Sensor Interface Circuit Using a Difference Amplifier

For ease of use, most sensors have been made into components, which already contain the corresponding amplification circuits and can be used by simply connecting to a power supply. Since the relevant parameters of the component are determined by the manufacturer, if the parameters of the circuit are to be changed, additional circuits have to be added to the outside of the component. The following is an example of a current sensor component with an operating voltage of 7.5V, a measurement range of 0A to 10A, a zero point voltage of 3.75V (at 0A), and a sensitivity of 30mV/A. At this time, the change in output voltage due to a 10A current is 30×l0(-3 power)×l0=0.3V. This voltage change is small and it is difficult to use without making changes. It is planned to change the zero point to 5V (at 0A) and the sensitivity to 400mV/A. The change in output voltage at a current of 10A is 4V.

The attached figure is a sensor interface circuit using a differential amplifier. In this example, since the sensitivity needs to be increased from 30mV/A to 400mV/A, the gain of the amplifier should be 400/30=13.3 times. The differential amplifier is composed of an operational amplifier IC1b, and its gain C can be expressed by the formula G=R2/RG, that is, it is determined by the ratio of R2 and RG. When R2=150kΩ, RG=150×l0 cubed/13.3=ll.3kΩ. In order to facilitate adjustment, RG is composed of a 9.lkΩ fixed resistor and a 3kΩ semi-variable resistor in series, which can adjust the gain of the amplifier.

Next, we need to solve the problem of zero point movement. Since the output voltage of the three-terminal voltage regulator 78075 used is 7.5V, a 3.75V voltage is generated by resistor voltage division. A semi-variable resistor VR2 for zero point adjustment is also connected in series in the voltage division circuit.

IC2a is a buffer amplifier, and the voltage REF2 at the output end is 3.75V.

In order to move the voltage corresponding to the zero point from 3.75V to 5V, a 5V reference voltage source needs to be set. The 5V voltage (REF1) is also obtained by splitting the components, and then buffered by the buffer amplifier IC2b. The buffer amplifier for the reference voltage REFI and REF2 uses LM358. The differential amplifier uses μPC842 with good frequency characteristics. If there is no requirement for frequency characteristics, LM358 can also be used.

The power supply voltage of the circuit is +12V, the current consumed is less than 20mA, the input voltage is 3.75V±0.3V, and the output voltage is 5V±4V.