LTC2431 Linear Technology, LTC2431 Datasheet - Page 35

• Manufacturer Part Number: LTC2431
• Description: (LTC2430 / LTC2431) 20-Bit No Latency Delta-Sigma ADCs
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
single variable element bridges, the nonlinearity of the half
bridge output can be eliminated completely; if the refer-
ence arm of the bridge is used as the reference to the ADC,
as shown in Figure 41. The LTC2430/LTC2431 can accept
inputs up to 1/2 V
must be at least 2 the highest value of the variable
resistor.
In the case of 100 platinum RTD’s, this would suggest a
value of 800
advisable due to self-heating effects. A value of 25.5k is
shown for R1, reducing self-heating effects to acceptable
levels for most sensors.
The basic circuit shown in Figure 41 shows connections
for a full 4-wire connection to the sensor, which may be
Figure 41. Remote Half Bridge Interface
PLATINUM
for R1. Such a low value for R1 is not
100
25.5k
0.1%
RTD
REF
U
R1
1
2
. Hence, the reference resistor R1
PLATINUM
3
4
U
Figure 42. Remote Half Bridge Sensing with Noise Supression on Reference
100
RTD
1
2
REF
REF
IN
IN
2.7V TO 5.5V
3
+
–
4
+
–
W
LTC2430/
LTC2431
GND
V
V
CC
S
2431 F41
10k, 5%
R1
0.1%
10k
R2
U
10k
5%
R3
1 F
located remotely. The differential input connections will
reject induced or coupled 60Hz interference, however, the
reference inputs do not have the same rejection. If 60Hz or
other noise is present on the RTD, a low pass filter is
recommended as shown in Figure 42. Note that you
cannot place a large capacitor directly at the junction of R1
and R2, as it will store charge from the sampling process.
A better approach is to produce a low pass filter decoupled
from the input lines with a high value resistor (R3).
The use of a third resistor in the half bridge, between the
variable and fixed elements gives essentially the same
result as the two resistor version, but has a few benefits.
If, for example, a 25k reference resistor is used to set the
excitation current with a 100
reference input is sampling the same external node as the
positive input, but may result in errors if used with a long
cable. For short cable applications, the errors may be
acceptably low. If instead the single 25k resistor is
replaced with a 10k 5% and a 10k 0.1% reference
resistor, the noise level introduced at the reference, at
least at higher frequencies, will be reduced. A filter can be
introduced into the network, in the form of one or more
capacitors, or ferrite beads, as long as the sampling
pulses are not translated into an error. The reference
voltage is also reduced, but this is not undesirable, as it
will decrease the value of the LSB, although, not the input
referred noise level.
+
–
LTC1050
5V
560
10k
10k
LTC2430/LTC2431
REF
REF
IN
IN
+
–
LTC2430/
LTC2431
+
–
GND
V
5V
CC
2431 F42
RTD, the negative
35
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