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LTC2309 : ADC analog inputs sinking current!!

I'm using the  ADC LTC2309 to convert analog signals coming from an opam. The analog voltage to measure ranges from 2 to 3.5 V. The signal has very low fluctuations so we can say that it is almost a DC signal.

In one of my experiments I was measuring 2.4V at input 7  and suddenly  the voltage went up by 0.7 V. Hence the measured voltage became 3.1V. I verified this value either by collecting the data via I2C or by measuring directly on  this input.

I thought it was the opam that was giving the wrong signal. Verifying the circuit, it was effectively the ADC pin that was giving the wrong signal.

The connection of components going from the opam output to the ADC input is a 100 Ohm resistor  with a 47pF capacitor from the ADC input  to ground, as recommended in the last figure in the datasheet.

The Vref pin is connected to ground while the Refcomp pin is connected to +5V. I retreive data from the ADC input via I2C using the command F8 that addresses the 7th channel in unipolar mode and seting the ADC in nap mode. Then leaving the ADC in sleep mode, the result is the same.

Observing this, I realize that the ADC input is sinking current which explains the increase of measured voltage at its input.

Another test that I made was removing the 100 Ohm resistor and connecting the output of the opam directly to the ADC input.

The read value was oscillating around  2.55V . Another observation is that the same value applied on input 8 is also present in other pins of the ADC which are not connected.

The question that arises is why the ADC would start sinking current with no apparent reason. ?

I will really appreciate any hint or advices you can provide.

Regards,

Fausto

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  • What I mean with I2C not working is that the ADC was not responding to I2C the commands sent from the microcontroller.  The presence of the ADC was not detected.

    After have connected all the ADC inputs to ground,  the ADC was detected again.

    As you mentioned, the fact of driving the I2C signals with voltage levels 0.8 below ground is producing the problem and will likely push the ADC to operate unpredictably.

    The 3.3V I2C signal provided by the microcontroller is around 0.5V below ground (for the low level). I will need to shift the signal up or clipping the signal below zero to avoid this issue. Do you think I can use a normal diode to clip the signal below zero? or should you recommend something else to do the job?

    Best regards,

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  • What I mean with I2C not working is that the ADC was not responding to I2C the commands sent from the microcontroller.  The presence of the ADC was not detected.

    After have connected all the ADC inputs to ground,  the ADC was detected again.

    As you mentioned, the fact of driving the I2C signals with voltage levels 0.8 below ground is producing the problem and will likely push the ADC to operate unpredictably.

    The 3.3V I2C signal provided by the microcontroller is around 0.5V below ground (for the low level). I will need to shift the signal up or clipping the signal below zero to avoid this issue. Do you think I can use a normal diode to clip the signal below zero? or should you recommend something else to do the job?

    Best regards,

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