DAC8574: DAC8574 IOVDD problem, shift analog ozput

Joachim Beushausen

Part Number: DAC8574

The task is to generate output voltages between -2.048 and +2.048 V with a DAC8574. For this purpose the DAC is operated with +/- 2.5 V.

The I2C signals have a swing of 5 V referred to -2.5 V. The signal transmission with I2C works in principle, but the output voltages are shifted

by about 0.45V (2.5 - 2.048 = 0.452 ?). According to the data sheet, IOVDD only is responsible for the level of the digital inputs.

IOVDD is currently at +2.5 V (IOVDD connected to VDD), which should not have any influence on the analog outputs. Is this circuit wrong?

If IOVDD has an influence on the output voltage, must IOVDD be stabilized?

over 4 years ago

0 Anbu Mani over 4 years ago

TI__Expert 4110 points

Hi Joachim,

Thanks for reaching out to us.

Yes, IO VDD is responsible for only digital inputs interface but the minimum IOVDD required 2.7V for proper operation. see power requirements section in datasheet. Also, we recommending to use separate rails for analog and digital rails for good signal integrity.

0 Joachim Beushausen over 4 years ago in reply to Anbu Mani

Prodigy 10 points

Hello and thank you for the answer. I apologize for the late reply. I had to stay in the hospital for a few days. The answer does not exactly hit the core of the problem. In the meantime, the question has been clarified. The DAC 8574 is operated with two voltages +2,5 and -2.5V. As reference voltage +/- 2.048V was used. Unfortunately the internal resistor of the output amplifier is at the GND pin, there are -2.5V here. This unfortunately results in a shift of the output voltage. It would be better if the resistor would be at VREFL.

0 Joachim Beushausen over 4 years ago in reply to Anbu Mani

Prodigy 10 points

0 Anbu Mani over 4 years ago in reply to Joachim Beushausen

TI__Expert 4110 points

Hi Joachim,

The part will not work with negative reference -2.5 V. The acceptable reference level is 0 to VDD . VDD level is 2.7 V to 5.5 V

Are you using opamp at output of DAC, convert unipolar to bi-polar ? please share your schematic including i2c interface. will review and provide our feedback.

0 Joachim Beushausen over 4 years ago in reply to Anbu Mani

Prodigy 10 points

0 Joachim Beushausen over 4 years ago in reply to Joachim Beushausen

Prodigy 10 points

I do not use an external OpAmp. It is about the internal OpAmp in the DAC.In the shown circuit I get bipolar voltages at the output referenced to Iso_GNDA. These voltages are shifted by (2.500 - 2.048)V

0 Anbu Mani over 4 years ago in reply to Joachim Beushausen

TI__Expert 4110 points

Hi Joachim,

Thanks for schematic. i will review the schematic and get back to you.

0 Anbu Mani over 4 years ago in reply to Anbu Mani

TI__Expert 4110 points

Hi Joachim,

Could you please share SPI host interface section too.

How host SPI lines are connected ? iam not see pullup resistor in I2C lines.

Regards

Anbu M

0 Joachim Beushausen over 4 years ago in reply to Anbu Mani

Prodigy 10 points

The circuit is a section of a larger project with 2 x DAC and 6 x ADC. Of course there are pull up resistors for the I2C lines. The data transmission works as well. There is only an offset of (2.500 - 2.048)V. This is because the internal OPAMP with its one resistor is connected to GND (pin 6) and not to VREFL (pin 5).