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# 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:

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

• Hi Wouter,

I can see that the spikes occur between ADC values from 500 to 800. Are you using external reference? If so please make sure that the reference source is stable. A not consistent reference will greatly affect ADC conversion. What is the value of the reference voltage?  From your schematic, there is low pass filter for IN1 and IN2, what value is the resistor on this inputs? I can see a 330pF for the capacitor. Can you send a clearer picture?

Regards,

Jonathan

• Hello Jonathan,

The spikes also occure on other values than the range 500 to 800 but they are not shown in the graph as it did not occure during the test measurement of the graph.

The reference voltage is 4,096V and is stable.

The resistor is 1500 ohm.

Below once again the schematic, I hope this one is better to read.

regards,

Wouter

• Hi Wouter,

There is a possibilty that the spikes usually occur near a major code transition or when ADC have heavier weighted most significant bit decision. From the data above, it looks like from code 575(dec) to 576(dec).

I am just curious, what reference are you using for the 4.096 Ref? Would it be possible to share your schematic, including the support circuitry, like the reference so can have a better understanding the possible cause of the problem.

Regards,

Jonathan

• Hello Jonathan,

I cannot share the full schematic with you but the 4,096V reference part should not be a problem. It is shown below. The 5V power supply is from a PC USB port.

Regards,

Wouter

• 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