LTC2444/LTC2445/
LTC2448/LTC2449
10
2444589fc
For more information www.linear.com/LTC2444
applicaTions inForMaTion
clears all internal registers. The conversion immediately
following a POR is performed on the input channel IN
+
= CH0, IN
–
= CH1 at an OSR = 256 in the 1X mode. Fol-
lowing the POR signal, the LTC2444/LTC2445/LTC2448/
LTC2449 start a normal conversion cycle and follow the
succession of states described above. The first conversion
result following POR is accurate within
the specifications
of the device if the power supply voltage is restored within
the operating range (4.5V to 5.5V) before the end of the
POR time interval.
Reference Voltage Range
These converters accept a truly differential external
reference voltage. The absolute/common mode voltage
specification for the REF
+
and REF
–
pins covers the entire
range from GND to V
CC
. For correct converter operation, the
REF
+
pin must always be more positive than the REF
–
pin.
The LTC2444/LTC2445/LTC2448/LTC2449 can accept a
differential reference voltage from 0.1V to V
CC
. The con-
verter output noise is determined by the thermal noise of
the front-end circuits, and as such, its value in microvolts
is nearly constant with reference voltage. A decrease in
reference voltage will not significantly improve the con
-
verter’s effective resolution. On the other hand, a reduced
reference voltage will improve the converter’s overall INL
per
formance.
Input V
oltage Range
Refer to Figure 4. The analog input is truly differential with an
absolute/common mode range for the CH0 to CH15 and COM
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 rap-
idly. Within these limits, the LTC2444/LTC2445/LTC2448/
LTC2449 convert the bipolar differential input signal, V
IN
=
IN
+
– IN
–
(where IN
+
and IN
–
are the selected input chan-
nels),
from –FS = –0.5 • V
REF
to +FS = 0.5 • V
REF
where
V
REF
= REF
+
– REF
–
. Outside this range, the converter
indicates the overrange or the underrange condition using
distinct output codes.
MUXOUT/ADCIN
There are two differences between the LTC2444/LTC2448
and the LTC2445/LTC2449. The first is the RMS noise
performance. For a given OSR, the LTC2445/LTC2449
noise level is approximately √2 times lower (0.5 effective
bits)than that of the LTC2444/LTC2448.
The second difference is the LTC2445/LTC2449 includes
MUXOUT/ADCIN pins. These pins enable an external buf
-
fer or gain block to be inserted between the output of the
multiplexer and the input to the ADC. Since the buffer is
driven by the output of the multiplexer
, only one circuit is
Figure 3. SDI Speed/Resolution, Channel Selection, and Data Output Timing
MSB
BIT 28 BIT 27 BIT 26 BIT 25 BIT 24 BIT 23 BIT 22 BIT 21 BIT 20 BIT 19 BIT 0
LSB
SIG
BIT 29
“0”
BIT 30
EOC
Hi-Z
CS
SCK
SDI
SDO
BIT 31
1 0 EN SGL A2 A1 A0 OSR3 OSR2 OSR1 OSR0 TWOXODD
1 2 3 4 5 6 7 8 9 10 11 12 13 14 32