LTC2431 Linear Technology, LTC2431 Datasheet - Page 32

• Manufacturer Part Number: LTC2431
• Description: (LTC2430 / LTC2431) 20-Bit No Latency Delta-Sigma ADCs
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC2430/LTC2431
BRIDGE APPLICATIONS
Typical strain gauge based bridges deliver only 2mV/Volt
of excitation. As the maximum reference voltage of the
LTC2430/LTC2431 is 5V, remote sensing of applied exci-
tation without additional circuitry requires that excitation
be limited to 5V. This gives only 10mV full scale, which can
be resolved to 1 part in 3500 without averaging. For many
solid state sensors, this is comparable to the sensor. Av-
eraging 128 samples however reduces the noise level by
a factor of eight, bringing the resolving power to 1 part in
40000, comparable to better weighing systems. Hysteresis
and creep effects in the load cells are typically much greater
than this. Most applications that require strain measure-
ments to this level of accuracy are measuring slowly chang-
ing phenomena, hence the time required to average a large
number of readings is usually not an issue. For those sys-
tems that require accurate measurement of a small incre-
mental change on a significant tare weight, the lack of history
effects in the LTC2400 family is of great benefit.
For those applications that cannot be fulfilled by the
LTC2430/LTC2431 alone, compensating for error in exter-
nal amplification can be done effectively due to the “no
latency” feature of the LTC2430/LTC2431. 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
32
–100
–120
– 60
–20
–40
–80
Figure 36. Measured Input Normal Mode Rejection
vs Input Frequency
0
0
15 30 45 60 75 90 105 120 135 150 165 180 195 210 225 240
U
INPUT FREQUENCY (Hz)
U
V
V
(150% OF FULL SCALE)
IN(P-P)
IN(P-P)
= 5V
= 7.5V
W
V
V
V
F
T
O
A
CC
REF
INCM
U
= GND
= 25 C
= 5V
= 5V
= 2.5V
2431 F36
BRIDGE
the polarity of excitation and dealing with the reversal of
input polarity mathematically. Alternatively, bridge excita-
tion 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 LTC2430/LTC2431 and developing excitation
via fixed gain, or LTC1043 based voltage multiplication,
along with remote feedback in the excitation amplifiers, as
shown in Figures 43 and 45.
Figure 38 shows an example of a simple bridge connec-
tion. Note that it is suitable for any bridge application
R1 AND R2 CAN BE USED TO INCREASE TOLERABLE AC COMPONENT ON REF SIGNALS
350
–100
–120
– 60
–20
–40
–80
Figure 37. Measured Input Normal Mode Rejection
vs Input Frequency
0
0
R1
R2
Figure 38. Simple Bridge Connection
25
0.1 F
50
REF
REF
IN
IN
+
–
LTC2430/
LTC2431
INPUT FREQUENCY (Hz)
+
–
GND
75
V
CC
V
V
(150% OF FULL SCALE)
IN(P-P)
IN(P-P)
SDO
SCK
CS
F
100
O
10 F
= 5V
= 7.5V
+
125
2431 F38
LT1019
150
V
V
V
F
T
O
A
CC
REF
INCM
= 5V
= 25 C
175
= 5V
= 5V
= 2.5V
2431 F37
200
24301f
0.1 F