EM357-RTR datasheet(178/241 Pages) SILABS

178 / 241 page

Rev 1.3

178

6. Wait until the INT_ADCULDFULL bit is set in the INT_ADCFLAG register, then read the result from

analogData.

To convert multiple inputs using this approach, repeat steps 4 through 6, loading the desired input configurations to

the ADC_CFG register in Step 5. If the inputs can use the same offset/gain correction, just repeat steps 5 and 6.

10.4. Calibration

Sampling of internal connections GND, VREF/2, and VREF allow for offset and gain calibration of the ADC in

applications where absolute accuracy is important. Offset error is calculated from the minimum input, and gain

error is calculated from the full-scale input range. Correction using VREF is recommended because VREF is

calibrated by the Ember software against VDD_PADSA. The VDD_PADSA regulator is factory-trimmed to 1.80 V

±20 mV. If better absolute accuracy is required, the ADC can be configured to use an external reference. The ADC

calibrates as a single-ended measurement. Differential signals require correction of both their inputs.

The following steps outline the calibration procedure:

1. Calibrate VREF against VDD_PADSA.

2. Determine the ADC gain by sampling independently VREF and GND. Gain is calculated from the slope of

these two measurements.

3. Apply gain correction.

4. Determine the ADC offset by sampling GND.

5. Apply offset correction.

Table 10.5 shows the equations used to calculate the gain and offset correction values.

Table 10.5. ADC Gain and Offset Correction Equations

Calibration

Correction Value

Gain

Offset (after applying gain correction)

Notes:

The ADC output is 2s complement. All N are therefore 16-bit 2s complement numbers.

Offset is a 16-bit 2s complement number.

Gain is a 16-bit number representing a gain of 0 to 65535/32768 in 1/32768 steps. The default value is 32768,

corresponding to a gain of 1.

NGND is a sampling of ground. Due to the ADC's internal design, VGND does not yield the minimum 16 bit 2s

complement value 32768 as the conversion result. Instead, VGND yields a value close to 57344 when the input buffer

is not selected. VGND cannot be measured when the input buffer is enabled because it is outside the buffer's input

range.

NVREF is a sampling of VREF. Due to the ADC's internal design, VREF does not yield the maximum positive 16-bit 2s

complement 32767 as the conversion result. Instead, VREF yields a value close to 8192.

NVREF/2 is a sampling of VREF/2. VREF/2 yields a value close to 0.

Offset correction is affected by the gain correction value. Offset correction is calculated after gain correction has been

applied.

32768

16384

NVREF NGND

–

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

57344 NGND

–

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

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