Datasheet

29
LTC2421/LTC2422
24212f
APPLICATIO S I FOR ATIO
WUUU
The absolute accuracy (less than 10 ppm total error) of the
LTC2422 enables extremely accurate measurement of
small signals sitting on large voltages. Each of the two
pseudo differential measurements performed by the
LTC2422 is absolutely accurate independent of the com-
mon mode voltage output from the bridge. The pseudo
differential result obtained from digitally subtracting the
two single ended conversion results is accurate to within
the noise level of the device (3µV
RMS
) times the square
root of 2, independent of the common mode input voltage.
Typically, a bridge sensor outputs 2mV/V full scale. With
a 5V excitation, this translates to a full-scale output of
10mV. Divided by the RMS noise of 8.4µV(= 6µV • 1.414),
this circuit yields 1190 counts with no averaging or ampli-
fication. If more counts are required, several conversions
may be averaged (the number of effective counts is in-
creased by a factor of square root of 2 for each doubling
of averages).
An RTD Temperature Digitizer
RTDs used in remote temperature measurements often
have long lead lengths between the ADC and RTD sensor.
These long lead lengths lead to voltage drops due to exci-
tation current in the interconnect to the RTD. This voltage
drop can be measured and digitally removed using the
LTC2422 (see Figure 35).
The excitation current (typically 200µA) flows from the
ADC through a long lead length to the remote temperature
sensor (RTD). This current is applied to the RTD, whose
resistance changes as a function of temperature (100Ω to
400Ω for 0°C to 800°C). The same excitation current flows
back to the ADC ground and generates another voltage
drop across the return leads. In order to get an accurate
measurement of the temperature, these voltage drops must
be measured and removed from the conversion result.
Assuming the resistance is approximately the same for the
forward and return paths (R1 = R2), the auxiliary channel
on the LTC2422 can measure this drop. These errors are
then removed with simple digital correction.
The result of the first conversion on CH0 corresponds to an
input voltage of V
RTD
+ R1 • I
EXCITATION.
The result of the
second conversion (CH1) is –R1 • I
EXCITATION.
Note, the
LTC2422’s input range is not limited to the supply rails, it
has underrange capabilities. The device’s input range is
–300mV to V
REF
+ 300mV. Adding the two conversion
results together, the voltage drop across the RTD’s leads
are cancelled and the final result is V
RTD
.
An Isolated, 20-Bit Data Acquisition System
The LTC1535 is useful for signal isolation. Figure 36 shows
a fully isolated, 20-bit differential input A/D converter imple-
mented with the LTC1535 and LTC2422. Power on the
isolated side is regulated by an LT1761-5.0 low noise, low
dropout micropower regulator. Its output is suitable for
driving bridge circuits and for ratiometric applications.
During power-up, the LTC2422 becomes active at V
CC
=
2.3V, while the isolated side of the LTC1535 must wait for
V
CC2
to reach its undervoltage lockout threshold of 4.2V.
Figure 35. RTD Remote Temperature Measurement
V
CC
LTC2422
FS
SET
ZS
SET
SCK
CH0
SDO
F
O
CS
CH1
GND
3-WIRE
SPI INTERFACE
1
5V
9
8
7
10
6
24212 F35
2
4
3
+
–
V
RTD
P
t
100Ω
5
I
DC
= 0
I
EXCITATION
= 200µA
I
EXCITATION
= 200µA
R2
R1
5k
25Ω
1000pF
5k25Ω
0.1µF