Analog to digital-iser accepting low level inputs?

Thread Starter


Joined Sep 18, 2010

I'm trying to make a circuit that takes analog and digital signals and converts them to digital. I.e. for an input voltage of -3v<in<3v the output becomes zero and everything else leads to 5v output. I would also like to isolate the output from the input.

The absolute value of the input is supposed to be <=35v and I'm not trying to protect against input of more than 35v. What I want to make sure is that the output will never be outside 0-5v and that the circuit will accept inputs as low as 3.5v. Also, I want the circuit response to be fast, i.e. much less then 1ms.

Currently, my circuit is as attached. The problem with this is that because of the three diode voltage drops a digital signal will not be able to be passed through because a 3.5v signal will already be 0 after the drops.

I've considered putting a switch where if you know the input is a digital signal you could skip the circuit or at least the bridge rectifier. However, I'm worried that by mistake I'd connect -30v to the input when the circuit is switched to skip the protection and destroy the next circuit in line. Also, if the input ressitor is too small I'd draw too much current from the logic source.

I've considered using a pre-gain stage with this circuit, or comparator instead of this circuit, but because they are powered they'd break the isolation.

Any ideas?




Joined Sep 9, 2010
Are your signals essentially DC? I mean, if you have AC you could use capacitive coupling and get a degree of isolation.

I think you'd be better off to use a comparator on the input for this, and then work on producing an isolated output if that's important. A comparator can have a wide common range and depending how much isolation you need, you might set up a differential comparison that's floating a bit relative to your circuit, as long as both input are still in the common range.


Joined Sep 9, 2010
Hmmm... perhaps that's directed at me?

Let me elaborate a tiny bit. I think you could set up two comparators as a window, to look for voltage ABOVE 3.5v or BELOW -3.5v. The comparators would share the same ground as the input. Then OR the two outputs together and - if you must - drive a transistor to power an LED that opto-couples to an isolated output. So any voltage outside the window turns on the LED to produce a 5v output.


Joined Sep 9, 2010
Reading shouldn't have to be painful. :D
Hah! On this forum, it can be half the challenge. :p No offense to this particular OP, but sometimes you wonder how the posters think we could possibly help them when so little information is offered, and when it is given without spell-checking, punctuation or grammar. I guess I should take it as a compliment that they think we are deities able to discern their questions with the vaguest of clues.