MSP430G2231 - ADC reference voltage = Vcc

Hi Everyone.

I am a noob to the MSP430 line... and I just picked up a few launchpad's. We chose the msp430g2231 for it's low power and small foot print and ADC lines. I want to put this microcontroller in to an existing product which has some op-amps and sensors connected to a 2.5V linear regulator and talk to an Ember chip. My question is I'd like to power the msp430 with 2.5V since the chip will use less power at that and it's available in the existing circuit... but, will I be able to use the internal 2.5V reference even though Vcc is 2.5V? Or would I have to connect the 2.5V to VREF+?

I guess I could use the internal 1.5V as well, right? Then some of my sensors may require a voltage divider.

thanks

Jeff

  • Take a look at MSP430G2231 datasheet on page 28 which has a table describing the electrical characteristics of the 10-bit ADC, Built-in Voltage reference.

    The minimum supply voltage for the 2.5V internal reference to operate is VCC=2.9V.  Therefore, if you are supplying VCC=2.5V, either use the internal reference at 1.5V, or use the external Vref+ pin as the input to the reference voltage.

  • In reply to BrandonAzbell:

    Thanks Brandon!

    I was thinking I'd have to use the external reference... I don't forsee any issues having VREF+ = Vcc.

    Thanks!

    Jeff

  • The ADC10 does nto need to use a stabilized reference. It can also use VCC directly as reference (SREF_0, default value). This is less reliable, as VCC might be subject to ripple (e.g. caused by MSP CPU execution or other preripherals), but makes no difference to routing VCC to VeREF+ and using it as external reference.
    Except if you decouple the 2.5V that are routed to the external reference (e.g. with 100Ohm series resistor and 10µF capacitor to ground. Then this is the better way to go, as there will be almost no more ripple on the VeREF+ pin.
    You can go even further and use SREF_6 and connect GND to VeREF- with a separate GND line that is connected to digital GND near the power supply. It's difficult to tell the normal layout program to NOT merge it into the common GND. I use a 'connector device' which has two pins, a GND supply input and a AGND power output. It's just a dummy piece of copper on the PCB, but allows the two GNDs to be handled separately by teh layout program. Of yourse you have to make sure you place this 'device' near the power supply manually.

  • In reply to Jens-Michael Gross:

    Thanks everyone... I've decided to run the chip off Vbatt (3.3v) and use the internal reference 2.5V. This will take a little more power in the long run but it's the solution I trust most.

    I have a PIR circuit with op-amp running at 2.5V but decided to leave it on it's own regulator since they tend to be fairly sensitive to ripple.

     

    Jeff