Bob Sawyer

Excessive crosstalk between ADC inputs on an HC08AP64

Discussion created by Bob Sawyer on Jun 6, 2006
Latest reply on Jun 9, 2006 by Bob Sawyer
I am using PA4-PA7 as analog inputs on an 'AP64.  I scan the inputs at an 8ms/channel rate.  On a prototype with an older date code part, I see a minimal and very short (50us) glitch on the PAx pins when switching channels.  On several pre-production boards, I have seen up to 100mv glitches that last the entire 8ms switching period.  The glitch shows a slight decay towards the end of the period.  The amplitude of the glitch seems to vary with the difference in input voltages to adjacent channels.  The bigger the voltage difference, the larger the glitch.  The input pins are being driven through a 1K resistor from LM224 op amps.  There is a 0.1uf filter from the micro pin to ground.  I tried eliminating the capacitor, but it did not solve the problem.
 
I have looked at all of the parts on the new boards, and they seem correct.  The board layout is virtually identical between the two boards.  The only thing I can surmise from this is that the later date code part has a leakage or crosstalk problem between ADC channels.
 
I have looked at the errata, and see no information that relates to this problem.
 
I will be replacing the newer part with an older part on the pre-production board to see if the problem is eliminated.
 
I can find no spec for crosstalk between ADC channels.  Have I just missed it in the spec sheet?
 
Has anyone experienced a similar problem with the 'AP64?
 
The date code for the older, working part is:  CTAW0420
This part has a Motorola logo.
 
The date code for the newer part is:  CTEL0541
This part has a Freescale logo.
 
Both are mask set 2L47P.
 
Thanks!

Outcomes