# measuring very low AC voltage

Hello,

I'm working on a project where I want to follow signals being sent to an LCD. measuring across an input pin and a backplane pin, I get a square wave of about -1v~1v when a symbol is off, and a more complicated signal that goes about -1.5v~1.5v when it's on. These go at about 500hz or so.

I want to be able to measure the voltage that the signal gets up to, so I know if a symbol is on or off. I've found some circuits online for AC measurement, but they seem to rely on diodes, and I think with such low voltage, the entire signal would get lost in the diode's forward voltage drop. Any ideas for how to measure this?

See if you can get hold of an oscilloscope. www.dpscope.com is a nice one for this kind of stuff, or try a soundcard scope such as Visual Analyzer, free download from http://www.sillanumsoft.org/ Make up a simple probe to plug into your sound card.

I actually probed out the signal on an oscilloscope, which is how I know the signal's voltage and frequency (it was kind of weird, actually, I had always thought of LCD as being a DC thing)

I want to be able to have the microcontroller follow this signal though, without a human intervening.

HOWEVER, detecting the signal on an LCD is a really last-ditch method. I would try VERY HARD to find a more suitable method. Even if you manage to amplify the signal, rectification could very easily mask any slight change in the signal. I would not consider this to be a viable method at all.

I think it's pretty much the best choice in my case. I'm trying to follow the state of an external device, and the controller is pretty much a black box, with no external communication, but all necessary information gets displayed on a purpose-built LCD.

In that case, buffer it with an op amp, give it some gain, then rectify it and run it thru an envelope follower or peak detecter circuit.

My favorite device for measuring low level AC (up to 500Mhz!) is the analog devices log amp, AD8307. While not a cheap device, around \$15 when I bought one, it has tremondous dynamic range and a easy to interface DC output voltage perfect for an arduino analog input pin.

Clearly you need what is referred to as a "precision rectifier" circuit. In these a simple opamp has a feedback loop which takes into account the normal forward voltage loss across the diodes.

Try googling "precision rectifier" or go directly to http://sound.westhost.com/appnotes/an001.htm for a write-up on the subject

jack