50Hz power frequency electromagnetic field interference is an unavoidable problem in hardware development, especially in sensitive measurement circuits, power frequency electromagnetic field will make the measurement signal submerged in the power frequency waveform, seriously affecting the measurement stability, so eliminating power frequency electromagnetic field interference is an inescapable challenge in the design of sensitive measurement circuits.
PT100 is currently the most widely used temperature measurement solution, will engineers encounter such problems when applying this solution?
Why does the PT100 temperature measurement circuit have small periodic fluctuations? How to fix it?
In fact, there are several reasons for this phenomenon:
- The influence of 50Hz power frequency electromagnetic field;
- Swarm pulse interference caused by switching actions such as surrounding motors or relays;
- Power-frequency common-mode interference conducted into the system.
Figure 1 Power-frequency electromagnetic field waveform
Due to the periodic fluctuation of the measurement circuit, the possibility of power frequency electromagnetic field disturbance is greater, and the power frequency electromagnetic field waveform observed with an oscilloscope is shown in Figure 1, and it is generally believed that the 50Hz power frequency electromagnetic field interference is caused by two reasons:
- 50Hz power frequency interference enters the system through conduction;
- 50Hz power frequency interference enters the system through spatial coupling.
In view of the above problems, the method of eliminating the interference of 50Hz power frequency electromagnetic field is also relatively clear, and there are the following four schemes for circuit designers to refer to:
- Electrical isolation is used to block the conduction path of power frequency interference;
- A common-mode suppression and filtering circuit is built at the sensitive circuit to filter out the power frequency disturbance entering the input channel;
- IIR notch or FIR band-hinder digital filter is constructed in the software to eliminate the influence of power frequency interference on the measurement results.
- Reduce the area of the measurement lead loop, increase the shielding, and reduce the spatial coupling effect.
ZLG launched a dual-channel RTD isolated temperature measurement module TPS02RAH, which is designed for sensitive circuits, fully considering 50Hz power frequency interference, as shown in Figure 2, our company uses a variety of schemes to suppress power frequency interference, so that the resolution of the TPS02RAH module can reach 0.01°C, and can run stably for a long time.
Figure 2 TPS02RAH system scheme
As shown in the system scheme in the figure above, for the 50Hz power frequency interference, a hardware filter circuit is used in the "reference buffer circuit" to reduce the impact of the 50Hz power frequency on the reference voltage of the ADC chip. As shown in Figure 3, it is essentially a voltage-following snubber circuit combined with a low-pass filter, with R1C1 filtering for 50Hz and R2R3C2C3 filtering for 50Hz higher-order harmonics.
Figure 3 Buffer filter circuit
The specific -3dB frequency response is calculated as shown in Equation 1
The internal PGA of the ADC chip adopts the instrumentation amplifier structure to greatly attenuate the common-mode power frequency interference, and the built-in digital processor is used to digitally filter the input signal, in which the frequency response of the digital filtering algorithm is shown in Figure 4, and the notch point of the digital filtering algorithm responds at an integer multiple of the frequency of 10Hz, 20Hz, 40Hz, and 80Hz, so the output of the 10Hz frequency can be attenuated to a certain extent by the 50Hz power frequency disturbance.
Fig.4. Digital filter frequency response
Combined with the galvanic isolation scheme, the 50Hz power frequency from the source is prevented from being conducted from the power supply into the system, affecting the sensitive signal acquisition end. The module adopts a four-layer board layout, and a large area of copper is grounded, so that the ground impedance is reduced to a very low level, and the signal loop of the system is shortened as much as possible, so as to suppress the generation of 50Hz power frequency interference.
The above four schemes are all used in our high-precision temperature measurement module TPS02RAH, for 50Hz power frequency electromagnetic field interference, TPS02RAH has strong adaptability, now our TPS02RAH and an RTD non-isolated measurement scheme to compare:
Figure 5 Measured data of related products
As shown in Figure 5, in the same environment, the Fluke 5520A source meter was used to simulate RTD test related products, the simulated temperature was 0°C, measured 145 times, and the relevant data were counted. The test uses the USB port of the PC to power the power, so there is a common 50Hz power frequency interference signal between the 5520A and the source meter. In the test data of Figure 5, the non-isolated RTD measurement scheme has a certain fluctuation range, the fluctuation range is [-0.44, 0.44] °C, while the fluctuation range of the TPS02RAH measurement is [0.046, 0.072] °C, and the influence of the 50Hz power frequency electromagnetic field is very small, which is used to verify the design effectiveness of the above resistance to the influence of the 50Hz power frequency electromagnetic field. In applications with complex or uncertain power supply conditions, the PT100 temperature measurement can be easily and stably measured using a TPS02RAH module with built-in galvanic isolation.