AD7671 Output has step

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

Hi Jonathan,

I found ADC have different operating range in one step, in Figure 1. The signal in circle A is the normal operating range, and the signal in circle B is the abnormal operating range. The step is form by the abnormal operating range.

But I don't know the source of the range B.

Thanks

BR

Henry ZHANG

Hi Jonathan,

I use ADR01 as the 10V reference. Resistance divider generate 2.5V reference voltage. I measured the reference pin. I think it is stable now.

I find voltage step when I change the input DC voltage, as in Figure 1. Blue line is the input DC voltage, and Green line is the voltage step of ADC output.

Thanks

BR

Henry ZHANG

Hi Jonathan,

I have a buffer with 47uF capacitor after the resistant divider to drive the 2.5V reference pin. I changed the sapmling rate to 1kHz and the result had no change.

I'll outoff office for Chrismas.I'll back next week.

Many Thanks

Merry Chrismas

Henry ZHANG

Hi Jonathan,

I found when I chang the input driver of ADC(AD7671) the steps will be different. When I use AD797 as the driver the output is in Figure 1. When I remove AD797 and drive the input directly from the generator the output is in Figure2.

I did not use AD8021 which is recommand in ADC's datasheet. I'm afraid the problem is the input driver. What's your opinion?

Many thanks

BR

Henry ZHANG

Hello

I think that we both have the same step problem 

https://ez.analog.com/message/334669-re-ad7671-step?commentID=334669&et=watches.email.thread#comment-334669