PC Based oscilloscope problems

[engcu90]

Hello
I want to construct PC based oscilloscope. I made a simple design and these are problems that face me:

1- My adc range from 0:5 v and it doesn't read -ve values. So I thought about make a dc offset to overcome this problem. But the question now is that i want to make Variable offset. I mean for example if signal from -2 to 3 so offset is +2, and if signal is from -1: 2 so the offset should be +1. So how can i make this Variable offset?

2- Signals may be more than 5 v in range. For example signal from 0:10 v. I thought about a cct to multiply the signal by 0.5. But remains the same problem, I want a Variable multiplier. How i can define when to multiply by 0.5 or 0.2 or 0.1 or 1?

Note: I am using pic16f877a and its internal adc.

I would be grateful if you have other comments or ideas of design about my project.

Thanks

 

[amjadali56]

what port u r using? RS32 USB or LPT?

need ur code for pic16.

 

[amjadali56]

what port u r using? RS32 USB or LPT:?: and also i need ur code for pic16.

this one i used and worked for me.

 

[betwixt]

I would suggest you leave the zero offset at 2.5V to get maximum ADC range and use switched amplifiers or attenuators to scale the signal as required. I do not understand why you want to change the offset to measure a wider input range as it does not improve accuracy.

Commercial systems work by trying to maintain the reading within range by "auto ranging", basically, if the reading is below a threshold, the amplifier gain is increased and if the reading is above a certain threshold the gain is reduced. If you do this to maintain a measurement in the region 0f 80% - 90% of full scale it should work well and still be bipolar.

Brian.

 

[elrayes]

you can use PWM output from the microcontroller to create variable offset, just you will need to connect a smoothing capacitor on the PWM output to get a smooth pure DC signal, the calculation of the offset value will be proportional to the duty cycle value which you can calculate.

let me know when it works with you