Datasheet
LTC2439-1
21
24391fa
Figure 12), a first order passive network with a time
constant τ = (R
S
+ R
SW
) • C
EQ
. The converter is able to
sample the input signal with better than 1LSB accuracy if
the sampling period is at least 11 times greater than the
input circuit time constant τ. The sampling process on the
four input analog pins is quasi-independent so each time
constant should be considered by itself and, under worst-
case circumstances, the errors may add.
When using the internal oscillator (F
O
= LOW), the
LTC2439-1’s front-end switched-capacitor network is
clocked at 69900Hz corresponding to a 14.3µs sampling
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
APPLICATIO S I FOR ATIO
WUUU
frequency 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).
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
18pF
(TYP)
R
SW
(TYP)
20k
I
LEAK
I
IN
+
V
IN
–
I
IN
–
I
REF
+
I
REF
–
24391 F11
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
Figure 12. LTC2439-1 Equivalent Analog Input Circuit
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
397 50 60
0 555 10
12
Ω