Fixing EasyDABv2 to prevent generating noise during cold start.

 

For all boards shipped before 1 March 2018 there is hardware bug found, when EasyDABv2 board starts-up at first time (so-called cold-start) it's producing wide-band noise during 1 second before FPGA boots-up. To fix this issue You need to solder 2K or 2.2K resistor to TXENABLE pin of DAC and to the GND (Ground pad).

Here is a video of turning-on the board (at the top-right corner) and it's spectrum with issue itselve BEFORE FIX and AFTER FIX:

   

Explanation

As You can see, when board is powered-on, the spectrum becames dirty immediatly (during first 1 second), until FPGA boots-up and take control of the DAC. The problem is that AD9957 starts working immediatly after power-up, and the MASTER_RESET pin just applies default register settings to the DAC, but when TXENABLE is logical 1, then it producing noise. Initially, before FPGA boots-up, that pin is pulled-up, and that was a problem. By adding resistor we pulling-down that pin, so TX is disabled before FPGA gets control of that pin. After fix applied, the initial noise level is highly decreased and will not overload output amplifiers.

 

 

How to Fix

Fixing up is simple, for revision of board, marked as "DAB6" You can install 2...2.2K resistor as seen on this image (click to see bigger size):

easydabv2_noise_fixup      easydabv2_noise_fixup_zoomed

For older board designs, just look for 680 OHm resistor that goes to red "UNDERFLOW" led. After finding it, You need to install pull-down resistor to opposite side (that goes not to LED but to DAC and FPGA).

After this fix applied, You will see that red led, marked as "UNDERFLOW" will be ON immediatly after power-up, so DAC will be off and not produce noise signal when powering-up the boad.

It is not recommended to use output amplifiers with boards that have no fix, described on this page!

NOTE: YOU CAN BURN YOUR OUTPUT AMPLIFIER IF THIS FIX IS NOT APPLIED TO YOUR BOARD!

 

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