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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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  • Hello mr Hoover,

    thank you for your feedback.

    Circuit Configuration until this morning

    Indeed I have unconnected pins. Some other pins are bridged to the input signal and the rest of them are directly connected to ground, as can be observed in the schematics of the figure "current version".

    Whenever the pin connected to the input resistance is not working, I remove the bridge and leave the pin disconnected. At today only  3 or 4 pins provide a correct reading.

    I also remarked that the I2C communication does not work . Do you think this is also produced by leaving unconnected pins?.

    Configuration after setting unconnected pins to ground

    I proceeded to set the unconnected pins to ground, as you suggested, and see if the problem of not correct readings was solved.

    I placed a 100 Ohms resistor between the input pins and ground and left only one pin connected to the input signal.

    The measured voltage in this pin is so far correct. Also the I2C starts working again although I don't know for how long. Nevertheless I'm not sure if the I2C stopped working in the previous configuration because of unconnected inputs.

    Regarding this subject, in the figure below I show a screen shot of the I2C clock signal that is applied to the  LTC2309. The low level of this clock signal is around -0.8V while the high level around 5V. This levels are provided by the I2C isolator. Do you see any issue using these voltage levels?

    I have the interest to use the LTC2309 to perform data acquisition for long periods of time. Do you know if the LTC2309  is resilient enough to support  being accessed for long periods of time.?

    best regards

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  • Hello mr Hoover,

    thank you for your feedback.

    Circuit Configuration until this morning

    Indeed I have unconnected pins. Some other pins are bridged to the input signal and the rest of them are directly connected to ground, as can be observed in the schematics of the figure "current version".

    Whenever the pin connected to the input resistance is not working, I remove the bridge and leave the pin disconnected. At today only  3 or 4 pins provide a correct reading.

    I also remarked that the I2C communication does not work . Do you think this is also produced by leaving unconnected pins?.

    Configuration after setting unconnected pins to ground

    I proceeded to set the unconnected pins to ground, as you suggested, and see if the problem of not correct readings was solved.

    I placed a 100 Ohms resistor between the input pins and ground and left only one pin connected to the input signal.

    The measured voltage in this pin is so far correct. Also the I2C starts working again although I don't know for how long. Nevertheless I'm not sure if the I2C stopped working in the previous configuration because of unconnected inputs.

    Regarding this subject, in the figure below I show a screen shot of the I2C clock signal that is applied to the  LTC2309. The low level of this clock signal is around -0.8V while the high level around 5V. This levels are provided by the I2C isolator. Do you see any issue using these voltage levels?

    I have the interest to use the LTC2309 to perform data acquisition for long periods of time. Do you know if the LTC2309  is resilient enough to support  being accessed for long periods of time.?

    best regards

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