LTC2414 LINER [Linear Technology], LTC2414 Datasheet - Page 36

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

LTC2414/LTC2418
APPLICATIO S I FOR ATIO
BRIDGE APPLICATIONS
Typical strain gauge based bridges deliver only 2mV/Volt
of excitation. As the maximum reference voltage of the
LTC2414/LTC2418 is 5V, remote sensing of applied exci-
tation without additional circuitry requires that excitation
be limited to 5V. This gives only 10mV full scale input
signal, which can be resolved to 1 part in 10000 without
averaging. For many solid state sensors, this is still better
than the sensor. Averaging 64 samples however reduces
the noise level by a factor of eight, bringing the resolving
power to 1 part in 80000, comparable to better weighing
systems. Hysteresis and creep effects in the load cells are
typically much greater than this. Most applications that
require strain measurements to this level of accuracy are
measuring slowly changing phenomena, hence the time
required to average a large number of readings is usually
not an issue. For those systems that require accurate
measurement of a small incremental change on a signifi-
cant tare weight, the lack of history effects in the LTC2400
family is of great benefit.
For those applications that cannot be fulfilled by the
LTC2414/LTC2418 alone, compensating for error in exter-
nal amplification can be done effectively due to the “no
latency” feature of the LTC2414/LTC2418. No latency
operation allows samples of the amplifier offset and gain
to be interleaved with weighing measurements. The use of
correlated double sampling allows suppression of 1/f
noise, offset and thermocouple effects within the bridge.
Correlated double sampling involves alternating the polar-
ity of excitation and dealing with the reversal of input
polarity mathematically. Alternatively, bridge excitation
can be increased to as much as ±10V, if one of several
precision attenuation techniques is used to produce a
precision divide operation on the reference signal. An-
other option is the use of a reference within the 5V input
range of the LTC2414/LTC2418 and developing excitation
via fixed gain, or LTC1043 based voltage multiplication,
along with remote feedback in the excitation amplifiers, as
shown in Figures 46 and 47.
Figure 41 shows an example of a simple bridge connec-
tion. Note that it is suitable for any bridge application
where measurement speed is not of the utmost impor-
tance. For many applications where large vessels are
36
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BRIDGE
weighed, the average weight over an extended period of
time is of concern and short term weight is not readily
determined due to movement of contents, or mechanical
resonance. Often, large weighing applications involve load
cells located at each load bearing point, the output of
which can be summed passively prior to the signal pro-
cessing circuitry, actively with amplification prior to the
ADC, or can be digitized via multiple ADC channels and
summed mathematically. The mathematical summation
of the output of multiple LTC2414/LTC2418’s provides the
benefit of a root square reduction in noise. The low power
consumption of the LTC2414/LTC2418 makes it attractive
for multidrop communication schemes where the ADC is
located within the load-cell housing.
R1 AND R2 CAN BE USED TO INCREASE TOLERABLE AC COMPONENT ON REF SIGNALS
A direct connection to a load cell is perhaps best incorpo-
rated into the load-cell body, as minimizing the distance to
the sensor largely eliminates the need for protection
devices, RFI suppression and wiring. The LTC2414/
LTC2418 exhibits extremely low temperature dependent
drift. As a result, exposure to external ambient tempera-
ture ranges does not compromise performance. The in-
corporation of any amplification considerably compli-
cates thermal stability, as input offset voltages and cur-
rents, temperature coefficient of gain settling resistors all
become factors.
350Ω
R1
R2
Figure 41. Simple Bridge Connection
0.1µF
11
12
21
22
REF
REF
CH0
CH1
LTC2414/
LTC2418
+
–
GND
V
CC
9
15
SDO
SCK
SDI
CS
F
O
10µF
20
18
17
16
19
+
LT1019
2414/18 F41
241418fa
0.1µF