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I am trying to understand working of the following circuit;
It has Input from 0 to 7V and based on the Input value Output A to D will be High or Low but ...
There seems to be an original idea in this "ladder" - the comparators are turned on sequentially and each of them allows the next.
Okay, here are some more tips to think about:
Is it Inverting Schmitt Trigger comparator circuit?
Yes, it is as the input voltage is applied to the inverting input.
Which Output will be High or Low at first? Out A or D?
It seems all the outputs will be HIGH at first.
How other Outputs will switch accordingly?
They should switch sequentially.
The voltage dividers are not firmly connected to Vcc and ground; instead their ends are "movable". Let's see what happens when Vin begins increasing above zero.
Vin = 0. In the beginning, the outputs of the all comparators are HIGH. It is convenient to check this by going from back to front.
Output D is HIGH since R31 is firmly connected to 7 V... and regardless of the previous output C, the input voltage of the non-inverting input is positive (> 0.7 V). We continue up and see that, for the same reason, the outputs C, B and A are HIGH.
Vin > VH. When the high threshold is exceeded, the first comparator switches and its output A becomes LOW; the 3 k R21 of the second voltage divider is grounded.
Since R23 is connected to HIGH, the second comparator switches and its output B becomes LOW; so the 3 k R26 of the third voltage divider is grounded.
Then, since R27 is connected to HIGH, the third comparator switches and its output C becomes LOW; so the 3 k R30 of the fourth voltage divider is grounded.
Finally, since R31 is connected to 7 V, the fourth comparator switches and its output D becomes LOW.
Vin < VL. When the low threshold is reached, the fourth comparator switches and its output D becomes HIGH; the 6.8 k R27 of the third voltage divider is connected to HIGH.
Since the 3 k R26 is connected to LOW, the third comparator switches and its output C becomes HIGH and R23 of the second voltage divider is connected to HIGH... and so on...
Like a domino effect, once the input voltage exceeds the upper threshold, the comparators start switching from HIGH to LOW in sequence A -> B -> C -> D. When the input voltage goes below the lower threshold, they start switching from LOW to HIGH in the reverse sequence D -> C -> B - > A.
The purpose of Schmitt Trigger hysteresis is to "denoise" the analog input into some binary output according to the spectrum and amplitude of the noise and the signal. Essentially to improve the SNR of the input Analog to Digital output to the fundamental frequency of the input.
Might also beg the question, what was it from? and what was intended? and can it perform any better than a single Schmitt trigger, that is well designed? IMHO, I would say no.
How does this circuit work?
It doesn't.
For Vin >0 and <=7V and R12 = N2V0 , the OUT A will always = N2V0 (i.e. the +ve input is always negative below the -ve = positive input Vin.)
But if R12 is connected to some Positive voltage around 7V, it will work.
I see no methodical filtering or advantage to this design.
Missing information in question "P7V0 = +7V and N2V0 = -2V "
Only makes the duty cycle closer to 55% due to Vin average = 3.5V and Avg{7V,-2V} = 2.5V (assuming correction made per above)
There is some slight LPF effect and denoising by cascading the circuits which could be better achieved by using a better analog "denoise" LPF front end.
