Lack of comparator, operational amplifier to save the field! (the op amp is recorded as a comparator circuit)

 Circuit class , Let's talk about an example of using an op amp as a comparator ... by  Due to Jin Chen 

Preface

Recently I did a small project , Because a comparator is out of stock , There's a little problem , It took some time and finally used an op amp to temporarily replace the comparator , Solved the problem .

Because of this problem , It reminds me of the previous example of using op amp instead of comparator , And then I thought This period's circuit class , Let's talk about how to use op amp as comparator as well as Things to pay attention to .

One 、 OP AMP and comparator

Those things like principle application instructions belong to basic problems , Not the focus of this article , We explain in a simple way .

1.1 brief introduction

The comparator ：

Let's start with a simple comparator , I really don't want to draw , Post your previous notes = =！ Make do with it ：

You can see it , The function of comparator is to compare The voltage of the two inputs is high or low , The general circuit application for comparing two voltages in a circuit is （ We haven't talked about the output here , Will the following article say , This is just a diagram ）：

The voltage comparator can be seen as having a magnification close to “ infinity ” Operational amplifier .

Operational amplifier ：

Operational amplifiers are relatively better than The voltage comparator is much more complicated , If you have the time , I will write a separate article to explain . This article will not do too much basic introduction .

Operational amplifier is an electronic integrated circuit with multi-stage amplification circuit , Its input stage is a differential amplifier circuit , High input resistance and ability to suppress zero drift ; The intermediate stage is mainly used for voltage amplification , With high voltage amplification , It is generally composed of common emitter amplifier circuit ; The output pole is connected to the load , It has strong carrying capacity 、 Low output resistance .

The operational amplifier uses differential input , And it has high gain , This is similar to the characteristics of the comparator , Therefore, it can be used as a low performance comparator in practical applications .

Use the op amp as a comparator , Then the operational amplifier works in an open loop way .

In fact, the most commonly used operational amplifier circuits are closed-loop circuits .

What is open loop and closed loop ？

 An open-loop circuit is a circuit without feedback , Output does not affect input .

 The closed-loop circuit is a circuit connected with feedback , That is, the output will affect the input .

 So for the op amp , The input feedback resistor is a closed loop circuit , No access is an open loop circuit .

We have drawn the application circuit diagram of the comparator above , For comparators , His common basic circuit is the closed-loop feedback circuit , Here's the picture ：

1.2 * Output terminal *

The op amp should be reasonably used as a comparator , Their outputs Vout Need special attention .

The output of operational amplifier is generally push-pull , The output of the comparator is open （ The opening episode ） More , There are also many Push-Pull Outputs .

Push pull output , It can easily output high and low levels ;

Open a leak （OC/OD door ） The most important characteristic of the output is that the high level has no driving ability , The external pull-up resistor is needed to really output high level .

therefore , When choosing op amp or comparator, you should pay attention to their output mode , The use of circuits with different output modes is also different .

Let's take a comparator as an example , such as TI Of LMV723X, I've never used , In fact, it is only for writing articles and searching on the Internet , There is a description of the model and output in its documentation ：

For open drain output ：

When we use it , The circuit generally adopts the following forms ：

For push-pull output ：

When we use it , The output terminal can be without a pull-down resistor （ In some cases, a diode can be added , For example, when the comparator output is in parallel ）：

So when we use op amp instead of comparator , First, check whether the original design of the circuit is open drain output or push-pull output , If it is a push-pull output, it can be directly replaced , In case of open drain output , It needs certain treatment .

1.3 Precautions for using op amp as comparator

Here is the description of Baidu Encyclopedia , In practice , Compared with using a dedicated comparator, using an op amp comparator has the following disadvantages ：

• The response speed is slower than the real comparator , The op amp is designed to operate in a linear segment with negative feedback , Therefore, saturated operational amplifiers generally have a slow flip speed . Most operational amplifiers have a compensation capacitor to limit the swing rate of high-frequency signals . This makes the op amp comparator have microsecond propagation delay , In contrast, the flip speed of the dedicated comparator is in the nanosecond order .
• The op amp has no built-in hysteresis circuit , A dedicated external network is required to delay the input signal .
• The static operating point current of the operational amplifier is stable only under the condition of negative feedback . DC bias occurs when the input voltages are unequal .
• The function of comparator is to generate input signal for digital circuit , It is necessary to consider the compatibility with the digital circuit interface when using the operational amplifier comparator .
• Interference may occur between different frequencies of multi section operational amplifiers .
• Many op amps have diodes in reverse series at their inputs . The inputs of the two poles of the op amp are generally the same , It won't cause problems . But the two poles of the comparator need different voltages , This may lead to unexpected diode breakdown .

actually , If we make some common civilian sensors , In addition to paying attention to the processing of the output circuit , Other problems are not problems .

At least some of the civilian sensor fields where bloggers are located , Some product cases haven't said anything about the problems .

Two 、 Circuit record

It says so much , Simply use a few program examples to illustrate , Also as my own plan record , Because it involves some problems , So I disposed of the resistance and capacitance in the circuit , But the explanation of principles and knowledge is not affected .

2.1 A proud alternative

Let's look at a circuit ：

In the circuit 2 A comparator ,2 An op amp , Finally, it is directly connected through the output of the comparator MCU Of IO mouth ：

It is also mentioned in the above figure , because LPV521 and MAX920 These two materials are expensive and short of materials , So the product has to be upgraded , Finally, I used a cheap 4 road Op amp directly replaces the above scheme ：

Combined with the analysis of the above article , Many problems can be explained by the above circuit ,

2.2 Some details are explained again

The above plan was completed a few years ago , Recently, when making a simpler plan , I didn't think much about it , Just draw the following picture , Yes , The same chip , The circuit is simpler ：

But I didn't think too much when designing , It is directly designed as follows ：

A micro power comparator is used RS8901XF , Everything tested is normal , When making small batch samples , because RS8901XF Shortage , Therefore, another signal comparator is selected , When you find out, something goes wrong .

The reason is that I didn't pay attention to the output of the comparator I chose later , If it is an open drain output, then this circuit cannot be used . Finally, an op amp is used to replace COP1 , The sample is tested to be normal ！

Now think about it. , This plan should not be designed like this , Use standard comparators directly , No diodes are required , Use only one pull-up resistor R4 , Directly connect the output of the comparator to MCU Corresponding IO Eloquence is the simplest way ！！！

Of course, the diode in the above figure is also a little redundant , The first is because 2 Circuit output in parallel , Diodes are necessary , And here, single channel output , Diodes are not needed ！

Conclusion

Okay , This article is mainly to explain the actual case that bloggers have used operational amplifiers as comparators .

Now look back , In general , If possible , Operational amplifier or operational amplifier , The comparator is a special comparator . But sometimes , For example, the first plan in my above record , Take the op amp as a comparator , It's also a good choice .

* Of course , Although I said that generally speaking, the general product is not a problem , But more tests are needed , Circuit design , Besides theoretical knowledge , Practice is essential ！！*

Okay , This article is here , Thank you. ！

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