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I have a digital, single-ended, signal that randomly varies between +400mV (logical ‘1’) and 0V (logical ‘0’). I am looking for a circuit to calculate the DC value that corresponds to the average between the maximum value of the signal (400mV) and the minimum value of the signal (0V). So in this case, 200mV. Please note that this does not necessarily correspond to the mean of the signal, because if there is a sequence of consecutive ‘1’s, the mean will be higher than 200mV and if there is a sequence of consecutive ‘0’s, the mean will be lower than 200mV. I am looking for a simple circuit that will output 200mV all the time.

Is this possible ?

My circuit needs to consume as little as possible, so I’d prefer it be implemented using only passive components ...

I don’t know whether this is possible. Do you have any ideas ?

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    \$\begingroup\$ Sounds like a low pass filter. Set the corner frequency so that it samples several (maybe hundreds) of pulses. \$\endgroup\$ – George Herold Mar 30 '15 at 16:36
  • \$\begingroup\$ This would have to be done with logic. Imagine getting a 6 second long string of zeros - you have no input. Why is this of value to you? Why not use a reference voltage supply of .2 volts? \$\endgroup\$ – Sean Boddy Mar 30 '15 at 16:38
  • \$\begingroup\$ If both the max and the min of the circuit are known, and the desired signal is the median, there is no lower power way of doing that than establishing a reference. \$\endgroup\$ – Sean Boddy Mar 30 '15 at 16:44
  • \$\begingroup\$ What do you need this for? Do you have +400 mV somewhere else? Can you use a simple voltage divider to get 200 mV? \$\endgroup\$ – Greg d'Eon Mar 30 '15 at 16:57
  • \$\begingroup\$ I'm sorry that I am not so sure that I get your problem exactly. but if you just need to make a simple source of 200mV, then I think you could easily adjust it by taking a higher voltage from any source of a accurately constant DC current through a resistor(in series) and then decrease and adjust the voltage by a variable resistant in parallel. \$\endgroup\$ – user70997 Mar 30 '15 at 20:38
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As has been implied in comments, if there is a digital 1 or zero event for some lengthy period of time any peak measurement made will need to rely on analogue values being held on a capacitor and no, there is no way of passively achieving this.

I have previously done this by using "peaks detectors" (an opamp with diode and capacitor) for "storing" the most positive voltage and the most negative voltage - two peak detector circuits are needed. Also, some modest amount of low-pass filtering is needed to be done on the raw signal to prevent noise spikes causing too much of an accuracy limit. Here is the general idea for detecting the most positive peak of the signal: -

enter image description here

For my application the detector needed to be done at very high speeds because I was trying to implement a special circuit for detecting and resolving corrupted high speed data. I used a MAX999 comparator for each peak detection circuit.

If your data rate is modest, a better way would be to use a microprocessor and an ADC and actually measure the levels.

Producing a third voltage (or number) that is the mean of the two levels is of course trivial in comparison.

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Here's a slightly different approach-

schematic

simulate this circuit – Schematic created using CircuitLab

Whenever the input signal exceeds the reference (shown as 200mV) the input is assumed to be 'high' and the capacitor C2 charges or discharges towards 1/2 the input signal with a time constant of 5msec through the CMOS HC4066 switch. U1B acts as a buffer. If U1 is a CMOS op-amp the droop can be kept very low, even if there are a lot of 0's in a row. U1A as a comparator needs to switch much faster than the time constant of C2*(R3||R4) or the accuracy will suffer.

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