
Analog / digital converter
UM0404
DocID13284 Rev 2
19.5.4
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:
•
Analog Signal Source Bandwidth (f
0
): 10 kHz
•
Conversion Rate (f
C
): 25 kHz
•
Sampling Time (T
S
): 1µs
•
Pin Input Capacitance (C
P1
): 5pF
•
Pin Input Routing Capacitance (C
P2
): 1pF
•
Sampling Capacitance (C
S
): 4pF
•
Maximum Input Current Injection (I
INJ
): 3mA
•
Maximum Analog Source Voltage (V
AM)
: 12V
•
Analog Source Impedance (R
S
): 100
Ω
•
Channel Switch Resistance (R
SW
): 500
Ω
•
Sampling Switch Resistance (R
AD
): 200
Ω
1.
Supposing to design the filter with the pole exactly at the maximum frequency of the
signal, the time constant of the filter is:
2.
Using the relation between C
F
and C
S
and taking some margin (4000 instead of 2048),
it is possible to define C
F
:
3.
As a consequence of step 1 and 2, RC can be chosen:
4.
Considering the current injection limitation and supposing that the source can go up to
12V, the total series resistance can be defined as:
5.
from which is now simple to define the value of R
L
:
6.
Now the three element of the external circuit R
F
, C
F
and R
L
are defined. Some
conditions discussed in the previous paragraphs have been used to size the
component, the other must now be verified. The relation which allow to minimize the
R
C
C
F
1
2
π
f
0
-------------
15.9
μ
s
=
=
C
F
4000 C
S
⋅
16nF
=
=
R
F
1
2
π
f
0
C
F
---------------------
995
Ω
1k
Ω
≅
=
=
R
S
R
F
R
L
V
AM
I
INJ
-------------
4k
Ω
=
=
+
+
R
L
V
AM
I
INJ
-------------
R
F
R
S
2.9k
Ω
=
–
–
=