Strange behavior ADC4 only.

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 18, 2016 by lpcware
Content originally posted in LPCWare by rody on Tue Mar 15 01:07:00 MST 2016
I use the ADC in burst mode on DMA, but I also test it without DMA (polling). The global ADC register is not used.
I excluded reentrancy problems, I'm sure that is not the issue.

The first problem was (but is now solved) was glitches in all ADC's. Like everybody else I just filtered it. BUT it is now solved!
The ADC clock must be <= 13MHz. Standard when using lpcopen the adc divider is 4. When the pclk is set to 96Mhz (like many will do) the adc clk will be to high (96/4=24MHz). I set the divider to 8 and the glitched are gone.

But the main reason I looked at the timing and init was because I had not only had glitches on all channels but particularly ADC4 was very bad.
This problem came with a new batch of hardware and was not before. So without changing the software only ADC4 has strange behavior on the new hardware.

[u]This leave me to the problem I have now[/u]:

I connected a voltage using a opamp (buffer), the output of the opamp is stable and is not oscillating. Each channel has is own opamp the input to all opamps is the same voltage.
1. All channels give a stable value but the value on ADC4 is fluctuating a view bits, even after filtering (with large T).
2. When the software is started by applying the voltage to the target, i have another value than when I reset the target (RST pin).
    - starting the target -> value = 2704
    - after resetting the target -> value = 2720
This can be easily reproduced!
When I cool the target a bit the value is the same a startup and reset. I warm it up (just to 30 decrease Celsius) the problem occurs again.

I did try many things in hardware and software, but nothing really works. Adding a small C (22pF) to the ADC input does help, the value then only changes 6 (2704, 2711)
If i connect a voltage directly to ADC, so no opamp, the value is also ok. I seems the opamp output is not hard enough, but measuring on the opamp output with a  6 1/2 digit meter or scope tells me that the output voltage is stable an is not dropping.

Anyone else with this problem on ADC4 only?