AS1543/44
Data Sheet - Detailed Description
Figure 22. Data Conversion
REFIN
CH0
CH1
CH2
CH3
CH4
CH5
CHOLD
13pF
Input
Switch
+
–
–
+
AIN+
AIN-
Control
Logic
CSWITCH
11pF
Sample
Switch
Comparator
CH7
AGND
Analog Input
Multiplexer
CHOLD
13pF
Input
Switch
RIN
–
+
CSWITCH
11pF
S&H and capacitive DAC
AGND
CSWITCH includes all parasitics
Analog Input
Figure 23 shows an equivalent circuit of one analog input. The two diodes, D1 and D2, provide ESD protection for the
analog inputs. Care should be taken to ensure that the analog input signal never exceeds the supply rails by more than
300mV. This will cause these diodes to become forward biased and start conducting current into the substrate. 10mA
is the maximum current these diodes can conduct without causing irreversible damage to the AS1543/44.
Figure 23. Equivalent Analog Input Circuit
C2
20pF
D1
R1
VINX
C1
D2
4pF
Open for Conversion;
Closed for Track
Capacitor C1 in Figure 23 is typically about 4pF and can primarily be attributed to pin capacitance. The resistor R1 is a
lumped component made up of the on-resistance of a switch (track/hold switch) and also includes the on-resistance of
the input multiplexer. The total resistance is typically about 400Ω. Capacitor C2 is the ADC sampling capacitor and typ-
ically has a capacitance of 20pF.
Track/Hold
The Track/Hold stage enters hold mode on the falling edge of CSN. For AC applications, removing high frequency
components from the analog input signal is recommended by use of an R/C low-pass filter on the relevant analog input
pin. In applications where harmonic distortion and signal-to-noise ratio are critical, the analog input should be driven
from a low impedance source. Large source impedances will significantly affect the ac performance of the ADC. This
may necessitate the use of an input buffer amplifier. The choice of the op-amp will be a function of the particular appli-
cation.
When no amplifier is used to drive the analog input, the source impedance should be limited to low values. The maxi-
mum source impedance will depend on the amount of total harmonic distortion that can be tolerated. The THD will
increase as the source impedance increases, and performance will degrade (see Figure 13 on page 10).
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