# How to eliminate the disturbance of 50Hz power frequency to the high-precision temperature measurement circuit?

50Hz power frequency electromagnetic field interference is an unavoidable problem in hardware development, especially in sensitive measurement circuits, the power frequency electromagnetic field will submerge the measurement signal in the power frequency waveform, which will seriously affect the measurement stability, so eliminating the power frequency electromagnetic field interference is a sensitive measurement. Inescapable challenges in circuit design.

50Hz power frequency electromagnetic field interference is an unavoidable problem in hardware development, especially in sensitive measurement circuits, the power frequency electromagnetic field will submerge the measurement signal in the power frequency waveform, which will seriously affect the measurement stability, so eliminating the power frequency electromagnetic field interference is a sensitive measurement. Inescapable challenges in circuit design.

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 solve it?

In fact, this phenomenon may be mainly due to the following reasons:

The influence of 50Hz power frequency electromagnetic field;

Group 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

Since there are periodic fluctuations in the measurement circuit, the possibility of power frequency electromagnetic field disturbance is greater. Use an oscilloscope to observe the power frequency electromagnetic field waveform as shown in Figure 1. 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;

The 50Hz power frequency interference enters the system through spatial coupling.

In view of the above problems, the method of eliminating 50Hz power frequency electromagnetic field interference is relatively clear. There are the following four schemes for circuit designers to refer to:

Use electrical isolation to block the conduction path of power frequency interference;

A common mode suppression and filter circuit is built at the sensitive circuit to filter out the power frequency disturbance entering the input channel;

Build an IIR notch or FIR band-stop digital filter in the software to eliminate the influence of power frequency interference on the measurement results;

Reduce the measurement lead loop area, increase the shielding, and weaken the spatial coupling effect.

ZLG has launched a dual-channel thermal resistance isolation temperature measurement module TPS02R, which is designed for sensitive circuits and fully considers 50Hz power frequency interference, as shown in Figure 2. 0.01℃, and can run stably for a long time.

Figure 2 TPS02R system scheme

As shown in the system scheme in the above figure, for the 50Hz power frequency interference, in the “reference buffer circuit”, a hardware filter circuit is used to reduce the influence of the 50Hz power frequency on the reference voltage of the ADC chip. As shown in Figure 3, it is essentially a voltage follower buffer circuit combined with a low-pass filter, R1C1 is for 50Hz filtering, and R2R3C2C3 is for 50Hz high-order harmonic filtering.

Figure 3 Buffer filter circuit

The specific -3dB frequency response is calculated as Equation 1

The internal PGA of the ADC chip adopts an instrumentation amplifier structure to greatly attenuate common-mode power frequency interference, and has a built-in digital processor to digitally filter the input signal. The frequency response of the digital filtering algorithm is shown in Figure 4, and the notch point of the digital filtering algorithm is shown in Figure 4. It responds at integer multiples of 10Hz, 20Hz, 40Hz, and 80Hz frequency, so choosing the output of 10Hz frequency can attenuate the 50Hz power frequency disturbance to a certain extent.

Figure 4 Digital filter frequency response

Combined with the electrical isolation scheme to prevent the 50Hz power frequency from being conducted from the power source into the system from the source to affect 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, thereby suppressing the generation of 50Hz power frequency interference.

The above four schemes are all used in our high-precision temperature measurement module TPS02R. For 50Hz power frequency electromagnetic field interference, TPS02R has strong adaptability. Now we compare our TPS02R with an RTD non-isolated measurement scheme:

Figure 5 Measured data of related products

As shown in Figure 5, in the same environment, use Fluke 5520A source meter to simulate RTD to test related products, the simulated temperature is 0℃, measure 145 times, and count related data. The test is powered by the USB interface of the PC, so there is a common 50Hz power frequency interference signal between the 5520A source meter. Among the test data in Figure 5, the non-isolated RTD measurement scheme has a certain fluctuation, and the fluctuation range is[-0.44, 0.44]°C, while the fluctuation range of the TPS02R measurement is[0.046, 0.072]°C , is very little affected by the 50Hz power frequency electromagnetic field, which is used to verify the effectiveness of the above design against the influence of the 50Hz power frequency electromagnetic field. In the application environment with complex power supply or uncertainty, using the TPS02R module with built-in electrical isolation, the temperature measurement of the PT100 can be easily and stably achieved.

Figure 6 TPS02R product picture