Initial ramblings

ADC

Half bridge strain gauges in each corner

Original spec is 0 - 180kg at 100g resolution

= 1800 data points = 11 bit ADC resolution

Assuming stain gauges energised at 3.3V;

Measured as ~ 0.03 mV/kg, so 3uV (0.003mV) resolution to meet original spec

5.4mV at full capacity (extrapolated)

11bit @ 200x gain would use full range of 1.1v vref

16bit @ x32 gain gives 100 LSB per kg

May be running directly from battery so require ratiometric operation, therefore need external Vref on ADC

Initial ADC thoughts;

atmega ADC is 10 bit, with 200x gain

 - would be 5uV LSB (if it were true)

 - could potentially achieve an extra bit of resolution with oversampling and decimation? Except resolution drops to 6 bits(?) with max gain!

MSP430 with SD16 ADC, 16bit with 32x gain

 - against a 1.1v reference (1v min Vref), 16bit resolution gives 0.5uV LSB

 - provides approx 10 LSB per 100g

LTC2433 - 16bit external ADC, with no gain

 - goes down to 100mV Vref, 16bit resolution gives 1.5uV LSB

ADS1231 - 24 bit external ADC with 128x gain (also 18bit ADS1131 option)

 - minimum Vref is 1.5v, example circuits tie to bridge +/-

 - so against 3.3v, with no gain gives 0.2uV LSB, with 128x gain gives 1.5nV per LSB !!

 - also has automatic low side bridge switching, will power up the circuit only when measuring

 - looks SPI compatible , although signals are named differently

Prototyped with MSP430F2013 ezUSB, bit banging the serial output. performed ok, noisy without proper board design so needed to oversample massively (500x) to get consistent output

LCD

No solderable conections on the glass, need to re-use zebra strip.

Observing the waveform, appears to be 4 mux with 1/3 bias, will require an LCD driver.

MS430 or dedicated chip?

Parts

MSP430s with 16it ADC, 2x serial, and LCD driver are all 100LQFP, start at ~£9

Linear tech 16bit ADC (SPI)
LTC2433
£2.90

TI 24bit ADC (SPI ish)
ADS1231
£4.95 (mouser)

TI 18bit ADC (SPI ish)
ADS1131
£3.46 (mouser)

NXP LCD driver (I2C)
PCF8576C
£1.43

NXP LCD driver (I2C)
PCF8566 - lower pin count version - available in DIP40 for prototyping
£1.72

Plus atmega168 @ ~£2.50

Radio

Assume Xbee footprint

Probably roving networks wireless module?