MSP430G2553: measure the voltage of a non-rechargeable 3.6V battery

Question

I am designing a circuit to measure the voltage of a non-rechargeable 3.6V battery. This battery powers the microcontroller (http://www.ti.com/lit/ds/symlink/msp430g2253.pdf) with which I want to measure its voltage.I simulated both circuits and they seem to work, which do you think is better? Thanks in advance.

^{simulate this circuit – Schematic created using CircuitLab}

Mosfet 1: http://www.onsemi.com/pub/Collateral/2N7002L-D.PDF

Mosfet 2: http://www.onsemi.com/pub/Collateral/BSS138-D.pdf

Mosfet 3: https://www.infineon.com/dgdl/Infineon-BSS84P-DS-v02_07-en.pdf?fileId=db3a304330f68606013118ac7a9b4549

Best regards,

Fran Martin

Answer 1

I see two advantages to Circuit 2, but neither are critical here. As with many design questions it ultimately comes down to weighing benefits against the cost, both of which happen to be minor in this case.

1. When PIN_CONTROL is inactive, the ADC pin sees a high impedance in Circuit 2, whereas in Circuit 1 the ADC pin sees an undivided 3.6 V. If your ADC were actually running at 2.5V that might cause issues, but in this case it's just a 2.5Vref and the MSP430 can accept VCC + 0.3V on any of its input pins. So we don't really care about that here.
2. If you were concerned about leakage current at the ADC input, Circuit 2 would prevent it from continuously drawing from the battery. I don't think the MSP430 ADC is well-characterized but I did find this forum post on the TI forum that quotes it at 50 nA static leakage current:

Input impedance of ADC in MSP430F5438

Answer 2

If the MCU is powered directly from the battery then you don't need any external circuit, because the MSP430G2553's ADC has an internal voltage divider for reading Vcc.

Example code (from Fix It Until It's Broken):-

/** Reads the MSP430 supply voltage using the Analog to Digital Converter (ADC).
On ez430 boards, this is approx. 3600mV
@return Vcc supply voltage, in millivolts
*/
unsigned int getVcc3()
{
ADC10CTL0 = SREF_1 + REFON + REF2_5V + ADC10ON + ADC10SHT_3;  // use internal ref, turn on 2.5V ref, set samp time = 64 cycles
ADC10CTL1 = INCH_11;                        
delayMs(1);                                     // Allow internal reference to stabilize
ADC10CTL0 |= ENC + ADC10SC;                     // Enable conversions
while (!(ADC10CTL0 & ADC10IFG));                // Conversion done?
unsigned long temp = (ADC10MEM * 5000l);        // Convert raw ADC value to millivolts
return ((unsigned int) (temp / 1024l));
}