How to set BIAS DC on analog output for ADAU1442X evaluation board?

Hello A2R,

I think you will have to do this externally. Here is why...

Adding a DC offset on the signal inside of SigmaStudio is easy:

This is adding 0x2F of offset with the arbitrary number I added. The actual DC level out of the DAC depends on the DAC itself. More on that later.

Then you could modify the eval board to bypass the output coupling caps.

In the schematic shown above, the output 0 and 1 you will see C53 and C57. If you short around these caps and remove the  49K9 resistor then you will have a DC coupled output.

You can see the large caps here on the eval board.

Now the problem is that the DAC runs on 3.3V AVDD.  The DAC sits at the common mode voltage with no signal in. This is roughly 1.5V. Then the signal swings up to around 3V with a full scale positive output and almost 0V with a full scale negative input. So if you bias up the output to give you a 1V DC output  then you would need to add a negative DC level and that will hurt your dynamic range. The signal will clip on the negative side far before it clips on the positive side. Then if you try to go more positive than the nominal 1.5V common mode voltage then the signal will clip on the positive side first. So I think you need to work up an external circuit to add the DC level you need using a circuit similar to how we bias up the ADC input. Here is what we do on the input side:

Here you see the input is Cap coupled. You would not need that cap since there already is a cap on the DAC output filter. Then it goes into an op-amp stage where it biases up the circuit to the VREF input.

This circuit also makes it differential which you probably don't need to do so half of this would be sufficient.

You will have to pay attention to the voltage rails on the Op-Amp. If you are going to add up to 10V of DC offset then the power rails need to be high enough to do that. I suppose you would run this Op-Amp single ended so the power rail would probably need to be 24V from ground. Then you can setup a pot  between 24V and ground to use as your reference for the Op-Amp. Then you can dial in whatever offset you like. If you have many channels then you would want to use an Op-Amp configured as a voltage follower to take this adjustment and provide a low impedance output to drive all the VREF pins from one place.

The design of all this is a longer discussion and you may be able to gain some insight either from a good Op-Amp reference book or I suppose in the linear amplifiers section of this forum.

So I suggest you take the AC coupled output of the eval board and add DC externally is my suggestion.

Dave T

Thank you very much Dave for your prompt response.

However, the DC-offset I want is around +0.25V, and I found a solution;
I simply took a battery box with 2x1.2V NiMH batteries with a Voltage dividing coupling using two resistor 1000 Ohm and 100 Ohm. Using the ~0.24V over the 100 Ohm resistor PARALELL OVER THE ADAU1442Z ANALOG OUTPUT appears to work fine. The AC from the ADAU board is the same, and there is a DC offset of 0.24V on the signal.

Current drawn should be ~2.4/1100 [A] = 2.2mA. With 1700mAh batteries I should be OK for several days operation.
Note that this is just a temporary setup for a technology demonstration. For the actual application the need for DC will be gone.

There may be limitations on the concept I use to get the DC on the ADAU1442 Evaluation Board (and other AD products with analog output) so be careful before applying to other boards and using other Voltages.

A2R

Hello A2R,

As long as you did this with the coupling caps still on the PCB then it would work fine. I am pretty sure that is what you did. So I am glad you worked it out.

Dave T