LTC2485 LINER [Linear Technology], LTC2485 Datasheet - Page 21

• Manufacturer Part Number: LTC2485
• Description: 24-Bit ?? ADC with Easy Drive Input Current Cancellation and I2C Interface
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Reference Voltage Range
The LTC2485 external reference voltage range is 0.1V to
V
thermal noise of the front-end circuits, and as such, its
value in nanovolts is nearly constant with reference volt-
age. A reduced reference voltage will improve the con-
verter performance when operated with an external con-
version clock (external F
output data rates (see the Output Data Rate section). V
must be ≥1.1V to use the internal temperature sensor.
The reference input is differential. The differential refer-
ence input range (V
the common mode reference input range is 0V to V
Input Voltage Range
The analog input is truly differential with an absolute/
common mode range for the IN
extending from GND – 0.3V to V
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
differential input signal, V
where FS = 0.5 • V
indicates the overrange or the underrange condition using
CC
SWITCHING FREQUENCY
f
f
SW
SW
V
V
I
V
V
I
REF
REF
I
I
REF
REF
. The converter output noise is determined by the
IN
IN
IN
IN
= 123kHz INTERNAL OSCILLATOR
= 0.4 • f
+
+
–
–
+
+
–
–
V
V
EOSC
CC
CC
V
V
EXTERNAL OSCILLATOR
I
I
I
I
LEAK
LEAK
LEAK
LEAK
CC
CC
I
I
I
I
LEAK
LEAK
LEAK
LEAK
REF
U
REF
R
R
R
R
= REF
SW
SW
SW
SW
. Beyond this range, the converter
10k
10k
10k
10k
(TYP)
(TYP)
(TYP)
(TYP)
O
U
IN
signal) at substantially higher
+
= IN
– REF
+
2485 F12
– IN
+
W
–
) is 100mV to V
CC
and IN
Figure 12. LTC2485 Equivalent Analog Input Circuit
–
, from – FS to +FS
C
12pF
(TYP)
+ 0.3V. Outside
EQ
–
I IN
I REF
R
R
R
D IS THE DENSITY OF A DIGITAL TRANSITION AT THE MODULATOR OUTPUT
WHERE REF
where
V
V
V
( )
( )
REFCM
IN
INCM
EQ
EQ
EQ
T
U
input pins
+
=
=
=
=
+
IN
AVG
:
=
2 71
2.98M
(
0.833 10
AVG
.
=
+
CC
−
IN
=
M
REF
CC
=
IN
I IN
+
–
Ω
Ω
( )
and
1 5
−
IS INTERNALLY TIED TO GND
+
2
REF
.
+
.
INTERNAL OSCILLATOR
INTERNAL OSCILLATOR 50Hz AND 60Hz MODE
IN
–
+
2
12
−
AVG
V
REF
)
REF
/ f
=
–
EOSC
0 5
−
.
V
V ,
IN CM
V
distinct output codes. Since the differential input current
cancellation does not rely on an on-chip buffer, current
cancellation and DC performance is maintained rail-to-rail.
Input signals applied to IN
300mV below ground and above V
fault current, resistors of up to 5k may be added in series
with the IN
mance 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 sec-
tions. In addition, series resistors will introduce a tem-
perature 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
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 capacitors.
Depending upon the relation between the differential input
voltage and the differential reference voltage, these capaci-
tors are switching between these four pins transferring
small amounts of charge in the process. A simplified equiva-
lent circuit is shown in Figure 12.
INCM
(
EXTERNAL OSCILLATOR
R
REF
0 5
EQ
.
)
+
=
−
R
V
V
REF
REF CM
REFCM
EQ
(
+
−
60
)
REF
−
Hz MODE
V
+
REF
–
and IN
V
IN
R
2
EQ
−
0 5
.
–
pins without affecting the perfor-
V
R
REF
EQ
D
+
T
and IN
≅
1 5
.
REF
V
REF
CC
+
–
= 5V. This error has a
(
. In order to limit any
0 5
V
pins may extend by
.
REF CM
(
R
EQ
LTC2485
)
–
V
IN CM
(
)
)
–
21
V
REF
V
IN
2485fa
2
R
EQ