Hi everyone,
I'm trying to get the AD9834 to work with an 8051 controller.
I soldered the device on a TSSOP20 breakout board, connecting it with the controller.
The problem: No signal on IOUT at all. Not even a "spurious" pre-configuration frequency. I just get a 300mV DC voltage after the first reset command.
Things that are working:
1) The 3.3V/GND supply (measured on the components)
2) The VCAP voltage is measured to be 2.5V
3) The REFOUT voltage is measured to be 1.18V
4) The resistance from FS_ADJUST to GND was measured to be 6.8kΩ
5) Same with IOUT and 200Ω
6) I do manage to get some response to commands - the IOUT DC voltage falls from about 500mV to about 300mV after a reset command and the SIGN BIT OUT pin is always high when I turn it on (DB5=1) and DB3=1 and always low when DB3=0.
What I'm doing "wrong":
1) The MCLK is 2MHz and is activated only after all SPI commands are sent.
2) The decoupling capacitors are pretty crappy. Only about 100nF for each rail.
3) There is no pretty GND star. At this point I'm just trying to see if stuff just work, later I'll do it right.
4) There is (see attached) a glitch in the Data and Clock signals betweed words, but CS is high during this time so it's not supposed to matter. EDIT: I found the cause of the glitch and fixed it. The problem remains.
The design is attached - note that the COMP capacitor is mistakenly parallel to ground. In the real circuit it is serial to VCC (just like in the reference design)
Also attached are the SPI signals. The words that I'm sending are:
0x2100
0x4000
0x6000
0xC000
0x2000