can ADC read varying offset voltage ?

[alarasi]

hello guys

i want to feed an analog bipolar signal (that has negative and positve values) into ADC of a PIC whic only receives positive values!
so i had an amplifier which bias the output signal into half of the supply voltage which means that if the supply voltage is 5v then the reference voltage will be 2.5v (instead of 0v) so that i can feed the signal into the ADC to acept the bipolar signal!

i'm using a mic to detect sound then preamplifiy it, my problem is that when i detect sound the reference voltage (let's assume 2.5v) varies with the sound detected -,+ 0.1-0.9 added to the reference voltage while when i remove the mic it becomes stable!

my question is , does the ADC reading get affceted if the reference voltage or offset voltage varies and unstable ?? or it stll can read and convert the voltage?

thank you

 

[karesz]

HI,
I must assume that your "ref voltage" miss all capacitor, especially a bigger electrolytic!? :-(
You must know pls; these sort of "Uref" is not a reference source!...
You need for a good ref. low impedances & an attenuator is not lo impedance source!
It will be better if you buffer it w. an OpAmp, but a real usable Uref is only from a dedicated Ref-voltage IC to become; ev. you can take a zener & buffer/amplify it, but it will be never so good as a reference element...
Your ADC can not sense a difference between variable Uref + added signal as you can not tell too what is the water in your river from the Nr 1 or Nr 5 smaller rivers their are flows in that...!
K.

 

[alarasi]

HI,
I must assume that your "ref voltage" miss all capacitor, especially a bigger electrolytic!? :-(
K.

i didn't understan what u mean here!

the output of my circuit is from a preamplifier circuit and then low pass active filter which has low impedance! it's a bipolar signal with a positive value above 2.5v and below 2.5v represented as negative voltage! it's like DC is carrying AC signal but the 2.5v is fluctuating up and down due to the AC signal! i don't know if it's normal and i can feed it to ADC now or no?

 

[karesz]

Maybe a schema(picture) tells more as thousand words ...

 

[alarasi]

sure here's the circuit
assume the supply voltage 5v

 

[karesz]

Hi,
I think your circuit is missing only a second (after filter) AC-coupling ...
K.

 

[alarasi]

if you mean before the PIC , yes because i don't want to set the offset voltage to 0v because in that way the ADC won't be able to read the negative voltage!

anyway my point is, if the offset voltage is 2.5v and fluctuating up and donw with the AC voltage of the signal, does it affect the reading process of the ADC or not ?

 

[nikhil_damle]

configure pic adc input with vref=2.5V as u say the input signal is varying u can have a 1Mohms resistance before the pin to scale down the voltage if it is AC 220V use a 6Mohms resistance