LTC2431_15 Datasheet, PDF (33/40 Pages) Linear Technology – 20-Bit No Latency ADCs with Differential Input and Differential Reference

LTC2430/LTC2431
APPLICATIO S I FOR ATIO
where measurement speed is not of the utmost impor-
tance. For many applications where large vessels are
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 LTC2430/LTC2431’s provide the
benefit of a root square reduction in noise. The low power
consumption of the LTC2430/LTC2431 make it attractive
for multidrop communication schemes where the ADC is
located within the load-cell housing.
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 LTC2430/
LTC2431 exhibit 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 complicates
thermal stability, as input offset voltages and currents,
temperature coefficient of gain settling resistors all be-
come factors.
The circuit in Figure 39 shows an example of a simple
amplification scheme. This example produces a differen-
tial output with a common mode voltage of 2.5V, as
determined by the bridge. The use of a true three amplifier
instrumentation amplifier is not necessary, as the LTC2430/
LTC2431 have common mode rejection far beyond that of
most amplifiers. The LTC1051 is a dual autozero amplifier
that can be used to produce a gain of 10 before its input
referred noise dominates the LTC2430/LTC2431 noise.
This example shows a gain of 34, that is determined by a
feedback network built using a resistor array containing
eight individual resistors. The resistors are organized to
optimize temperature tracking in the presence of thermal
gradients. The second LTC1051 buffers the low noise
input stage from the transient load steps produced during
conversion.
The gain stability and accuracy of this approach is very
good, due to a statistical improvement in resistor match-
ing due to individual error contribution being reduced. A
gain of 34 may seem low, when compared to common
5V_REF
350Ω
BRIDGE
5V
3₊ 8
U1A
2_–
4
0.1µF
1
15
14
4 5 12
1
RN1
16 ^{6 11 7 10 8 9}
2
3
13
6_–
7
U1B
5₊
0.1µF
5V
2_– 8
1
U2A
3₊
4
6_–
7
U2B
5₊
0.1µF
V_CC
REF⁺ SDO
REF^–
SCK
IN⁺
CS
LTC2430/
LTC2431
IN^–
GND
F_O
RN1 = 5k × 8 RESISTOR ARRAY
U1A, U1B, U2A, U2B = 1/2 LTC1051
Figure 39. Using Autozero Amplifiers to Reduce Input Referred Noise
2431 F39
24301f
33