AD7671 Output has step

Hi,

 When I use AD7671 to convert signal (1Hz to 1kHz), I found steps in the output.

Attachment figure 1 is the waveform, where green line is input of AD7671;yellow line is the output of AD7671 (a DCA output, the DAC operated well).

  • Hi daydayzhq26 ,

       

          You mentioned that the yellow line is measured on the DAC. At what throughput rate the was the AD7671 ? and what is the rate of the DAC capture? What do you mean by DCA? If it is okay to share the schematic of your application so we may have a better understanding of the problem.

    Regards,

    Jonathan

  • Hi Jonathan,

    AD7671 has 64kHz sample rate (see Figure 4). It operates with a MCU (see figure 3). The DAC sample rate is 64kHz as well.

    The schematic is in Figure 1,2. The MCU just receive data from AD7671 and send the row data to DAC. The data has no process.

    The height and period of these steps is in Figure 5.

    Figure 6 is the overview of AD7671 input (green line) and DAC output (yellow line).

    Figure1

    Figure 2

    Figure 3

    Figure4

    Figure 5

    Figure 6

  • Hi Henry Zhang,

      Just some clarification, where is the voltage reference coming from? The reference needs to be very stable. Unstable reference would result in huge error on conversion result. I can see that, you used a opamp to drive the reference, but where did the reference voltage come from? I would recommend using the ADR421 and AD780 as mentioned in the datasheet and it is known to work with AD7671.

    Regards,

    Jonathan

  • Hi Henry Zhang,

       You mentioned that you did a voltage divider on the ADR01 to have an output of 2.5V as reference to the AD7671. Where was the resistor connected? The total impedance on the reference output can interact with the ADC required reference voltage. With the impedance and the 200uA reference current of the AD7671 would affect the stability of the reference voltage during conversion. The AD7671 technology is a  switch cap dac. During transition from acquisition phase there is a large current requirement from the reference.

       On one of the snapshot, the time between each level is 62khz which is almost equal to the sampling rate. Would you be able to check the result when lowering the sampling rate.

    Regards,

    Jonathan

  • Hi Jonathan,

    Could you updata my question? Give me new reply?

    Thanks

    BR

    Henry ZHANG