ADC troubles

[andy257]

Hi All,

I am starting a project using an FPGa to control a ADC. The input to the ADC will be audio. My problem comes with connecting the microphone up to the ADC. The ADC chip is a single supply chip going from VCC to GND.

Now i am a little confussed here whether i have chosen the correct ADC for my application. If we imagine the audio signal input will be a sine wave it will have a positive cycle and a negative cycle (obvious). However my ADC only deals with positive signals as there is no -ve VREF.

My other confussion is even if i did use a ADC chip with a +ve and -ve REF how does the digital bits correspond to a negative voltage?

the ADC chip i am using is the ADCS7476.

datasheet
www.national.com/pf/AD/ADCS7476.html

does anyone have any experience with interfacing ADC, i would appreciate any comments,

Thank You

Andy

 

[Nigel Goodwin]

You bias the ADC so it's at half voltage with no input, then positive values increase the reading, and negative ones decrease it.

 

[andy257]

Thanks for the reply nigel.

How do i bias the input, will a simple resistor divider do the trick or is there a better solution. op amps? I want to keep noise to a minimum.

Thanks

Andy

 

[Nigel Goodwin]

Two resistors would be fine, but bearing in mind you're probably feeding it from an opamp anyway?, then simply design the opamp to have half supply on it's output (no spilt supply, just a divider to bias it).

 

[audioguru]

The opamp will be the preamplifier for the microphone. Because the output level from a microphone is very small (unless it is inside a bass drum in a band).