I am making a digital oscilloscope using a PIC18F26K22 and a fast ADC chip. Currently I'm testing a CA3306 (6 bit ADC).

I have the clock out of the PIC feeding the clock-in of the ADC chip at 16MHz and I have managed to read and display traces on my 320x240 SPI LCD.

Instead of having switches on the scope to select v/div, I want to use 2 digital pots to make a voltage divider at the start of the scopes front end. Does anybody know of any SPI digital pots that allow the voltage applied to the top of the voltage divider ladder in the pot to be greater than the supply voltage of the chip?


Thanks for any help

Craig

 

I don't think you will find any digital pots that allow signal voltages to go outside the supply rails. However, I believe both Maxim and Analog Devices offer digipots capable of operating from +/- 15V supplies, if that helps any.

 

Can you show us a block diagram or schematic of what you have in mind? I doubt that will get the performance you want at several MHz with a standard digital pot.
It seems to me that the tricky part will be obtaining a 1 Meg ohm input impedance with digital pots.

By the way, Tektronix published a number of books on scope concepts. This was a long time ago but the concepts still apply.

http://www.davmar.org/TE/TekConcepts/TekVertAmpCircuits.pdf

You can start reading on page 39 but the pages leading up to that have a lot of basic information.