SX 868 ADC with PT1000

Question

Hello,

has anybody successfully read out a PT100 or PT1000 temperature sensor on a XBee ADC yet? There is a 12-Bit ADC on the Cortex M3 µC, the XBee has only a 10-Bit ADC at 4 PINs, so the resolution would be 1.25V/1023digit=1.22mV/digit (with 1.25 V_Ref).

To read out a PT1000 sensor it is recommended to connect the supply voltage of the voltage divider with reference resitor and PT1000 to the V_Ref Port at the µC, but this isn't possible at XBee's because V_Ref isn't led out to the PINs. Why isn't V_Ref of the µC led out to the XBee PINs?

So my idea was to read out the actual supply voltage of XBee SX 868 with DIO1_AD1 (three 10K resistors as voltage divider) and the voltage over the PT1000 with DIO3_AD3 (reference resistor with PT1000 as voltage divider). The accuracy is about +/-2 K at 0...100°C, this is a very bad accuracy for a 10-Bit ADC.

Answer 1

Maruis,

That modules processor uses an internal VREF. You have the ability to set it to either 1.25 or 2.5V using the AV command.

Comments

The problem is that the internal reference voltage is between two values (like 1.25V is between 1.2 and 1.3V). Shouldn't the XBee SX 868 modules have a Cortex-M3 EFM32LG230F256 integrated processor?
Edit: With a small low pass filter before the ADC input i get an accuracy of +-0.5 Kelvin, that's enough so far.

Answer 2

Because of the battery power consumption i need to set the end device to synchronous sleep and a minimal wake time. The communication between the end device (reading the ADC value and trasmit this value) and the coordinator (io sample callback function) work's well.
But how can i change GPIOs of the remote device from my coordinator in synchronous sleep with minimal wake time? I always get an TimeoutException error and XCTU can't find the remote device (wake time ~350ms) over RF, only if i set the wake time to 1s or higher.

Comments

Marius,

Try using Hardware flow control and API mode. Then you can send your API frames till the buffer is full. Then when the network wakes, they will be sent in order.

Eric

Answer 3

One last question about the SX 868 ADC PIN, what is the maximum (input)resistance i can connect to this PIN for good measurement results?

Comments

https://www.digi.com/resources/documentation/Digidocs/90001538/#reference/r_specs_gpio.htm%3FTocPath%3DTechnical%2520specifications%7C_____6

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Thanks for your answer. I'm missing some detailed information in the XBee datasheet about the ADC, like is there any input RC filter active on the microcontroller PIN of the XBee module? Because there are no information about the internally configuration of the microcontroller unit it's hard to find out the best hardware cuircitry around that ADC PIN.