Datasheet

27
LTC2421/LTC2422
24212f
APPLICATIO S I FOR ATIO
WUUU
re sistance R
P1
in series with the half-bridge sensor are
removed by the FS
SET
input to the ADC. The absolute full-
scale output of the ADC (data out = FFFFF
HEX
) will occur
at V
IN
= V
B
= FS
SET
, see Figure 31. Similarly, the offset
errors due to R
P2
are removed by the ground sense input
ZS
SET
. The absolute zero output of the ADC (data out =
00000
HEX
) occurs at V
IN
= V
A
= ZS
SET
. Parasitic resistors
R
P3
to R
P5
have negligible errors due to the 1nA (typ)
leakage current at pins FS
SET
, ZS
SET
and V
IN
. The wide
dynamic input range (–300mV to 5.3V) and low noise
(1.2ppm RMS) enable the LTC2421 or the LTC2422 to
directly digitize the output of the bridge sensor.
The LTC2422 is ideal for applications requiring continu-
ous monitoring of two input sensors. As shown in
Figure 32, the LTC2422 can monitor both a thermocouple
temperature probe and a cold junction temperature sen-
sor. Absolute temperature measurements can be
performed with a variety of thermocouples using digital
cold junction compensation.
Figure 31. Transfer Curve with Zero-Scale and Full-Scale Set
Figure 30. Half-Bridge Digitizer with
Zero-Scale and Full-Scale Sense
Figure 29. Errors Due to Excitation Currents
V
FULL-SCALE ERROR
SENSOR SENSOR OUTPUT
R
P1
I
EXCITATION
+
–
V
OFFSET ERROR
+
–
+
–
R
P2
24212 F29
V
CC
LTC2421
FS
SET
GND
SCK
V
IN
SDO
F
O
CS
ZS
SET
3-WIRE
SPI INTERFACE
1
9
8
7
10
24212 F03
2
3
5
R
P2
R
P5
I
DC
≅ 0
R
P1
V
B
V
A
6
R
P4
I
DC
≅ 0
I
EXCITATION
R
P3
I
DC
≅ 0
00000
H
12.5%
UNDER
RANGE
ADC DATA OUT
FFFFF
H
ZS
SET
FS
SET
V
IN
24212 F31
12.5%
EXTENDED
RANGE
Figure 32. Isolated Temperature Measurement
V
CC
F
O
FS
SET
ZS
SET
SCK
CH1 SDO
GND
CS
THERMOCOUPLE
COLD JUNCTION
ISOLATION
BARRIER
PROCESSOR
CH0
–+
110
12k
THERMISTOR
100Ω
9
8
7
6
2
3
4
5
2.7V TO 5.5V
LTC2422
24212 F32