Isolated ADC Acquisition Solution for Analog Signals

zjqmyron

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GP9303 analog signal isolation ADC acquisition solution for your reference GP9303_cn_V1.9.pdf (527.97 KB, downloads: 66) 2020-1-16 16:39 上传 点击文件名下载附件

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maychang

From "0-5V voltage" to MCU output, what is the resolution and accuracy? I'm afraid it won't be very high. The error caused by the optocoupler will not be very small.

zjqmyron

maychang posted on 2020-1-16 18:21 From "0-5V voltage" to MCU output, what is the resolution and accuracy? I'm afraid it won't be very high. The error caused by the optocoupler will not...

If it is to tell the optocoupler, the speed is actually OK. The resolution can be 16Bit

johnsonxjq

This thing is good, good

dwwzl

It is a new idea

maychang

The title of this article is "Isolated ADC Acquisition Solution for Analog Signals", which is a bit problematic.

The chip used in this circuit converts the voltage into a PWM signal. However, the PWM signal is still an analog signal, just as the frequency signal is an analog signal. There is no analog-to-digital conversion in this process.

maychang

The real analog-to-digital conversion occurs in the first microcontroller.

After the PWM signal is transmitted by the optocoupler and low-pass filtered, it is still a voltage signal. This voltage signal should be proportional to the original "0-5V voltage" (the disproportion is due to the error in the transmission process). The microcontroller still needs to use the internal ADC channel to measure this voltage signal in order to complete the work of converting the "0-5V voltage" into a digital signal.

maychang

Instead of using optocouplers to isolate and measure analog signals, it is better to use the voltage-frequency conversion chip LM331.

Optocouplers transmit PWM signals. Due to the different delay times of the rising and falling edges of the optocouplers, the duty cycle will change after passing through the optocouplers, causing additional transmission errors. However, when using the voltage-frequency conversion chip LM331, the optocouplers transmit frequency. The different delay times of the rising and falling edges of the optocouplers will not change the frequency, and will not cause additional transmission errors.

PowerAnts

PFM is the only way to ensure accuracy. In the early days, there were special VF and FV conversion circuits.

PowerAnts

The transmission delay of the optocoupler is affected by temperature, which is a serious problem for PWM that transmits analog quantities by pulse width. This problem can be avoided by changing to period.

maychang

Moreover, the single-chip microcomputer measures frequency by using the on-chip counter, and measures voltage by using the on-chip ADC channel. The counter is a must-have part of any single-chip microcomputer, but not every single-chip microcomputer has an ADC.

Conclusion: Using the voltage-frequency conversion chip LM331 to isolate and measure voltage is more convenient and more accurate than the original poster's solution.

zjqmyron

maychang published on 2020-1-17 09:18 Moreover, the microcontroller measures the frequency by using the on-chip counter, and measures the voltage by using the on-chip ADC channel. The counter is a must-have for any microcontroller...

Reply to the above:

1. Have the people who mentioned LM331 above really used this chip? Look at the peripheral circuit in the figure below. It is quite complicated. Look at the frequency formula again. It is not easy to achieve 1% of its absolute value. It is even more difficult to have a temperature coefficient less than 50PPM. So those who say that LM331 is better either don’t know LM331. Or don’t know APC chips. The advantage of voltage-to-frequency conversion is that low-speed optocouplers can be used for signal isolation. If the frequency of PWM transmission is low, the requirements for optocouplers are not high. If the frequency is high, there are many options for high-speed optocouplers. Let’s do a high and low temperature test to tell that optocouplers basically do not affect the accuracy. In addition, we also have distortion-free capacitors and magnetic isolation solutions (off topic).

2. For those who say that PWM needs to be collected by ADC, I think you may have a misunderstanding of PWM. Don't say that each STM32 MCU now has a PWM capture function. Even if it is 51, you can also convert PWM data into data by counting and then dividing.

3. Some people say that PWM signal is an analog signal. This statement is incorrect. PWM should be regarded as an intermediate signal between digital and analog. It is a signal that is very convenient to simulate and digitize.

zjqmyron

PowerAnts published on 2020-1-17 09:17 The transmission delay of the optocoupler is affected by temperature, which is a serious problem for PWM that transmits analog quantities with pulse width. This problem can be avoided by changing to a period...

You can take a look at our customized high-speed optocoupler, which can reach 15MHz and ns-level delay. Even taking into account the temperature coefficient, the error is basically negligible compared to the millisecond-level PWM signal.

zjqmyron

PowerAnts published on 2020-1-17 09:15 Only by using PFM can the accuracy be guaranteed. In the early days, there were special VF and FV conversion circuits

Yes, chips like the LM331 are pretty good.

However, we now use high-speed optocouplers, and the PWM transmission is basically distortion-free.

We can also use technologies such as magnetic isolation and capacitive isolation to achieve distortion-free transmission.

zjqmyron

maychang published on 2020-1-17 09:15 Using optocouplers to isolate and measure analog signals, it is better to use the voltage-frequency conversion chip LM331. Optocouplers transmit PWM signals. Due to the rise of optocouplers...

LM331 is indeed an option, but I really want to know what kind of applications people are using this chip in. I really want to discuss it. In my opinion, this chip is difficult to use, and there are some peripheral components. We are also preparing AFC chips, Analog to Frequency Convertor. I just don't see such applications very often. If you have friends who know about it, please discuss it with me. Thank you.

PowerAnts

zjqmyron published on 2020-1-18 10:40 Yes, chips like LM331 are indeed good. However, we now use high-speed optocouplers, and the transmission PWM is basically undistorted. And we...

To achieve the same performance, the period method is simpler, more reliable, more stable and cheaper than the pulse width method

zjqmyron

PowerAnts published on 2020-1-18 11:59 To achieve the same performance, the period method is simpler, more reliable, more stable and cheaper than the pulse width method

Is your example LM331? The price of this chip is not low.

PowerAnts

zjqmyron posted on 2020-1-18 12:25 Is your example LM331? The price of this chip is not low

Why must it be 331? It is not difficult to find an MCU with Chinese specifications that can meet the VF conversion requirements.

bigbat

Instead of going through so much trouble, it is better to directly use SPI's external AD digital quantity to ensure authenticity.

PowerAnts

Regarding whether PWM is analog or digital, I think it is analog in most cases.