7. Final Report and Issues

posted Feb 11, 2010, 11:45 PM by Bill Bai
The basic voltmeter functionality of the board worked very well.  All of the measurements in all ranges had less than <1.3% error.  

However, the auto-ranging function encountered some issues. The auto-ranging clock is based off of a square wave generated at the Cref+ pin.  The square wave is supposed to be a 500mVpp signal, but it actually turned out to be 100mVpp, likely due to output loading.  A set of transistors was supposed to step this square wave up to TTL logic levels.  But since the signal was too small, this didn't work and thus the auto-ranging clock never actually clocked, defeating the auto-ranging.

There were also two small details in the PCB layout that caused problems. 
I had put this chip on the wrong side of the board so I put the socket on the right side and bent the pins of the IC back to fit in the socket.  As you can see, that is the bottom of the middle chip rather than the top.

In this other case, the IC's turned out to be a bit bigger than was expected.  I had to bend the pins again to make both IC's fit.
Bill Bai,
Feb 12, 2010, 1:05 AM