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Major Circuit Board Re-design/ Updated Gerber files and Design Spark project files

A project log for DAV5 V3.01 Raman Spectrometer

The only thing worth doing, is the thing worth doing right!

david-h-haffner-srDavid H Haffner Sr 05/02/2017 at 21:410 Comments

I have been working with David Allmon on this CCD driver circuit for a couple of months now and there seems to be a unique problem with the circuit and I do not want to prematurely post the results nor post erroneous build files so there needs to be some explanation and also some help from anyone out here with mucho experience in electrical engineering, especially with ADC circuit design and operation.

Apparently, David Allmon didn't have this problem until he started using the AD7667 ADC, even though the data sheet calls for the AD8021 op amp as it's pair up component. We have been kickin' ideas around and this guy is brilliant no doubt, I just think the more grey matter on this the better, it would be so greatly appreciated!

Well here is part of the problem;

"The ADC is prone to latchup when OVDD is brought up at the same time as DVDD. OVDD needs to be off until DVDD has reached 2.4V or so. There is a fix for this. I cut the trace around pin 18 of the ADC in two places to isolate it from the 5V supply. Then I run a wire from pin 1 of the ATmega1284 to pin 18 of the ADC. A little firmware tweak and the OVDD line is controlled by the MCU, which brings OVDD up after its own 5V comes up. OVDD on the ADC only draws around 200µA, so it is safe to power through the MCU. OVDD is the voltage supply for the MCU interface on the ADC.

Even though Dave made the fix for this both on the board and in the firmware, there can still be a problem with latchup if you have the USB plugged in before powering up the board. The board will be powered by the current going through receive line and the protection diodes. It will be powered and the MCU can't prevent latchup on power up. Not sure why. It is probably already latched up at that point. On all boards I do with a USB-TTL converter I have begun putting a 2-transistor buffer that leaves no path for the USB-TTL transmit line to get to VCC."

Ok, so that's that little tidbit. Here is the link to his website for the full explanation; http://davidallmon.com/pages/ad7667-spectrograph - board re-design and circuit changes

Also you will find the new gerber files and Design Spark project files there also, I will have them available here and on Github also.

Ok so that's the status update we are workin' hard on this but it never hurts to get a little help from UR friends!

Thanks David H Haffner Sr :)

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