DAC38RF83: SYSREF and no DAC output questions

Scott Greeley1

Part Number: DAC38RF83

What are the min and max Vcom for the DAC38RF38 SYSREF input? Datasheet only says 0.5V for typical. Also, what is the min Vi(dPP)? The datasheet only gives typ (800mv) and max(2000 mv). We are driving this with a signal that has 1.2V Vcom, and 0.66Vdpp. Will that work? We do not want to AC couple the SYSREF signal, because we are driving it with pulses (ideally 2 per the datasheet) and not a constant SYSREF clock.

Also, how can I test the DAC output, without having a working JESD interface and SYSREF? It appears that page1/2 registers 0x2F and 0x30 can be used to set a constant input to the multi-DUC input (instead of JESD). But when I do that, and set the NCO to a non-zero frequency, and don't see any output of the Vout(1 or 2). I've configured the DAC, and there are no alarms being set & the internal PLL is locking. My application is fairly straightforward, I have DAC input rate of 720Msps, 8x interpolation, using the internal PLL, NCO enabled and set to 0x3000_0000_0000.

If I send you my configuration register settings, can you verify the settings?

thanks,

Scott

over 4 years ago

0 Kang Hsia over 4 years ago

TI__Guru* 78884 points

Hi Scott,

SYSREF related answers can be found in the following post:

https://e2e.ti.com/support/rf-microwave-group/rf-microwave/f/rf-microwave-forum/941686/dac38rf89-minimum-sysref---and-dac_clk---input-differential-voltage-and-min-max-input-common-mode-voltage/3479210?tisearch=e2e-sitesearch&keymatch=sysref%2525252520vcm#3479210

You can use DAC constant input to send the tone at NCO frequency. If you set to 2's complement, you may send up to 0x7FFF of code. I recommend sending 0x4FFF for the constant code in 2's complement as there are 3dB in phase gain with sin/cos with the same amplitude (i.e. sqrt(I^2+q^2)).

0 Kang Hsia over 4 years ago in reply to Kang Hsia

TI__Guru* 78884 points

Scott,

you may need to follow up with a toggle of the SIF_SYNC to initialize the NCO. Please refer to the SYNC_NCO registers to check which signals are needed to reset the NCO accordingly.

-Kang

0 Scott Greeley1 over 4 years ago in reply to Kang Hsia

Intellectual 585 points

Yes, that was my problem. This isn't very well documented (or maybe I overlooked it). I used the GUI (in Simulation mode) to see what registers are written and their order when hitting the 'Update NCO' button. It does the following: writing the Frequency registers (0x1E - 0x20), followed by the Phase registers (0x1C, 0x1D), then followed by toggling the SPI_SYNC bit (reg 0x28 = 0x2 then back to 0x0). It worked when I did this (but not if I wrote the Phase registers before the Frequency registers -- are those all a part of some AUTO_SYNC register set?). I recommend adding that to the datasheet if it isn't already.

thanks!

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DAC38RF83: SYSREF and no DAC output questions