The ADC module of this MCU is based on a 12-bit Analog-to-Digital Converter (ADC) managed by an ADC Controller providing enhanced resolution up to 14 bits. It also integrates a 12-to-1 analog multiplexer, making possible the analog-to-digital conversions of 12 analog lines. The conversions extend from 0V to the voltage carried on pin ADVREF.
As explained in the SAMA5D2 Series datasheet
65.2 Embedded Characteristics
The block diagram shows logic parts involved in A/D convertion and the name of the pins multiplexed for this purpose.
Table 65-1: ADC Pin Description
|VDDANA||Analog power supply|
|AD0–AD11||Analog input channels|
The ADC can be configured to operate in Single-ended mode (the default mode after a reset) or Differential mode. In Differential mode, the ADC requires differential input signals having a VDD/2 common mode voltage.
|Instance||Signal||I/O Line||Peripheral||Differential mode|
The maximum voltage level that can be applied to an analog input must be equal or lower than the analog voltage power supply: VADVREF.
66.10.2 External Reference Voltage
VADVREF is an external reference voltage applied on the pin ADVREF. The quality of the reference voltage VADVREF is critical to the performance of the ADC. A DC variation of the reference voltage VADVREF is converted to a gain error by the ADC. The noise generated by VADVREF is converted by the ADC to count noise.
Table 66-29: ADVREF Electrical Characteristics
|VADVREF||Voltage Range||Full operational||2||–||VDDANA||V|
|VADVREF||RMS Noise||Bandwidth 10 kHz to 1 MHz||–||–||100||μV|
|RADVREF||Input DC Impedance||ADC reference resistance bridge||6||8||10||kΩ|
|IADVREF||Current||VADVREF = 3.3V||-||-||460||μA|
Because of these constraints the voltage under measure must be well conditioned before being applied to the ADC pin. In the schematic example below there is a Transient Voltage Suppressor that, together with the input series resistor, limits the maximum voltage for both permanent signals or spikes (ESD). In this example the reference voltage is 1V. A voltage divider is applied to extend the measurement range. Two different full-scales, 5 or 10V, can be selected inserting different resistors with the switch. The low-pass RC filter composed by the input resistor and the capacitor limits the frequency, and therefore the noise, to 1.5Hz, usefull to measure slow fluctuating signals as a power supply voltage or temperature sensors.