Part Number:MSP430FR5969
Hi all,
I am working on a design which includes a full-bridge strain gauge to measure tank pressure. It does not need to be rad-hard at this time, but I would like to be able deliver a rad-hard version in the future with minimal redesign. For this reason, the MSP430FR5969 (and its -SP version) are my best choice for the microcontroller. I am trying to decide if I can get away with using the internal 12 bit SAR ADC.
Unlike some MSP430s (such as MSP430F6736A), the ADC on the FR5969 does not have a programmable gain amplifier. Because of that, I am looking at using the THS4513 (fully differential) to provide the gain necessary for my strain gauge into the MSP430FR5969 ADC12 input. Have I made any obvious mistakes in setup here?
My intended supply voltage is 3V (single ended). According to the THS4513 datasheet, the input and out ranges are +/-400mV and +/-450mV around mid-rail.
Maximum bridge output voltage (with 3V supply): 486uV differential
Minimum bridge output voltage to be resolved: 1.6uV differential
ADC Vref+: 1.2V internal reference voltage (ADC12VRSEL = 0001b)
ADC Vref-: AVSS
ADC ENOB with differential input: 10.7 bits typical (Table 5.26 in datasheet - 2.5V Vref, not sure how much this drops with a 1.2V reference)
ADC settings: 12 bit unsigned, differential inputs (ADC12DIF =1b, ADC12DF = 0b, ADC12RES = 10b)
ADC Voltage resolution based on ENOB: 721uV
My calculations tell me that I need about 55dB of gain to match the 1.6uV differential output of the strain gauge to the 721uV sensitivity of the ADC. Part of my concern is that the open loop gain of the THS4513 is only 68dB, and shooting for 55dB of gain will require a large resistor (Rf of 267k with Rg of 47 ohms).
This design seems borderline to me. I could go ahead and order the THS4513 EVM and try it all out, but if my chances of success are low, I should go ahead and start looking for an external ADC which works well with the MPS430. As a side note, I don’t have any other external chips which will require an I2C/SPI/etc interface, which is one reason I am hesitant to add an external ADC.
Thanks,
Paul