ST7L09 datasheet(92/104 Pages) STMICROELECTRONICS

92 / 104 page

ST7L05, ST7L09

92/104

ADC CHARACTERISTICS (Cont’d)

Figure 69. RAIN max. vs fADC with CAIN =0pF

Figure 70. Recommended CAIN/RAIN values

Notes:

1.CPARASITIC represents the capacitance of the PCB (dependent on soldering and PCB layout quality) plus the pad ca-

pacitance (3pF). A high CPARASITIC value will downgrade conversion accuracy. To remedy this, fADC must be reduced.

2. This graph shows that depending on the input signal variation (fAIN), CAIN can be increased for stabilization and to allow

the use of a larger serial resistor (RAIN). It is valid for all fADC frequencies ≤ 4 MHz.

13.11.0.1 General PCB Design Guidelines

To obtain best results, some general design and

layout rules must be followed when designing the

application PCB to shield the noise-sensitive, ana-

log

physical

interface

from

noise-generating

CMOS logic signals.

– Properly place components and route the signal

traces on the PCB to shield the analog inputs.

Analog signals paths should run over the analog

ground plane and be as short as possible. Isolate

analog signals from digital signals that may

switch while the analog inputs are being sampled

by the A/D converter. Do not toggle digital out-

puts on the same I/O port as the A/D input being

converted.

010

CPARASITIC (pF)

4 MHz

2 MHz

1 MHz

0.1

100

1000

0.01

0.1

fAIN(KHz)

Cain 10 nF

Cain 22 nF

Cain 47 nF

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