How to collect the analog signal 4-20mA input and input it to the ADC pin of STM32?

chuzhaonan

How to collect the analog signal 4-20mA input and input it to the ADC pin of STM32? [Copy link]

How to collect analog signal 4-20mA input and input it to the ADC pin of STM32?

huixianfxt

After being converted into voltage through a sampling resistor, the voltage signal is properly processed and then sent to the STM32 ADC for acquisition.

wudianjun2001

Connect a resistor in series to collect the voltage across the resistor

maychang

The most common method is to connect a 250 ohm precision resistor in series in a 4-20mA loop to convert it into a 1-5V voltage. The resistor must not only have an accurate value, but more importantly, a small temperature coefficient. Of course, if 20mA does not require 5V, other smaller values of resistors can also be used.

chuzhaonan

maychang posted on 2018-7-5 14:17 The most common way is to connect a 250 ohm precision resistor in series in a 4-20mA loop to convert it into a 1-5V voltage. The resistor must not only have an accurate value, but more importantly, the voltage...

Isn't it a good idea to do this directly? Wouldn't it be better to add an ESD and a voltage follower?

maychang

chuzhaonan posted on 2018-7-5 14:39 Isn't it not good to do it directly? It would be better to add an ESD and a voltage follower, right?

In actual use, of course, various protection circuits, filter circuits, impedance conversion circuits, etc. need to be added. The sampling resistor (this is how the 2nd and 3rd floors call this resistor) is connected to both ends with a fairly long wire (maybe a kilometer or even more), which may carry a fairly high common-mode interference voltage. Directly connecting to the microcontroller is simply asking for trouble.

chuzhaonan

maychang posted on 2018-7-5 15:20 In actual use, of course, it is necessary to add various protection circuits, filter circuits, impedance conversion circuits, etc. The sampling resistor (the second and third floors call it this...

Filtering generally uses RC low-pass filtering, right? What is the point of impedance conversion?

maychang

chuzhaonan posted on 2018-7-5 15:53 Filtering generally uses RC low-pass filtering, right? What is the point of impedance transformation?

The voltage follower you mentioned on the 5th floor is a kind of impedance transformation circuit. Simply put, the filter circuit, impedance transformation circuit... are all based on your needs, such as how many orders of filter circuits to use, what is the turning frequency, etc. These circuits should not increase the 4-20mA current acquisition error, and meet your usage requirements.

chunyang

250 ohm dedicated conversion resistor + TVS or diode clamp (all 5V system is best to use clamp protection) + active low-pass filter built by op amp (the turning frequency is determined by the signal frequency and the frequency of the main interference signal, and the 5V power supply requires a rail-to-rail op amp). If the electromagnetic environment at the work site is clean, the front-end signal cable is transmitted with a shielded wire and the shielded wire is reliably connected to the ground, then a 250 ohm resistor IV conversion and an op amp follower can be added. Impedance matching here generally does not require special consideration. Adding an op amp buffer is to avoid voltage drops caused by insufficient input impedance of the ADC, which is also matching.

jameschen2015

The resistor is connected in series to the current loop, and the differential amplifier measures the voltage at both ends. Selecting the appropriate resistor ratio can adapt to higher common-mode voltages; at the same time, the ADC is driven; appropriate filtering can be added between the op amp and the ADC to stabilize the input.

han0097

chunyang published on 2018-7-5 17:02 250 ohm dedicated conversion resistor + TVS or diode clamp (all 5V system is best to use clamp protection) + active low-pass filter built by op amp (the turning frequency is based on...

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