Datasheet

LTC2485
21
2485fc
APPLICATIONS INFORMATION
Reference Voltage Range
The LTC2485 external reference voltage range is 0.1V
to V
CC
. The converter output noise is determined by the
thermal noise of the front-end circuits, and as such, its
value in nanovolts is nearly constant with reference voltage.
A reduced reference voltage will 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). V
REF
must
be ≥1.1V to use the internal temperature sensor.
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 LTC2485 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.
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 LTC2485 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 simplifi ed
equivalent circuit is shown in Figure 12.
Figure 12. LTC2485 Equivalent Analog Input Circuit
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
–
2485 F12
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
IIN IIN
VV
R
I REF
VV V
R
V
VR
VD
R
VV V
R
V
VR
where
AVG AVG
IN CM REF CM
EQ
AVG
REF INCM REFCM
EQ
IN
REF EQ
REF T
EQ
REF REF CM IN CM
EQ
IN
REF EQ


REF

REF
–
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v

v



–
() ()
() ()
.
.
.
.• •
.–
.•
–
•
05
15
05
05
15
05
2
2
:
.
V
VININ
V
IN IN
R M INTERNAL OSCILLATOR Hz MODE
REFCM
IN
INCM
EQ
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


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§
¦
´
¶
µ


v



V,
REF

REF

REF
–
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2
2
271 60
R 2.98M INTERNAL OSCILLATOR 50Hz AND 60Hz MODE
R 0.833 10 / f EXTERNAL OSCILLATOR
D IS THE DENSITY OF A DIGITAL TRANSITION AT THE MODULATOR OUTPUT
EQ
EQ
12
EOSC
T
WHERE REF
–
IS INTERNALLY TIED TO GND