Keywords: A/D converters, ADCs, A/D converter calibration, offset flipping, handheld devices
VOUTA = VREF + RB/RA[(VINA- - VS1-)] - RC/RD[(VINA+ - VS1+)]Where
VOUTA = VREF + 30[(VS1+ - VS1-)]Precision divider RN3 is connected between VLi and ground (GND) to generate the offset voltage (VREF). RE = 30RF. Note that you can alter the magnitude of VREF (from VLi/31 to 30VLi/31) by toggling the switches U1X and U1Y, which swaps the divider connections to VLi and GND. For VLi = 3.6V, VREF is:
VREF = [RF/(31RF)](VLi) = VLi/31 = 0.116VThis offset voltage simplifies calibration by ensuring that the U2A output remains positive at VLi/31. VREF is buffered by U2B to eliminate the effects of RN2 loading. Thus, we have a millivolt sensor amplified by a difference amplifier with a gain of 30 and zero offset of VLi/31. Now to calibrate the ADC: Section U1Z of the triple SPDT switch (MAX4783) is used to short sensor outputs VS1+ and VS1- together. Its low impedance (1.2Ω maximum) is negligible compared to that of the sensor bridge (300Ω to 500Ω). Otherwise, the resistor network of the differential amplifier would impose an excessive load on the sensor. The output of U2A therefore equals VREF plus the net effect of offset and gain errors.
VREF = (30/31)VLi. For VLi = 3.6V, VREF = 3.484VThe output of amplifier A now "flips" to VREF = 3.484V, plus the net effect of offset and gain errors. An ADC conversion in this configuration provides a span reading of (30/31)VLi.
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APPLICATION NOTE 4539,AN4539, AN 4539, APP4539, Appnote4539, Appnote 4539
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