaa868.pdf

Hi Kevin,

Thanks for the note! 

I was looking a pairing a DAC80508 with a OPA388 and NTA7002N to accomplish this. I'm attaching a simulation of this below.

I have multiple rails on the regulator however the rail I'm simulating in the attached schematic is a 4V rail referenced to 3V. The Vin is simulating the DAC input with using a gain of x2 5V fullscale output.

Would you be able to advise if you see any issues with this schematic? Do I need Rfb, Riso and Cf for this?

Thanks!

NikSimulation_4.0-3.0VV.TSC