Datasheet

x V

SENSE

x V

REF

OUT

LMV861, LMV862

SNOSAZ5C –FEBRUARY 2008–REVISED MARCH 2013

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Load Cell Characteristics

The load cell used in this example is a Wheatstone bridge. The value of the resistors in the bridge changes when

pressure 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.

Load Cell Example

Figure 51 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 amplified 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 resistors of the sensor from

affecting the gain of the following gain stage. The third buffer (A3) is used to create a reference voltage, to

correct for the offset in the system.

Given the differential output voltage V

of the load cell, the output signal of this op amp configuration, V

OUT

equals:

(2)

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 needed. 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 51, 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 configuration

of Figure 51 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 display 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.

Product Folder Links: LMV861 LMV862