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

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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steveastrouk (author)2015-12-20

+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.

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user

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.

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user

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

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Downunder35m (author)2015-12-20

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?

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