TLC2543C, TLC2543I, TLC2543M

12-BIT ANALOG-TO-DIGITAL CONVERTERS

WITH SERIAL CONTROL AND 11 ANALOG INPUTS

SLAS079D – DECEMBER 1993 – REVISED MAY 1997

21

POST OFFICE BOX 655303

APPLICATION INFORMATION

simplified analog input analysis

Using the equivalent circuit in Figure 18, the time required to charge the analog input capacitance from 0 V to

The capacitance charging voltage is given by

(1)

where

V

C +

V

S

1–e

–tc RtCi

The final voltage to 1/2 LSB is given by

(2)

(3)

and

(4)

V

S *

V

S

8192

+ V

S

1–e

–tc RtCi

Therefore, with the values given, the time for the analog input signal to settle is

(5)

This time must be less than the converter sample time shown in the timing diagrams.

Rs

ri

VS

VC

60 pF Max

1 k

Ω Max

Driving Source†

TLC2543

Ci

VI

VI = Input Voltage at AIN

VS = External Driving Source Voltage

Rs = Source Resistance

ri = Input Resistance

Ci = Input Capacitance

VC= Capacitance Charging Voltage

† Driving source requirements:

• Noise and distortion for the source must be equivalent to the

resolution of the converter.

• Rs must be real at the input frequency.

Figure 18. Equivalent Input Circuit Including the Driving Source