LMV862MMEVAL National Semiconductor, LMV862MMEVAL Datasheet - Page 16

• Manufacturer Part Number: LMV862MMEVAL
• Manufacturer: National Semiconductor

Specifications of LMV862MMEVAL

• Lead Free Status / Rohs Status: Supplier Unconfirmed

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DECOUPLING AND LAYOUT
Care must be given when creating a board layout for the op
amp. For decoupling the supply lines it is suggested that
10 nF capacitors be placed as close as possible to the op
amp. For single supply, place a capacitor between V
V
the board ground, and a second capacitor between ground
and V
Even with the LMV861 and LMV862 inherent hardening
against EMI, it is still recommended to keep the input traces
short and as far as possible from RF sources. Then the RF
signals entering the chip are as low as possible, and the re-
maining EMI can be, almost, completely eliminated in the chip
by the EMI reducing features of the LMV861 and LMV862.
LOAD CELL SENSOR APPLICATION
The LMV861 and LMV862 can be used for weight measuring
system applications which use a load cell sensor. Examples
of such systems are: bathroom weight scales, industrial
weight scales and weight measurement devices on moving
equipment such as forklift trucks.
The following example describes a typical load cell sensor
application that can be used as a starting point for many dif-
ferent types of sensors and applications. Applications in en-
vironments where EMI may appear would especially benefit
from the EMIRR performance of the LMV861 and LMV862.
Load Cell Characteristics
The load cell used in this example is a Wheatstone bridge.
The value of the resistors in the bridge changes when pres-
sure is applied to the sensor. This change of the resistor
values will result in a differential output voltage depending on
the sensitivity of the sensor, the used supply voltage and the
applied pressure. The difference between the output at full
scale pressure and the output at zero pressure is defined as
the span of the load cell. A typical value for the span is
10 mV/V.
The circuit configuration should be chosen such that loading
of the sensor is prevented. Loading of the resistor bridge due
to the circuit following the sensor, could result in incorrect
output voltages of the sensor.
−
. For dual supplies, place one capacitor between V
−
.
FIGURE 8. Load Cell Application
+
+
and
and
16
Load Cell Example
Figure 8 shows a typical schematic for a load cell application.
It uses a single supply and has an adjustment for both positive
and negative offset of the load cell. An ADC converts the am-
plified signal to a digital signal.
The op amps A1 and A2 are configured as buffers, and are
connected at both the positive and the negative output of the
load cell. This is to prevent the loading of the resistor bridge
in the sensor by the resistors configuring the differential op
amp circuit (op amp A4). The buffers also prevent the resis-
tors of the sensor from affecting the gain of the following gain
stage. The third buffer (A3) is used to create a reference volt-
age, to correct for the offset in the system.
Given the differential output voltage V
output signal of this op amp configuration, V
To align the pressure range with the full range of an ADC the
correct gain needs to be set. To calculate the correct gain, the
power supply voltage and the span of the load cell are need-
ed. For this example a power supply of 5V is used and the
span of the sensor, in this case a 125 kg sensor, is 100 mV.
With the configuration as shown in Figure 8, this signal is
covering almost the full input range of the ADC. With no
weight on the load cell, the output of the sensor and the op
amp A4 will be close to 0V. With the full weight on the load
cell, the output of the sensor is 100 mV, and will be amplified
with the gain from the configuration. In the case of the con-
figuration of Figure 8 the gain is R3/R1 = 51 kΩ/100Ω = 50.
This will result in a maximum output of 100 mV x 50 = 5V,
which covers the full range of the ADC.
For further processing the digital signal can be processed by
a microprocessor following the ADC, this can be used to dis-
play or log the weight on the load cell. To get a resolution of
0.5 kg, the LSB of the ADC should be smaller then 0.5 kg/125
kg = 1/1000. A 12-bit ADC would be sufficient as this gives
4096 steps. A 12-bit ADC such as the two channel 12-bit
ADC122S021 can be used for this application.
30024069
S
of the load cell, the
OUT
, equals: