Datasheet

TLC542C, TLC542I
8-BIT ANALOG-TO-DIGITAL CONVERTERS
WITH SERIAL CONTROL AND 11 INPUTS
SLAS075C – FEBRUARY 1989 – REVISED JUNE 2001
8
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
simplified analog input analysis
Using the equivalent circuit in Figure 8, the time required to charge the analog input capacitance from 0 to V
S
within 1/2 LSB can be derived as follows:
The capacitance charging voltage is given by
V
C
= V
S
1–e
–t
c
/R
t
C
i
( )
(1
)
where
R
t
= R
s
+ r
i
The final voltage to 1/2 LSB is given by
(2)V
C
(1/2 LSB) = V
S
– (V
S
/512)
Equating equation 1 to equation 2 and solving for time t
c
gives
V
S
–(V
S
/512) = V
S
1–e
( )
(3
)
–t
c
/R
t
C
i
and
t
c
(1/2 LSB) = R
t
× C
i
× ln(512) (4
)
Therefore, with the values given the time for the analog input signal to settle is
(5)
t
c
(1/2 LSB) = (R
s
+ 1 kΩ) × 60 pF × ln(512)
This time must be less than the converter sample time shown in the timing diagrams.
R
s
r
i
V
S
V
C
50 pF MAX
1 kΩ MAX
Driving Source
†
TLC542
C
i
V
I
V
I
= Input Voltage at INPUT A0–A10
V
S
= External Driving Source Voltage
R
s
= Source Resistance
r
i
= Input Resistance
C
i
= Input Capacitance
†
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 8. Equivalent Input Circuit Including the Driving Source