Most engineers encounter numerical fluctuations when using the 331-7KF02 to measure signals. These fluctuations are mostly caused by external interference entering the measurement channel or the influence of common-mode voltage. To address this issue, we can optimize the measurement from the following aspects to make the measured values more stable. The following diagram shows the wiring diagram of the 7KF02 current sensor. It can be seen that the internal circuitry of the 7KF02 includes:
1. Internal power supply circuit: This circuit converts the 24V power supply into the voltage required by the internal analog circuit.
2. Compensation circuit: This circuit is used to compensate for the measured values. It is divided into internal compensation and external compensation.
3. Multiplexer: This circuit switches the channels in turn. The 7KF02 has a total of 8 channels. The ADC can only measure one channel at a time. After the measurement is completed, it switches to the next channel through this multiplexer. In the specifications of the 7KF02, this is reflected in the channel measurement time.
4. Range card: This is self-explanatory. It is used to select the type of measurement signal, such as 2-wire current, 4-wire current, RTD, thermocouple, etc.
In addition to signal pins 2-9 and 12-29, it's worth noting that the 7KF02 module also provides pins 10 and 11 for optimization. So what are the main functions of pins 10 and 11? And how should we wire them in practice to achieve optimal performance?
As seen in the internal diagram, pin 10 primarily provides external compensation. If external compensation is not required, pin 10 can be connected to pin 11. See the diagram below:
If external compensation is required, the compensation voltage can be input to pin 10. When is external compensation needed? It's only needed when measuring thermocouples. Why?
Because thermocouples are composed of different metals connected together, they can sense a tiny voltage value when there is a temperature difference. Using a 7KF02 thermocouple for temperature measurement indirectly measures this induced voltage value to obtain the temperature value. During measurement, the cold junction temperature must remain constant so that the generated voltage value has a proportional relationship with the measured temperature. If the cold junction temperature changes during measurement, it will severely affect the accuracy of the measurement. Therefore, cold junction compensation is required. Using a 10-pin external compensation system serves this purpose, as shown in the diagram below:
As seen in the internal diagram, pin 11 is connected to ground, but to what ground? Actually, pin 11 is connected to the reference ground of the ADC power supply via a 400-ohm connection. All measured data is taken with reference to the ADC power supply. We call the ground connected to pin 11 analog ground, as shown in the diagram below:
Some readers might ask, what's the purpose of this 400-ohm resistor? Actually, it's not a resistor, but a ferrite bead. The ferrite bead's function is to provide high-frequency isolation between analog and digital ground. This isolation channel is added to prevent spikes and glitches generated by the digital circuitry during high-frequency switching from affecting the analog measurement circuitry and interfering with the measured values. However, in actual measurements, the wires from the sensor to the module are often quite long. The longer the wire, the stronger the coupling ability to interference, which will affect our final measurement value. To minimize this impact, in actual wiring, we need to connect the analog ground of pin 11 to the sensor power ground together, so that the ADC measurement reference potential directly references the sensor power ground. See the diagram below:
If a certain amount of common-mode voltage inevitably enters at the sensor end, we can connect pin 11 analog ground to the 24V ground, and at the same time connect them to a clean ground line at the module end to eliminate the influence of common-mode interference at the sensor end, as follows:
For more services, please follow Yiwei's WeChat service account (unimat2014)!
