Introduction

This chapter describes the usage of the 12-bit analog-to-digital converter (ADC).

ADC is a successive-approximation register (SAR) ADC and the conversion result of ADC sample is stored in the result register and can be read on demand.

ADC characteristics are as follows:

• Resolution: 12 bit

• ENOB: 10 bit

• Sample Frequency: 31.25 kHz ~ 250 kHz, 166.67 kHz is recommended (default)

• Sample Capacitance: 2.4 pF ~ 2.93 pF

• Reference Voltage: Fixed, cannot be externally supplied

• Internal Impedance: About 500 kΩ

• FIFO size: 64 x 16 bit

• Channel switch list length: 1 ~ 16

• Description of channels:

RTL8721DxRTL8720ERTL8710ERTL8726ERTL8713ERTL8730ERTL8721F

RTL8721Dx:

Channel	ADC channel ID	Pin name	Input voltage (V)
Normal channel	CH0~CH6	PB13~PB19	0~3.3
BAT_MEAS channel	CH7	BAT_MEAS	0 ~ 5

Note

ADC multi-channel sample is time-division multiplexing on the same circuit. The sample frequency value listed above is specific to a single channel. When multiple channels are set in the channel switch list, the sample frequency will decrease accordingly.

ADC Calibration

ADC Voltage Calibration

• To improve the linearity of ADC input/output characteristics, each chip undergoes nonlinear calibration in factory to reduce ADC gain errors and offset errors.

• Users can directly retrieve voltages via the following APIs:

  • ADC_GetVoltage(u32 chan_data): Get calibrated normal channel voltage.

  • ADC_GetVBATVoltage(u32 vbat_data): Get calibrated BAT_MEAS channel voltage.

ADC Internal Resistor Calibration

• ADC internal resistor R varies among chips.

• Users can directly retrieve internal resistor value via ADC_GetInterR()