ꢇ ꢍꢚ ꢛꢀ ꢀꢁ
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SBAS286A − JUNE 2003 − REVISED MARCH 2004
a differential in/differential out amplifier with a gain of
VESSEL WEIGHING WITH FOUR LOAD
CELLS
499V/V (G = 1 + 2R /R ). Gain on the load cell gives
F
G
the amplifier a full-scale output of 5V.
In vessel weighing systems, four load cells are
frequently employed to measure the weight of the
vessel and its contents. The output of the load cells are
usually combined in an external summing junction box
that balances the load cells’ sensitivities for accuracy.
Each load cell input uses an external amplifier. The
outputs of the amplifiers connect to the analog inputs of
the ADS1224 through a low-pass filter. The cut-off
frequency is set to 360Hz, allowing full settling in a
single measurement cycle. A lower cut-off frequency
can be used to reduce noise from mechanical
vibrations, but at the expense of filter settling time.
The four differential inputs of the ADS1224 allow for
direct measurement of the four load cells individually. In
this way, the mechanical adjustments performed inside
the summing junction box are eliminated and are
replaced by digital summing of the load cells in
software. Figure 32 shows an example of such a
system.
The internal buffer of the ADS1224 is disabled, allowing
the VREFN pin to be grounded. Note that the loading
from the reference inputs will change the effective
reference voltage. The effective input impedance into
the VREFP and VREFN pins will lower the reference
voltage seen at these pins. At 2MHz, input impedance
is approximately 500kΩ. For the reference circuit shown
in Figure 32, this lowers the effective reference voltage
by approximately 0.1%.
The reference voltage of the ADS1224 is derived by
dividing down the AVDD supply voltage to 2.5V, while
the load cell has a positive full-scale output of 10mV. In
the figure, a low drift, dual op amp (OPA2335) provides
5V
+3V
µ
µ
µ
µ
0.1 F
0.1
F
0.1
F
1
F
RFI
Filter
(1)
Ω
1k
1k
AVDD
DVDD
DVCC
AVCC
VREFP
BUFEN
(1)
Ω
µ
0.1
F
5V
Ω
350
Shielded
Cable
TEMPEN
ADS1224
VREFN
Load Cell
1/2
Ω
100
2R
MSP430F41x
RFI
Filter
Ω
OPA2335
1k
1k
AINP1
AINN1
µ
0.22
F
Ω
(1)
R
F
F
G = 1 +
Ω
Ω
2.49k
R
G
AINP2
AINN2
AINP3
AINN3
AINP4
AINN4
CLK
P1.1/TA0/MCLK
P1.2/TA1
(1)
R
RFI
Filter
(1)
G
R
F
Ω
XIN
10
SCLK
2.49k
NOTE: (1) Low−drift resistors.
DRDY/DOUT
MUX0
P1.0/TA0
P1.6/CA0
P1.7/CA1
XOUT/TCLK
Ω
100
MUX1
RFI
Filter
1/2
GND
GND
OPA2335
Replicate for Channels 2, 3 and 4
Figure 32. Vessel Weighing System with Four Load Cells
19
BB [ BURR-BROWN CORPORATION ]
