Any suggestions for this design?

Slick05 · Joined: 05 Feb 2005 Posts: 14

Hi, I have a circuit using the 18f458 where I have a microphone going to an amplifer , going to the A/D of the PIC. The microphone generates a signal of amplitude 50mV (roughly). Could you recommend me an amplifer that will give it a 5V peak to peak amplification with a 2.5V DC value? The reaon I need this is I am building a frequency counter, so I need to count the peaks of the incoming signal. Thanks!

dvsoft · Joined: 28 Nov 2003 Posts: 46

hello

you can use a histeresis comparator

look at www.cypress.com

seek: AN2108 - Standard - Hysteresis Comparator with PSoc

Alain

Guest · Posted: Sat Feb 12, 2005 7:45 am

For a frequency counter you probably want to count the zero crossings rather than the peaks. Could you capacitor couple the signal to get rid of DC, compare it to zero with a comparator chip, and bring the result into the PIC through a digital pin? Or use an internal comparator in the PIC if your chip has one. I don't see the need for an A/D in a frequency counter.

Sherpa Doug

Slick05 · Joined: 05 Feb 2005 Posts: 14

Well I have already wrote the software, I just need an amplifier for the input signal....

dyeatman · Joined: 06 Sep 2003 Posts: 1923 Location: Norman, OK

Sounds like a job for a rail-to-rail opamp. You can adjust the input differential to get the DC offset you need.

Slick05 · Joined: 05 Feb 2005 Posts: 14

Yeah, I tried implementing the frequency counter by sampling the signal with the PIC 18F458 and then counting the peaks, but the processor does not have enough RAM to do this robustly. I like the idea of a comparator and counting zero-crossings and sending a pulse to the PIC every zero-crossing. Can anyone recommend me a comparator chip? Thanks!

dyeatman · Joined: 06 Sep 2003 Posts: 1923 Location: Norman, OK

You can use a simple capactive discriminator circuit to get spikes at zero crossing to use for triggering interrupts..

Slick05 · Joined: 05 Feb 2005 Posts: 14

Would I be able to use the comparator built into the 18f458? I've never used the comparator before so I'm not sure how it works...