LTC2414 LINER [Linear Technology], LTC2414 Datasheet - Page 39

• Manufacturer Part Number: LTC2414
• Description: 8-/16-Channel 24-Bit No Latency TM ADCs
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
The basic circuit shown in Figure 44 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 45. 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 and 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
U
PLATINUM
100Ω
Figure 45. Remote Half Bridge Sensing with Noise Suppression on Reference
U
RTD
W
10k, 5%
R1
0.1%
10k
R2
U
10k
5%
R3
1µF
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 45 shows a more rigorous
example of Figure 44, with increased noise suppression
and more protection for remote applications.
Figure 46 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 introduce 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 alterna-
tives, 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
be a source of error. The fact that input offset voltage has
relatively little effect on overall error may lead one to use
low performance amplifiers for this application. Note that
the gain of a device such as an LF156, (25V/mV over
temperature) will produce a worst-case error of –180ppm
at a noise gain of 3, such as would be encountered in an
inverting gain of 2, to produce –10V from a 5V reference.
–
+
LTC1050
5V
560Ω
10k
10k
LTC2414/LTC2418
11
12
21
22
REF
REF
CH0
CH1
LTC2414/
LTC2418
+
–
GND
V
5V
CC
9
15
2410 F51
39
241418fa