ST STM32 Application Note

DocID15067 Rev 3 17/49
AN2834 ADC errors
48
2.2 Errors due to the ADC environment
2.2.1 Reference voltage noise
As the ADC output is the ratio between the analog signal voltage and the reference voltage,
any noise on the analog reference causes a change in the converted digital value. V
DDA
analog power supply is used on some packages as the reference voltage (V
REF+
), so the
quality of V
DDA
power supply has influence on ADC error.
For example, with an analog reference of 3.3 V (V
REF+
= V
DDA
) and a 1 V signal input, the
converted result is:
(1/3.3) × 4095 = 0x4D9
However, with a 40 mV peak-to-peak ripple in the analog reference, the converted value
becom
es:
(1/3.34) × 4095 = 0x4CA (with V
REF+
at its peak).
Error = 0x4D9 – 0x4CA = 15 LSB
The SMPS (switch-mode power supply) usually embeds internal fas
t-switching power
transistors. This introduces high-frequency noise in the output. The switching noise is in the
range of 15 kHz to 1 MHz.
2.2.2 Reference voltage / power supply regulation
Power supply regulation is very important for ADC accuracy since the conversion result is
the ratio of the analog input voltage to the V
REF+
value.
If the power supply output decreases when connected to V
DDA
or V
REF+
due to the loads on
these inputs and to its output impedance, an error will be introduced in the conversion result.
Digital code
V
AIN
2
N
1–()
V
REF+
--------------------------------- -=
, where N is the resolution of the ADC (in our case N = 12).
If the reference voltage changes, the digital result changes too.
For example:
If the supply used is a reference voltage of 3.3 V and V
AIN
= 1 V, the digital output is:
Digital
output
12
12
1–()×
3.3
---------------------------------- - 0x4D9==
If the voltage supply provides a voltage equal to 3.292 V (after its output connection to
V
REF+
), then:
Digital
output
12
12
1–()×
3.292
---------------------------------- - 0x4DC==
The error introduced by the voltage drop is: 0x4DC – 0x4D9 = 3 LSB.

ST STM32 Specifications

General

Series	STM32
Category	Microcontrollers
Core	ARM Cortex-M0, Cortex-M0+, Cortex-M3, Cortex-M4, Cortex-M7, Cortex-M33
Clock Speed	Up to 480 MHz
GPIO Pins	Up to 144
Communication Interfaces	I2C, SPI, USART, USB, CAN, Ethernet
Package Types	LQFP, BGA, WLCSP
ADC Resolution	12-bit, 16-bit (varies by series)
DAC Resolution	12-bit (varies by series)
Operating Temperature	-40°C to +85°C or +105°C