Datasheet
LTC2433-1
20
24331fa
period. Thus, for settling errors of less than 1LSB, the
driving source impedance should be chosen such that τ ≤
14.3µs/11 = 1.3µs. When an external oscillator of fre-
quency f
EOSC
is used, the sampling period is 2/f
EOSC
and,
for a settling error of less than 1LSB, τ ≤ 0.18/f
EOSC
.
Input Current
If complete settling occurs on the input, conversion re-
sults will be unaffected by the dynamic input current. An
incomplete settling of the input signal sampling process
may result in gain and offset errors, but it will not degrade
the INL performance of the converter. Figure 12 shows the
mathematical expressions for the average bias currents
flowing through the IN
+
and IN
–
pins as a result of the
sampling charge transfers when integrated over a sub-
stantial time period (longer than 64 internal clock cycles).
The effect of this input dynamic current can be analyzed
using the test circuit of Figure 13. The C
PAR
capacitor
includes the LTC2433-1 pin capacitance (5pF typical) plus
the capacitance of the test fixture used to obtain the results
shown in Figures 14 and 15. A careful implementation can
bring the total input capacitance (C
IN
+ C
PAR
) closer to 5pF
thus achieving better performance than the one predicted
Figure 13. An RC Network at IN
+
and IN
–
Figure 14. +FS Error vs R
SOURCE
at IN
+
or IN
–
(Small C
IN
)
C
IN
24331 F13
V
INCM
+ 0.5V
IN
R
SOURCE
IN
+
LTC2433-1
C
PAR
≅20pF
C
IN
V
INCM
– 0.5V
IN
R
SOURCE
IN
–
C
PAR
≅20pF
Figure 12. LTC2433-1 Equivalent Analog Input Circuit
V
REF
+
V
IN
+
V
CC
R
SW
(TYP)
20k
I
LEAK
I
LEAK
V
CC
I
LEAK
I
LEAK
V
CC
R
SW
(TYP)
20k
C
EQ
6pF
(TYP)
R
SW
(TYP)
20k
I
LEAK
I
IN
+
V
IN
–
I
IN
–
I
REF
+
I
REF
–
24331 F12
I
LEAK
V
CC
I
LEAK
I
LEAK
SWITCHING FREQUENCY
f
SW
= 69900Hz INTERNAL OSCILLATOR (F
O
= LOW)
f
SW
= 0.5 • f
EOSC
EXTERNAL OSCILLATOR
V
REF
–
R
SW
(TYP)
20k
IIN
VV V
R
IIN
VV V
R
I REF
VV V
R
V
VR
I REF
VV V
R
V
VR
where
AVG
IN INCM REFCM
EQ
AVG
IN INCM REFCM
EQ
AVG
REF INCM REFCM
EQ
IN
REF EQ
AVG
REF INCM REFCM
EQ
IN
REF EQ
+
−
+
−
()
=
+−
•
()
=
−+ −
•
()
=
•− +
•
−
•
()
=
−• − +
•
+
•
05
05
15
05
15
05
2
2
.
.
.
.
.
.
::
./
./
V REF REF
V
REF REF
VININ
V
IN IN
R M INTERNAL OSCILLATOR Hz Hz Notch F LOW
R f EXTERNAL OSCILLATOR
REF
REFCM
IN
INCM
EQ O
EQ EOSC
=−
=
+
=−
=
−
==
()
=•
()
+−
+−
+−
+−
2
2
11 9 50 60
167 10
12
Ω
R
SOURCE
(Ω)
1 10 100 1k 10k 100k
+FS ERROR (LSB)
24331 F14
3
2
1
0
V
CC
= 5V
REF
+
= 5V
REF
–
= GND
IN
+
= 5V
IN
–
= 2.5V
F
O
= GND
T
A
= 25°C
C
IN
=
0.01µF
C
IN
=
0.001µF
C
IN
=
100pF
C
IN
=
0pF
APPLICATIO S I FOR ATIO
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