Hi Friends,

About two months back I had asked a query regarding interfacing a standalone MCU equipment to PC running a custome VB program to plot the characteristics of a Proportional valve.(http://www.8052.com/forum/read.phtml?id=74125 ).

Thanks to your inputs, the project is through and the first plots have come in. The enclosed plot is a good sample except for the problem described below.



I am getting spikes in the ADC readout. What is peculiar is that the spikes seem to be of almost equal magnitude ( as compared to that instant's normal value ) and happening at fairly regular intervals. I have some plots where the number of spikes in +ve half match the ones in -ve half.

Some details :

1. The command (blue) curve is the output of the 8 bit DAC feeding the proportional valve electronics.

2. The pink curve is the feedback from the valve mounted LVDT sensor. ( basically the spool position)

3. The ADC is a AD574 operating in bipolar mode. The circuit layout is as recommended by AD in thier data sheet. The +5, +12, -12 supplies are decoupled by 10 Ohm + 100n+ 10 mfd capacitiors. The input is fed from a voltage follower made of 3140 amp.

4. Pin 9(AGND) and 15(DGND) linked by a short thick track on the PCB near the IC. The +/-12V ground is routed separatley and joins the PCB at this point.

5. The DC supply is from a switcher.

6. The PCB is double sided and has no good ground plane. But similiar layouts earlier have been peaceful without spikes. ( not defending the case but just mentioning it for records ;-)

Things so far tried out :

- Change of ADC.
- Try with another proportional valve.
- Instead of feedback from valve, feed a variable voltage from a 5K pot connected to the +/- 12 V supply of the board.

In all the above cases the spikes remain as they are.

While I fully agree that these spikes can be the outcome of a badly laid out PCB, some offending component on the PCB etc, does the pattern of spikes ring a bell somewhere ? Members with more experience with ADCs can find something at first glance ?

Any help is most welcome.

Thanks

Raghu