Errors Due To The Adc Environment; Reference Voltage Noise; Reference Voltage / Power Supply Regulation - ST STM32 Application Note

AN2834
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
analog power supply is used on some packages as the reference voltage (V
quality of V
For example, with an analog reference of 3.3 V (V
converted result is:
(1/3.3) × 4095 = 0x4D9
However, with a 40 mV peak-to-peak ripple in the analog reference, the converted value
becomes:
(1/3.34) × 4095 = 0x4CA (with V
Error = 0x4D9 – 0x4CA = 15 LSB
The SMPS (switch-mode power supply) usually embeds internal fast-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
If the power supply output decreases when connected to V
these inputs and to its output impedance, an error will be introduced in the conversion result.
Digital code
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
If the voltage supply provides a voltage equal to 3.292 V (after its output connection to
V ), then:
REF+
The error introduced by the voltage drop is: 0x4DC – 0x4D9 = 3 LSB.
power supply has influence on ADC error.
DDA
N
( )
V 2 1
–
AIN
, where N is the resolution of the ADC (in our case N = 12).
= --------------------------------- -
V
REF+
Digital
output
Digital
output
DocID15067 Rev 3
= V
REF+
at its peak).
REF+
value.
REF+
DDA
= 1 V, the digital output is:
AIN
12
× ( )
1 2 1
–
= ---------------------------------- - =
3.3
12
× ( )
1 2 1
–
0x4DC
= ---------------------------------- - =
3.292
ADC errors
REF+
) and a 1 V signal input, the
DDA
or V due to the loads on
REF+
0x4D9
DDA
), so the
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