adc opamp interface

[5282]

i'm poor man and i not have money to buy new  oscilloscope so i will make one
pleas help me
my adc is 5v reference 5v/255=about20mv 
in some case the signal not swing from 0 to 5v   may be from 4v to 5v
and as example i want opamp amplifying  the signal swing  from 4v to 5v to swing  from 0v to 5v

this picture will explain what i mean
first as example the sine wave swing from 4v to 5v   that's mean 1vpp



and i want opamp convert the signal from 1vpp to 5vpp





what must i learn to do this

[Brutte]

You need a so called "analog front-end".

This is basically something that can:
a) shift a signal (offset)
b) attenuate/amplify the amplitude
c) match impedances of source and sink
d) filter out unnecessary components.

As for examples:
a) sine that is 3V centred can be offset by -1V to be 2V centred.
b) 1 V square wave can be amplified twice to become a 2V square wave.
c) if the source of the signal is a tiny potentiometer and you need to control a 100W DC motor with that then even though voltages ranges may match, direct connection won't work without something that can accept a signal of kohm input resistance and can output a mohm signal for powering DC motor. BTW, ADCs usually need <1kohm signals and many sensors do not offer less than 10k output resistance so even though the span is smaller - same rules apply.
d) a 0V:5V 1kHz sine fed to the ~1sps ADC returns junk. You need to get rid of everything that sticks well below 0.5Hz to be able to get a reasonable 2.5V on the output.

Now a simplest analog front end can be made with passives but unfortunately it is very limiting w.r.t the gain (only attenuation possible) and matching impedances.

I suggest you should get a rail2rail output opamp (like MCP6004) and study differential amplifier. You can make any offset, any gain and any reasonable input and output impedance with that configuration, it is only a matter of picking right resistor values.

[5282]

thank you 
i want simple circuit to study it.

[danadak]

Here is a sound card oscilloscope, freebee. You need to protect
sound card inputs, so google "sound card input protection", a number
of hits. Its is also a function generator, you might need to use OpAmp
scaling on sound card output so you can offset it for use in digital
circuit work.

https://www.zeitnitz.eu/scope_en

http://www.zelscope.com/



Regards, Dana.

[Rerouter]

What your describing is 2 parts, 1 an AC coupling measurment (generally done with a capacitor), and an amplifier.

[rstofer]

Chapter 4 of Op Amps For Everyone

https://focus.ti.com/lit/an/slod006b/slod006b.pdf