Datasheet

LTC2410
39
APPLICATIO S I FOR ATIO
WUU
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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 51. 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
acceptalby 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 51 shows a more rigorous
example of Figure 50, with increased noise suppression
and more protection for remote applications.
Figure 52 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.
Figure 49. Bridge Amplification Using a Single Amplifier
0.1µF
5V
REF
+
REF
–
IN
+
IN
–
GND
V
CC
3
3
2
4
6
7
4
350Ω
BRIDGE
5
6
2410 F49
1, 7, 8, 9,
10, 15, 16
2
LTC2410
–
+
LTC1050S8
5V
0.1µV
R2
46.4k
20k
20k
175Ω
1µF
10µF
R1
4.99k
()
A
V
= 9.95 =
R1 + R2
R1 + 175Ω
+
+
1µF
+