st10f272z2 STMicroelectronics, st10f272z2 Datasheet - Page 153

• Manufacturer Part Number: st10f272z2
• Description: 16-bit Mcu With 256 Kbyte Flash Memory And 20 Kbyte Ram
• Manufacturer: STMicroelectronics

ST10F272Z2
Figure 45. Anti-aliasing filter and conversion rate
The considerations above lead to impose new constraints to the external circuit, to reduce
the accuracy error due to the voltage drop on C
above, it is simple to derive the following relation between the ideal and real sampled
voltage on C
From this formula, in the worst case (when V
assuming to accept a maximum error of half a count (~2.44mV), it is immediately evident a
constraints on C
In the next section an example of how to design the external network is provided, assuming
some reasonable values for the internal parameters and making hypothesis on the
characteristics of the analog signal to be sampled.
Example of external network sizing
The following hypothesis are formulated in order to proceed in designing the external
network on A/D Converter input pins:
●
●
●
●
●
●
●
●
●
●
●
Anti-Aliasing Filter (f
Analog Source Bandwidth (V
Analog Signal Source Bandwidth (f
conversion Rate (f
Sampling Time (T
Pin Input Capacitance (C
Pin Input Routing Capacitance (C
Sampling Capacitance (C
Maximum Input Current Injection (I
Maximum Analog Source Voltage (V
Analog Source Impedance (R
Channel Switch Resistance (R
Sampling Switch Resistance (R
S
:
f
f
0
F
F
value:
F
= RC Filter pole)
Noise
S
C
):1 µs
):25 kHz
f
f
A
P1
)
S
-----------
V A2
):4 pF
):5 pF
V A
S
SW
):100 Ω
AD
=
Sampled Signal Spectrum (f
):500 Ω
P2
------------------------------------------------------------
C P1
):200 Ω
C F 2048 C S
T
f
2 f
INJ
F
0
C
):1 pF
):10 kHz
AM)
= f
0
C P1
≤ 2 R
):3 mA
>
≤ f
0
+
:12 V
A
(Anti-aliasing Filtering Condition)
C
C P2
f
0
+
is maximum, that is for instance 5 V),
(Nyquist)
F
S
C P2
C
; from the two charge balance equations
⋅
F
+
(Conversion Rate vs. Filter Pole)
C F
+
C F
+
C S
f
C
C
= conversion Rate)
Electrical characteristics
f
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