
Electrical characteristics
ST10F296E
24.7.3
Analog reference pins
The accuracy of the ADC converter depends on the accuracy of its analog reference. A
noise in the reference results in the same proportion of error in a conversion. A low pass
filter on the ADC converter reference source (supplied through the VAREF and VAGND pins),
is recommended to clean the signal thereby minimizing the noise. A simple capacitive
bypassing may be sufficient in most cases. In the presence of high RF noise energy,
inductors or ferrite beads may be necessary.
In the ST10F296E architecture, the VAREF and VAGND pins also represent the power supply
of the analog circuitry of the ADC. An effective DC current is required from the reference
voltage to the internal resistor string in the R-C DAC array and to the rest of the analog
circuitry.
An external resistance on VAREF could introduce error under certain conditions. For this
reason, series resistance is not advisable. Any series devices in the filter network should be
designed to minimize the DC resistance.
24.7.4
Analog input pins
To improve the accuracy of the ADC, analog input pins must have low AC impedance.
Placing a capacitor with good high frequency characteristics at the input pin of the device
can be effective. The capacitor should be as large as possible, ideally infinite. This capacitor
contributes to attenuating the noise present on the input pin. Moreover, the source of the
capacitor charges during the sampling phase, when the analog signal source is a high-
impedance source.
A real filter is typically obtained by using a series resistance with a capacitor on the input pin
(simple RC filter). RC filtering may be limited according to the value of the impedance
source of the transducer or circuit supplying the analog signal to be measured. The filter at
the input pins must be designed to account for the dynamic characteristics of the input
signal (bandwidth).
Figure 99.
ADC input pins scheme
1.
Legend:
RS: Source impedance
RF: Filter resistance
CF: Filter capacitance
RL: Current limiter resistance
RSW: Channel selection switch impedance
RAD: Sampling switch impedance
cp: Pin capacitance (two contributions, CP1 and CP2)
CS: Sampling capacitance
VA: Source voltage
RF
CF
RS
RL
RSW
CP2
CS
VDD
Sampling
Source
Filter
Current limiter
External circuit
internal circuit
CP1
RAD
Channel
selection
VA