Datasheet

LTC2483
17
2483fc
the thermal noise of the front-end circuits, and as such,
its value in nanovolts is nearly constant with reference
voltage. Since the transition noise (600nV) is much less
than the quantization noise (V
REF
/2
17
), a decrease in the
reference voltage will increase the converter resolution. A
reduced reference voltage will also improve the converter
performance when operated with an external conversion
clock (external F
O
signal) at substantially higher output
data rates (see the Output Data Rate section).
The reference input is differential. The differential reference
input range (V
REF
= REF
+
– REF
–
) is 100mV to V
CC
and the
common mode reference input range is 0V to V
CC
.
Input Voltage Range
The analog input is truly differential with an absolute/
common mode range for the IN
+
and IN
–
input pins
extending from GND – 0.3V to V
CC
+ 0.3V. Outside
these limits, the ESD protection devices begin to turn on
and the errors due to input leakage current increase rapidly.
Within these limits, the LTC2483 converts the bipolar differ-
ential input signal, V
IN
= IN
+
– IN
–
, from –FS to +FS where
FS = 0.5 • V
REF
. Beyond this range, the converter indicates
the overrange or the underrange condition using distinct
output codes. Since the differential input current cancella-
tion does not rely on an on-chip buffer, current cancellation
and DC performance is maintained rail-to-rail.
applicaTions inFormaTion
I nput signals applied to IN
+
and IN
–
pins may extend by
300mV below ground and above V
CC
. In order to limit any
fault current, resistors of up to 5k may be added in series
with the IN
+
and IN
–
pins without affecting the performance
of the devices. The effect of the series resistance on the
converter accuracy can be evaluated from the curves
presented in the Input Current/Reference Current sections.
In addition, series resistors will introduce a temperature
dependent offset error due to the input leakage current.
A 1nA input leakage current will develop a 1ppm offset
error on a 5k resistor if V
REF
= 5V. This error has a very
strong temperature dependency.
Driving the Input and Reference
The input and reference pins of the LTC2483 converter
are directly connected to a network of sampling capaci-
tors. Depending upon the relation between the differential
input voltage and the differential reference voltage, these
capacitors are switching between these four pins transfer-
ring small amounts of charge in the process. A simplified
equivalent circuit is shown in Figure 7.
For a simple approximation, the source impedance R
S
driving an analog input pin (IN
+
, IN
–
, REF
+
or REF
–
) can
be considered to form, together with R
SW
and C
EQ
(see
Figure 7), a first order passive network with a time constant
τ = (R
S
+ R
SW
) • C
EQ
. The converter is able to sample the
V
REF
+
V
IN
+
V
CC
R
SW
(TYP)
10k
I
LEAK
I
LEAK
V
CC
I
LEAK
I
LEAK
V
CC
R
SW
(TYP)
10k
C
EQ
12pF
(TYP)
R
SW
(TYP)
10k
I
LEAK
I
IN
+
V
IN
–
I
IN
–
I
REF
+
I
REF
–
2483 F07
I
LEAK
V
CC
I
LEAK
I
LEAK
SWITCHING FREQUENCY
f
SW
= 123kHz INTERNAL OSCILLATOR
f
SW
= 0.4 • f
EOSC
EXTERNAL OSCILLATOR
V
REF
–
R
SW
(TYP)
10k
Figure 7. LTC2483 Equivalent Analog Input Circuit