XTR111: sensor interfacing problem

Shibin Varghese

Part Number: XTR111
Other Parts Discussed in Thread: TLV9001

Hi all
We are trying to interface this capacitive moisture sensor with XTR111AIDGQT. But we are unfortunate to get a proper result we are expecting.
can anyone help me by providing necessary information and cares that have to be taken while interfacing with such a sensor.
NB: We have successfully obtained the output for a resistive sensor for the same PCB board.

over 6 years ago

0 kai klaas69 over 6 years ago

Guru 140200 points

Hi Shibin,

what have you done so far? Can you show a schematic?

Kai

0 Shibin Varghese over 6 years ago in reply to kai klaas69

Expert 1130 points

Sure.

Here it is.
Thank you for your reply.
The ADC out of the sensor is directly connected to the 6th pin of the chip. The 5V needed for the sensor is directly connected from the 5th pin of the chip.

0 kai klaas69 over 6 years ago in reply to Shibin Varghese

Guru 140200 points

Hi Shibin,

eventually, the sensor is drawing too much supply current from the XTR111.

What Rset have you chosen? And what Rload have you mounted?

Kai

0 Shibin Varghese over 6 years ago in reply to kai klaas69

Expert 1130 points

Thank you for your reply.
Rset=2.2K, and the output is connected to a microcontroller, so a voltage divider is used to make the output to be between minimum voltage(4mA) to 3.3V (20mA).

0 kai klaas69 over 6 years ago in reply to Shibin Varghese

Guru 140200 points

Hi Shibin,

the XTR111 must see a load resistor at its output to GND. A 100R load resistor gives a voltage drop of 2V with an output current of 20mA. Your microcontroller ciruit might not be adequate.

Kai

0 Shibin Varghese over 6 years ago in reply to kai klaas69

Expert 1130 points

This circuit works fine for me.
Is there any problem with the circuit?

0 kai klaas69 over 6 years ago in reply to Shibin Varghese

Guru 140200 points

Hi Shibin,

yes, there is a problem. Is it possible that you misunderstood what the XTR111 is intended to be used for? The XTR111 is generating a current signal at its output not a voltage signal. You have connected R4 and C10 as if the drain of U3 is generating a voltage signal. But it is a current signal and you need a low ohmic resistor (burden resistor) from the drain of U3 to signal ground to produce a voltage signal from this current signal.

Me thinks that you don't need the XTR111 at all. Why not directly connecting the output signal of U3 to the ADC input, or at least using a voltage follower to drive the ADC input low-ohmically enough? I see no reason why you should use the XTR111 at this place.

Kai

0 Shibin Varghese over 6 years ago in reply to kai klaas69

Expert 1130 points

Hi kai klaas69,
When I connected the R4 (15 Ohm as in the datasheet) to ground, my output has no changes even if my sensor output changes. that's why I connected in series with the controller.
The sensor is far away from the controller so there may be voltage loss as the signal travels over a large distance, that's why we choose a current to voltage converter since loss of current is much small as compared to voltage

0 kai klaas69 over 6 years ago in reply to Shibin Varghese

Guru 140200 points

Hi Shibin,

yes, that's true, you cannot connect a 15R resistor from the output to signal ground. This will short circuit the output of sensor. But why not buffering the sensor output signal with the TLV9001?

Kai

0 Shibin Varghese over 6 years ago in reply to kai klaas69

Expert 1130 points

Thank you

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XTR111: sensor interfacing problem