voltage divider for oscilloscope.

[fugrammer]

i have made a lcd oscilloscope with my pic. However, it is only able to measure voltage from 0 to 5V. I wish to measure a wider range of voltage. What is the best way to achieve this or simply a potentiometer will do the job?

Thanks.

 

[Nigel Goodwin]

Use an input attenuator for higher voltages, or an amplifier for lower ones - just like an oscilloscope does.

If you check my PIC analogue tutorial hardware it uses that method to give 0-10.23V using a 2.5V reference.

 

[fugrammer]

thanks nigel. your circuit is helpful. i do not understand why the source impedence must be low (<2.5k for pic18f4480). wouldn't this means when a voltage measure is taken it would not be so accurate? is this the reason to place a buffer?

 

[Nigel Goodwin]

thanks nigel. your circuit is helpful. i do not understand why the source impedence must be low (<2.5k for pic18f4480). wouldn't this means when a voltage measure is taken it would not be so accurate? is this the reason to place a buffer?

The source impedance needs to charge the sample and hold cpacitor in the A2D, so it has to be low, or it won't charge fast enough and give incorrect readings. The buffer provides a low impedance to feed the A2D, and a fairly high input impedance so as not to load what you're measuring.