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AS1543/44

Data Sheet - D e t a i l e d D e s c r i p t i o n

Figure 22. Data Conversion

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

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 400

Ω. 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).

+

–

+

Comparator

11pF

13pF

–

CH0

CH1

CH2

CH3

CH4

CH5

CH7

AGND

REFIN

AIN+

11pF

+

13pF

–

Sample

Switch

Analog Input

Multiplexer

AIN-

Input

Switch

Input

Switch

AGND

S&H and capacitive DAC

Control

Logic

Open for Conversion;

Closed for Track

R1

C1

4pF

C2

20pF

D1

D2