The recommended calibration procedure
is as follows:
- Preset the offset and gain
calibration registers to 000000h and 400000h, respectively.
- Perform offset calibration by shorting the ADC inputs, or short
the inputs at the system level to include the offset error of the external
amplifier stages. Acquire conversion data and write the average value of the
data to the offset calibration registers. Averaging the data reduces conversion
noise to improve calibration accuracy.
- Perform gain calibration by applying a calibration signal to
the ADC input or at the system level to include the gain error of the external
buffer stages. For the standard input range mode, choose the calibration voltage
to be less than the full-scale input range to avoid clipping the output code.
Clipped output codes result in inaccurate calibration. For example, use a 3.9-V
calibration signal with VREF = 4.096 V. When operating in the
extended range mode, the calibration signal can be equal to VREF
without causing clipped output codes. Acquire conversion data and average the
results. Use Equation 19 to calculate the gain calibration value.
Equation 19. Gain
Calibration Value = (expected output code / actual output code) ·
400000h
For example, the expected output code of a 3.9-V calibration voltage using
a 4.096-V reference voltage is: (3.9 V / 4.096 V) · 7FFFFFh =
79E000h.