Need to adjust analog signal to digial. (bad description, look inside)

[iso9001]

I have a multiplexed 5V signal that has will only ever have vaules of ~ 4.25, 3.75, 2, 1.25, and .50v

This is all going to a schmitt trigger on a micro. What I'd like to do is set it up so that when the signal gets to 2.0 my micro detects a falling edge. Problem is that the schmitt will only detect a low of 1.0V (5V VDD * .2) and won't reset to high state until 4V (5V VDD * .8).

Also, I can't modify the original signal at all. I though about using a diode or two to the micro, and that would get me low enough, but not ever reset to 4V so thats out.

This is in a power managment mode so I can't use any external parts that draw more then ~200uA (thats about what I've alotted myself).

One idea, i was told to try a mosfet inverter and figure out how to set the trigger point on it. But I have no idea what they meant (set trigger part, mosfet part is easy).

Another idea is that since I already have a free rail to rail opamp I figured maybe I could use that, but its just a thought, nothing has really come to me yet.

Any ideas ?

 

[Papabravo]

A micropower single supply comparator with an open collector output is just the thing you need. You can set the threshold anywhere within the common mode range of the device with a pair of resistors and it will easily interface to a micro with any reasonable Vcc that you care to name.

Start with the LM393 dual or the LM339 quad. If those require too much power then check out "Single supply Comparator" on google or the websites of manufacturers like Maxim or TI or others. Let us know what you find.

 

[iso9001]

with any reasonable Vcc that you care to name.

... lol

I appreciate the help,

I'm looking into compators now. Microchip has some really low Iq ones. Cheap enough.

They have a op amp + comparator that I'd love to get but they want over $2.00 at 500 units... too bad.

I think I can even get away with no hysterisis if I'm tricky.

 

[Papabravo]

Hysteresis is easy to add to a comparator. Google for Schmitt Trigger Comparator Circuits. The idea is to use the output pulled to Vcc or driven to GND to determine the threshold. Search the application notes for the LM393 or LM339. There is a wealth of information out there.