Datasheet

LTC2413
38
sn2413 2413fs
APPLICATIO S I FOR ATIO
WUU
U
Figure 44. Using Autozero Amplifiers to Reduce Input Referred Noise
0.1µF
8
0.1µF
0.1µF
REF
+
REF
–
SDO
SCK
IN
+
IN
–
CS
GND
V
CC
F
O
312
5V
REF
4
350Ω
BRIDGE
13
5
6
2413 F44
11
1, 7, 8, 9,
10, 15, 16
2
14
LTC2413
RN1 = 5k × 8 RESISTOR ARRAY
U1A, U1B, U2A, U2B = 1/2 LTC1051
–
+
3
2
8
4
U1A
4
5V
+
–
6
5
RN1
1
16
15
2
611
7
1
14
3
710
4
13
89
512
U1B
+
–
2
3
U2A
5V
1
+
–
6
5
U2B
7
The basic circuit shown in Figure 46 shows connections
for a full 4-wire connection to the sensor, which may be
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 reference input, a low pass
filter is recommended as shown in Figure 47. 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 100Ω RTD, the negative refer-
ence 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 re-
placed 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 intro-
duced 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.
The circuit shown in Figure 47 shows a more rigorous
example of Figure 46, with increased noise suppression
and more protection for remote applications.
Figure 48 shows an example of gain in the excitation circuit
and remote feedback from the bridge. The LTC1043’s
provide voltage multiplication, providing ±10V from a 5V
reference with only 1ppm error. The amplifiers are used at
unity-gain and, hence, introduce a very little error due to
gain error or due to offset voltages. A 1µV/°C offset voltage
drift translates into 0.05ppm/°C gain error. Simpler alter-
natives, with the amplifiers providing gain using resistor
arrays for feedback, can produce results that are similar to
bridge sensing schemes via attenuators. Note that the
amplifiers must have high open-loop gain or gain error will