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Use of reference voltage outside of CC430F6137

John Schnellenberger
Prodigy 130 points

Hi,

We are planning on using the CC430F6137 Reference Voltage (pins 59/60) as a reference for a DAC7513 digital to analog converter.  Specifically, we're planning on connecting pin 59 (VREF+) on the CC430 to pin 3 (VREF) on the DAC7513.  Could you please answer the following questions when you get a chance?

1)  We were planning on having the CC430 and DAC7513 share the same ground.  Is it alright if we connect pin 60 (VREF-) on the CC430 to this shared ground plane?  Or, should we have a seperate analog ground for VREF- and the DAC7513?

2)  What capacitance is recommended between VREF+ and VREF-?  We were thinking of putting a 470nF capacitor between these pins (59/60) near the CC430 and then putting a 100nF capacitor between VREF and ground near the DAC7513.  Does this sound reasonable?

3)  How much current can the CC430 VREF+ pin supply?

 

Thanks for your time.

John

  • 0
    Guru 227245 points

    1) Yes, if your negative reference point is GND, then this is okay. If providin an external reference to the MSP, you can set VREF- to something higher, so you can measure 0..x for voltages of VeREF-..VeREF+. It is useful for eliminating some constant offset from the result (increasing the resolution). Some MSPs have a separate AVcc pin. For better results, you should route AVcc and VeREF- separate from the digital GND to the supply. This avoids offset voltages on teh GND line caused by the other circuitries current consumption.

    In addition, you should decouple the AVcc terminal from DVcc with a 10 Ohm resistor and decouple it to AVss with an identical 10µF/100nF combo.

    2) Typical capacitance for stabilizing the reference output is 10µF tantalum parallel with 100nF ceramic. This combo is small, has a low leakage, low noise and high stablization capability. We too use it with good results for years.

    3) Surprisingly, this is not clearly listed in the datasheet (on others, it is). However, the other parameters give a hint. IREF+ would be my first candidate, but the description describes it as operating supply current into AVcc terminal. However, the reference oputput cannot be higher than its supply current minus the internally required. So I'd guess typically 0.8mA and 1.35mA max, with a maximum voltage drop of 2.5mV/mA.
    ~1mA is a typical value for other devices too.

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