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Simplified Analog Input Analysis

V

C + VS 1 * e

*tc RtCi

(1)

VC (1/2 LSB) = VS − (VS/8192)

(2)

V

S * VS 8192 + VS 1 * e

*tc RtCi

(3)

tc (1/2 LSB) = Rt × Ci × ln(8192)

(4)

tc (1/2 LSB) = (Rs + 1 kΩ) × 60 pF × ln(8192)

(5)

Rs

ri

VS

VC

60 pF Max

1 k

Ω Max

Driving Source(A)

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

TLC2543-EP

12-BIT ANALOG-TO-DIGITAL CONVERTER

WITH SERIAL CONTROL AND 11 ANALOG INPUTS

SGLS125A – JULY 2002 – REVISED NOVEMBER 2006

APPLICATION INFORMATION (continued)

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

within one-half LSB can be derived as follows:

The capacitance charging voltage is given by:

Where:

R

t = Rs + ri

The final voltage to 1/2 LSB is given by:

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

and

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

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

A.

Driving source requirements:

• Noise and distortion for the source must be equivalent to the resolution of the converter.

• R

s must be real at the input frequency.

Figure 18. Equivalent Input Circuit Including the Driving Source

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