TLC548C datasheet(7/12 Pages) TI

7 / 12 page

TLC548C, TLC548I, TLC549C, TLC549I

8-BIT ANALOG-TO-DIGITAL CONVERTERS

WITH SERIAL CONTROL

SLAS067C – NOVEMBER 1983 – REVISED SEPTEMBER 1996

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATIONS INFORMATION

simplified analog input analysis

Using the equivalent circuit in Figure 2, the time required to charge the analog input capacitance from 0 to VS

within 1/2 LSB can be derived as follows:

The capacitance charging voltage is given by

VC = VS 1–e

–tc/RtCi

(

)

(1)

where

Rt = Rs + ri

The final voltage to 1/2 LSB is given by

(2)

VC (1/2 LSB) = VS – (VS/512)

Equating equation 1 to equation 2 and solving for time tc gives

VS –(VS/512) = VS 1–e

(3)

–tc/RtCi

(

)

and

tc (1/2 LSB) = Rt × Ci × ln(512)

(4)

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

tc (1/2 LSB) = (Rs + 1 kΩ) × 60 pF × ln(512)

(5)

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

1 k

Ω MAX

Driving Source†

TLC548/9

VI = Input Voltage at ANALOG IN

VS = External Driving Source Voltage

Rs = Source Resistance

ri = Input Resistance

Ci = Input Capacitance

† 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.

55 pF MAX

Figure 2. Equivalent Input Circuit Including the Driving Source

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