DAC7728: Clarification regarding a question about the datasheet

Hi Osamu-san,

What is the state of your USB/BTC pin? This pin modifies how the input code is read, which could cause some unexpected output readings.

Additionally, what code are you sending to the DAC channels? Are they at 0-code?

Thanks,
Erin

Hi Erin, thank you for your prompt reply.

The USB/BTC pin is connected to IOVDD via a 4.7 kΩ resistor.

Also, regarding the question, "Additionally, what code are you sending to the DAC channels? Are they at 0-code?", I would like to clarify something.
Could you please let me know if you are asking about the registers (e.g., the Configuration Register) or the DAC output data?

Thank you for your continued support.

Best regards,

Osamu

Hi Osamu-san,

Thank you for letting me know the state of your USB/BTC pin, this means you should be sending data in 2's complement format.

I would like to check a few specific things:

-Set the OFFSET DAC code to the default AABh. Measure the OFFSET DAC voltage, do you get the 3.3V as defined by the datasheet? If you set the OFFSET DAC code to 0h, do you get the 0V on the OFFSET DAC?

-Set the DAC output data to zero-scale (800h with USB/BTC pulled to IOVDD). You should ideally see minimum DAC voltage, and it should be negative.

If the DAC is not negative, there is a good chance there's an issue with your power startup sequence. I took a look through older e2e threads, and it seems like not starting up the device properly can sometimes cause the DAC to default to a unipolar mode. It looks like specifically IOVDD and DVDD need to come up before the other supplies.

Thanks,
Erin

Hi Erin, thank you for your prompt reply.

Here is the response to your inquiry:

-Set the OFFSET DAC code to the default AABh. Measure the OFFSET DAC voltage; do you get 3.3V as defined in the datasheet? If you set the OFFSET DAC code to 0h, do you get 0V at the OFFSET DAC?

--->With the code set to "AABh," the OFFSET DAC voltages are 0.96V for OFFSET-A and 0.88V for OFFSET-B. With the code set to "0h," the voltages are 2.64V for OFFSET-A and 2.56V for OFFSET-B.

-Set the DAC output data to zero-scale (800h with USB/BTC pulled to IOVDD). Ideally, you should see the minimum DAC voltage, and it should be negative.

--->It measured -2.5V. Since the minimum voltage is supposed to be -10V, I suspect something is incorrect.

Incidentally, with the code set to "2ABh," the OFFSET DAC voltages were 3.36V for OFFSET-A and 0.24V for OFFSET-B. I am unsure what this means.

Thank you for your continued support.

Best regards,

Osamu

Hi Osamu-san,

Getting 2.5V on the OFFSET DAC when using 0h is very odd. I suspect something may be going on with your communication interface that's causing the device to receive incorrect data. Could you double check your address and data pins, and the input on them to make sure the data is exactly what you expect? Send me a screenshot of your schematic as well if you are able.

Thanks,
Erin

Hi Erin, thank you for your prompt reply.

I have attached the peripheral circuit diagram for the DAC.
I have also attached the register settings.

There were no issues regarding the data itself.

When the setting is 0x2AB (equivalent to -2.5V), the offset becomes 0V.
After that, the output matches the settings, and the gain is correct.

Therefore, we are currently using the value 0x2AB without any problems.
However, the discrepancy with the datasheet is puzzling.

I would appreciate your assistance.

Best regards,

Osamu

Hi Osamu,

Your schematic image is too small for me to read unfortunately, it may have been limited by the forum.

With an OFFSET DAC code of 0x2AB, you should read around 0.83V on the OFFSET DAC output (pin 12).

I believe the discrepancy could be found in the supply sequencing. If the supplies are not sequenced correctly, it's possible that the negative bias could not be fully turning on. If you go through the range of DAC codes, do you get the correct output?

Thanks,
Erin

Hi Erin, thank you for your prompt reply.

I am resending the circuit diagram in a clearer format.
Could you please take a look at it?

Thank you for your continued support.

Best regards,

Osamu

6283.Schematic.pdf

Hi Osamu,

Your schematic looks fine as far as I can tell. Let me know if you run into any other issues.

Thanks,
Erin