How to give analog input from 3.5 mm jack to FPGA (Spartan 3e)?

I am working on a voice transmission project by using Spartan 3e. My code works just fine. My problem is that i give analogue input to adc side by using a potentiometer and see the changes (as digital) on leds ( digital output is assigned to leds). Here is the problem, i cut a headphone into two and connected 3.5mm jack side to my phone to play music. There are 3 cable which are golden, blue and red. Golden one is connected to "GND" pin , Red one goes to "Va" pin (at adc input). i didnt touch the blue one.

When i play music, i see no change on leds despite the fact that i did see by using potentiometer. What am i missing ? Do i have to setup an external circuit before giving the analog signal to Va input ?

Thanks in advance.

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+1 to Downunder, unless the ADC on the Spartan accepts a bipolar input, you will need a level shifter on your input.

Also, the signal from your microphone is going to be VERY small. Have you built a pre-amplifier into your circuit.

AND can your code handle audiofrequencies - you can't modulate a pot at 4kHz. Connect a sound card to the input to your circuit BUT SEE MY COMMENTS BELOW

The simplest method is to bias the input of the ADC with a high value resistor network, say two 100K resistors, 1 tied to Vcc, one tied to ground, and the common to your ADC input.

Use a capacitor, like a 100nF from your sound card to the common point.

CuriousPuppy (author)  steveastrouk1 year ago

I am adviced the built the same external circuit to amplify the signal. I'm going to buy the necessary equipments and write the result here. Thanks for the help.

Don't forget to mark the Best answer then...

CuriousPuppy (author)  CuriousPuppy1 year ago

to *

The potentiometer provides a changing resistor - if it is connected directly to your inputs.
Music provides a +- voltage - did you account for a voltage input or just for a resistor?