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RF430FRL152H: Supply Voltage for analog sensors

Adam Collins1
Prodigy 60 points
Part Number: RF430FRL152H
Other Parts Discussed in Thread: MSP-EXP430G2ET, ,

Hi All,

I have a question about the voltage supply from the chip.

I've connected up a thermistor and voltage divider to the chip and I am getting incorrect resistance readings for the thermistor. I think the error is coming from the value I'm using for the input voltage.

The supply voltage SVss is 125mV on the datasheet (section 5.23) but my calculated temperature values are incorrect when using this value. When I measure SVss using a DMM I am reading ~300mV. The temperature values are close to the correct values if I use 300mV for SVss in my calculations.

Have I misinterpreted the datasheet? Is 300mV the correct value for SVss?

I am using 0.9V for the reference voltage for the 14-bit ADC, this value is taken from the datasheet also.


Appreciate any help.

Thanks,
Adam

  • 0
    TI__Mastermind 18265 points

    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

  • 0 in reply to Helfried Vollbrecht
    Prodigy 60 points

    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.

  • +1 in reply to Adam Collins1
    TI__Mastermind 18265 points

    Hello Adam,

    the formula to calculate the temperature from the ADC readings can be found in the RF430FRL152H FAQ chapter 4.1:

    http://www.ti.com/lit/pdf/sloa247

    Best Regards,

    Helfried

  • 0 in reply to Helfried Vollbrecht
    Prodigy 60 points

    Hi Helfried,

    Thanks, I have it now. Didn't spot that on the FAQ.

    Regards,

    Adam.