AD7888ARUZ generating spikes on digital output

Hello Everyone

We are having a problem with the AD7888ARUZ. This problem is that the results of the ADC sometimes jump approxemetly 64 bit while the input stays relatively constand, and no more than 1-4 value differences are expected.

In the below graph we has set the input value of the ADC to drop from a high signal to zero. On each step we take 10 measurements from the ADC:

Y-as: ADC value

X-as: sample number (1ms time in between)

In the above graph can be seen that the jump always is the same size and happens around the same ADC values.

Taking a closer look at one of the spikes, the circled one, we get the following graph:

It can be seen that the value jumps approximately 64 bit each time above and below the expected value.

If we take a closer look to the values from the first spike:

dec

hex

bin

576

240

000101000000

576

240

000101000000

579

243

000101000011

515

203

000100000011

639

27F

000101111111

581

245

000101000101

576

240

000101000000

580

244

000101000100

It appears that the MSB and LSB of the conversion is done is 2 steps. And that the voltage on the ADC input differs slightly when measuring the LSB and MSB. Meaning that of the first conversion the value is below 64 so the 7th bit is set low and when the second conversion is done the value is above 64 so all lower bits are set to 0 instead of 1. And on the next read the other way around.

The connection of the ADC in our product is shown in the below image:

Is there anyone here that knows why the spikes happen and perhaps also knows a solution to it? or  otherwise point me in the direction of solving the issude.

Kind regards,

Wouter

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  • Hi Wouter,

         I can see that there is a 100nf capacitor on the ref pin. Can you try changing to larger capacitor value, like 10uf. From your reference circuit, the ADR3440 is the source for the 4.096V and 2.418V reference. right? so the 47k and 82k and 390k resistor are use to divide the voltage to come up with the 2.418V?

         The Vbridge and Vblock, that are connected on the AD7888 input, are they connected to another circuit that have a driver amplifier circuit?

        

    By the way at what speed the AD7888 is operating? and what power down option is it using?

    Regards,

    Jonathan

Reply
  • Hi Wouter,

         I can see that there is a 100nf capacitor on the ref pin. Can you try changing to larger capacitor value, like 10uf. From your reference circuit, the ADR3440 is the source for the 4.096V and 2.418V reference. right? so the 47k and 82k and 390k resistor are use to divide the voltage to come up with the 2.418V?

         The Vbridge and Vblock, that are connected on the AD7888 input, are they connected to another circuit that have a driver amplifier circuit?

        

    By the way at what speed the AD7888 is operating? and what power down option is it using?

    Regards,

    Jonathan

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