MSP430F6721A: IR Thermometer application questions

Hi Franz,

Thanks for posting these questions to E2E. I'll do my best to address them all. If there are follow up questions, I'll address them individually to reduce confusion.

Franz Schellhase said:

Do you have an application circuit for infrared thermopile measurement ?

Refer to the following IR-related documents.

• Designing a Low-Cost, High-Accuracy Infrared Thermometer (uses external op-amp)

• Low-noise and long-range PIR sensor conditioner circuit with MSP430™ smart analog combo (uses internal op-amps)

• Ultra-Low-Power Motion Detection Using the MSP430F2013 (no op-amps are used, but the external impedance must be considered since it impacts the ADC's minimum settling time)

Franz Schellhase said:

How to measure positive and negative voltage of the thermopile directly with the ADC24 without additional OPAMP ?

The documents above describe several approaches. One design uses an external op-amp, another uses the op-amps integrated inside the MSP430 and one uses just passives connected to the ADC inputs.

Franz Schellhase said:

How would it be possible to drive the µC directly via a LDO without a buck LDO ? Supply will be via 2xAAA batteries. Which LDO voltage do you recommend?

You can use the 2xAAA battery voltage to power the MSP430 since it's max voltage is ~3.0V. However, if you use a LDO between the MSP430 and the batteries, you can achieve greater efficiency since the current consumption is constant. You can learn more by reading through the Using power solutions to extend battery life in MSP430(TM) MCU applications report. Keep in mind though, the lower the supply voltage, the lower the maximum CPU frequency. Deciding on the supply voltage must factor in these aspects.

Franz Schellhase said:

What is the minimum LCD voltage in practice ? Are there any limitations regarding MUX (number of common lines) ?

Details about the LCD voltage can be found in Table 5-33 of the datasheet. To learn more about the MUX, refer to the LCD section in the user's guide. Another good resource is the Designing With MSP430™ MCUs and Segment LCDs application report.

Franz Schellhase said:

Could you tell me the temperature drift of the ADC24 in µV/°C ?

The datasheet provides a max gain error for the internal reference as 50ppm/C. Divide 50 by 1e6 to get a percentage which equals 0.005%/C. VREF is ~1.2V (typically), so multiplying these two would equal ~60uV/C.

Regards,

James