Transcription
Hey friends, welcome to the YouTube channel ALL ABOUT ELECTRONICS.
So, in this video, we will learn about counter type ADC. So, this counter type ADC is a very simple type ADC and the reason it is known as the counter type ADC because it uses the binary counter for the conversion. So, apart from the binary counter, it consists of the comparator, the digital to analog converter, the control circuit, the AND gate, and the latches. And this ADC is also known as the digital ramp ADC. Because here, the output of the counter is given to the DAC. And as the counter increments its count, the output of the DAC increases in the ramp fashion. But actually, if you see the DAC output then it looks like a staircase.
So, now let's understand the working of this counter type ADC. So, here the input voltage is applied at the non-inverting terminal, while the output of the DAC is given at the inverting terminal of the comparator. And here the output of the counter is given as an input to this digital to analog converter. So, here initially when the conversion starts then the counter is RESET. And that's why the output of the DAC is equal to zero. So, initially, this input voltage is greater than the output of the DAC. And that's why the output of the comparator is high.
Now, if you notice over here, these clock pulses are applied to this counter using the AND gate. So, whenever the output of the comparator is high, at that time these clock pulses will be applied to the counter. And due to that, the counter starts counting. And as the counter increments its count, the output of the DAC will also increase in the staircase fashion. And here the output of the DAC is continuously compared with the input voltage. So, as far as this Vdac is less than the input voltage, the output of the comparator will be high. And due to that these clock pulses will be applied to the counter. And hence, the output of the DAC will increase gradually in the staircase fashion. And as soon as this Vdac is greater than the input voltage, then the output of the comparator will become low. And now, no clock pulses will be applied to this counter. So, as soon as the output of the comparator will become low, through this control circuit, the output of the counter will get latched and this counter will get RESET. So, now this latched output is directly proportional to the input voltage. And after this entire procedure, once again the new conversion will start. That means the new input value will get sampled and once again the new conversion will start.
But the disadvantage of this counter is that every time the new conversion starts, then this counter will get RESET. That means every time, the counter starts from 0 onwards. So, we can say that the conversion time depends on the magnitude of the input voltage. That means the larger will be the input voltage, the more time ADC will take for the conversion. And in this figure, the typical conversion pattern is shown for this counter type ADC. So, as you can see, the output of the DAC increases, until it is less than the input voltage. And once it crosses the input voltage, then this counter is getting RESET and then after the new conversion starts. So, this will be the total conversion time for one conversion. And as you can see over here, as the input signal increases, then the conversion time will also increase.
And in the worst case, the maximum conversion time will be equal to (2^N -1 ) Tclk. Where this Tclk is the duration of the clock pulse. And N is the number of bits of the ADC. So, this worst case will happen whenever the input voltage is equal to the full-scale output voltage of the DAC. And the DAC will give the full-scale output voltage whenever all the input bits are one. So, for the counter, to reach from all zeros to all 1's, it will take 2^N -1 clocks. And therefore, in the worst case, the total conversion time will be equal to (2^N - 1) Tclk.
Now, if you notice over here, the conversion time depends on the resolution as well as the clock frequency. so, as the number of bits increases, then the conversion time will also increase. And speaking of the resolution, the resolution of this type of ADC depends on the resolution of the DAC. That means if the resolution of the DAC is 8-bit, then the resolution of this entire ADC will be equal to 8-bit. And as I said, if we increase the resolution beyond a certain extent, then the conversion time of the ADC will get affected. So, there is a trade-off between the resolution and the conversion time. Of course, by increasing this clock frequency we can reduce this conversion time. But that too cannot be increased beyond a certain limit. As the maximum allowable clock frequency is restricted by the response time of the components of the ADC.
So, basically, this type of ADC is slow, because the counter gets RESET after each conversion. But if the counter doesn't RESET after the conversion, and rather if it tracks the input by going up or down from its current position, then the conversion would be much faster. And exactly, the same principle is used in the tracking type ADC. So, we will discuss more about it in the next video.
So, I hope in this video, you understood the working of this counter type ADC. So, if you have any question or suggestion do let me know here in the comment section below. If you like this video, hit the like button and subscribe to the channel for more such videos.