Started working on a new USB PC oscilloscope.
It uses a pair of A2D's and 50MHz PIC microcontroller.
I wanted to proceed slowly so I started off with channel 1 A2D IC fitted and it worked a treat.
I added channel 2 A2D and just got terrible noise.
The circuit looked fine to me.
Then spotted A2d on channel 2 enable was always low so making 2 A2D's data buses interfere with each other.
Turned out the A2D enable pin was also used as USB pin.
So disabled it as USB pin.
Still noise.
So back to datasheet on AD876 A2D.
Read it through slowly and carefully until I came across the tri state pin which enables databus output.
Then spotted delay from tri state changing state requires 150nS.
I was expecting just a few nS so that completely messed things up.
So moral of the story is read whole of data sheet first before using an IC.
It uses a pair of A2D's and 50MHz PIC microcontroller.
I wanted to proceed slowly so I started off with channel 1 A2D IC fitted and it worked a treat.
I added channel 2 A2D and just got terrible noise.
The circuit looked fine to me.
Then spotted A2d on channel 2 enable was always low so making 2 A2D's data buses interfere with each other.
Turned out the A2D enable pin was also used as USB pin.
So disabled it as USB pin.
Still noise.
So back to datasheet on AD876 A2D.
Read it through slowly and carefully until I came across the tri state pin which enables databus output.
Then spotted delay from tri state changing state requires 150nS.
I was expecting just a few nS so that completely messed things up.
So moral of the story is read whole of data sheet first before using an IC.
