Single chip microcomputer: a/d differential input signal

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A/D Differential input signal

As mentioned in the previous article , Control word no 4 Position and number 5 Bits are used to control PCF8591 Is the analog input pin of the single ended input or differential input . Differential input is a common technique in analog circuits , Here we give a brief introduction to the relevant knowledge .

Strictly speaking , In fact, all signals are differential signals , Because all voltages can only be relative to another voltage . But most systems , We are all systematic GND As a benchmark . And for A/D Differential input for , Usually, except for GND outside , The other two routes have the same amplitude , Input signal with opposite polarity , In fact, it's easy to understand , Just like a seesaw . As shown in the figure .

Differential input , It's not a single input , But by the 2 A group of inputs consisting of inputs .PCF8591 Is the total 4 Analog inputs , It can be configured as 4 Patterns , The most typical is 4 A two-way differential mode composed of two input terminals

When the first... Of the control word 4 Position and number 5 All places are 1 When , that 4 The path simulation is configured to 2 Differential mode input channel 0 and channel 1. We use channel 0 For example , among AIN0 Is the forward input ,AIN1 Is the reverse input , The signal input between them is of the same amplitude , Signals of opposite polarity , After passing the subtracter , What you get is the difference between the two input channels

Usually , The neutral line of the differential input is half of the reference voltage , Our reference voltage is 2.5 V, If 1.25 V As the center line ,V+ yes AIN0 Input waveform ,V- yes AIN1 Input waveform ,Signal Value Is the waveform after the subtracter . quite a lot A/D They are all input in a differential way , Because differential input mode is better than single ended input , It has stronger anti-interference ability .

Single ended input signal , If there is interference change on the first line , For example, the amplitude increases 5 mv,GND unchanged , The measured data will deviate ; When the differential signal is input , When there is interference signal from outside , As long as the wiring is reasonable , Most of them are coupled to two lines at the same time , Amplitude increase 5 mv Will increase at the same time 5 mv, What the receiver cares about is the difference between the two signals , Therefore, the external common mode noise can be completely eliminated . Because the polarity of the two signals is opposite , The electromagnetic fields radiated by them can counteract each other , Effectively restrain the electromagnetic energy released to the outside world .

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