CN0540 + CN0532 LSB to g conversion

Question

Hi, according to the AD7768-1 datasheet (with a 4.096 V reference as is the case in CN0540) 1 LSB size is equal to 488.28 nV. 
 Also, according to the CN0532 and ADXL1002 datasheet, this sensor produces 40 mV/g. 
 
 That would mean that 81920 LSB at the output of AD7768-1 equals 1 g measured by the CN0532, assuming that 1 V at the output signal of CN0532 is equal to 1 V at the input of AD7768-1.

But when using CN0540 + CN0532, I roughly measure that 1 g equals 63200 LSB (I've checked by fixing the accelerometer positive axis downward, flat and upward). 
 
 I'm using the CN0540 (AD7768-1) default settings.

Could somebody please explain to me the reason for that difference? I'm missing something in the middle between CN0530 and CN0540 when making that calculation?

Regards,

malcolm_kwok · Accepted Answer

Hi FelipeCook , 
 With the CN0532's ADXL1002 sensitivity of 40 mV/g, 1 g = 40 mV. 
 The CN0540 has a total gain = 0.8 from sensor to ADC. 
 40 mV * 0.8 = 32 mV. 
 You are correct that the AD7768-1 itself has an LSB = 488 nV. 
 32 mV * 1 LSB / 488 nV = 65574 LSB. Your measurement of 63200 LSB is close, and there might be some offset remaining 65574 - 63200 = 2374 LSB = 1.16 mV referred to ADC output. With the CN0540's DC bias compensation algorithm, ideally, the remaining offset is less than 1 LSB of the DAC referred to the ADC output ("Total LSB DAC"). 
 However, there can be some other sources of error such as nonlinearities. 
 All-in-all, I think most of the mismatch from your expected and actual measurements is due to the system gain. I hope this sufficiently answer your question. 
 Regards, Malcolm)

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CN0540 + CN0532 LSB to g conversion