RF430FRL152H: Supply Voltage for analog sensors

Hello Adam,

the SVSS voltage of 300mV is definitely to high. The SVSS voltage is specified for a maximum current of -5uA. My assumption is that the current is to high. For the thermistor and the reference resistor measurement the resistive bias interface must be enabled to ensure a constant current of ~2.4uA.

Because you wrote of a voltage divider, can you please describe your setup in detail.

Best Regards,

Helfried

Hi Helfired,

Thanks for the reply.

I am trying to recreate the circuit used in the NFC temperature sensing patch application note (RF430-TMPSNS-EVM) on a breadboard.
I am taking the sensor reference potential from SVSS and passing it through a 100kOhm resistor and into pin ADC1, and a 100kOhm NTC thermistor and into pin ADC2. I am using a MSP-EXP430G2ET plus the TRF7970ABP and the windows GUI to read the device.

When I run the demo mode on the windows GUI application the correct temperature is displayed on the graph (~17°C). I'm trying to take the  ADC reading directly and it them to temperature in a custom software application. I can't get my calculations to agree with the texas instruments software.

Maybe it's easier to consider these readings taken from the evaluation board RF430FRL152HEVM:

The software is reading 17°C (which is correct). My calculations are as follows:

3512h = 13586 decimal

ADC voltage (VADC) is: 13586/16384 * Vref. Assuming Vref is 0.9 V then the ADC voltage is is 0.75V.

The thermistor resitance is Rtherm = Rref x (VADC/SVss -1)

With SVss = 0.125V this gives a thermistor resistance of 497kOhm which corresponds to ~-5°C.

Have I made some error in the values I'm using for Vref or SVss? How does the windows GUI arrive at the value of 17°C?

Appreciate the help!

Adam.

Hi Helfried,

Hi Helfried,