I try to use two parallel AD9707 on a mixed signal FPGA board for signal generation @ about 5MHz. Whilst @ the ouputs of both AD9707 a strange behavior can be noticed.
AVDD = 1.8V
DVDD = 1.8V
Diff_CLK = 65MHz
FS_ADJ 6k4 to GND
OTCM to GND
Differential Buffered Output setting using an ADA4807-4
REFIO to AD8656 to ADA4807-4
See schematics:
Register settings
Mnemonic Addr Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
SPI CTL 0x00 1 0 0 0 0 0 0 0 (default)
Data 0x02 0 - - 0 0 1 - 0
Version 0x0D - - - - 1 1 0 1
CALMEM 0x0E - - 0 0 - 1 0 1
MEMRDWR 0x0F 0 0 - - 0 0 - 0 (default)
The ILA curves of the FPGA are the same as what can be measured at the DAC data inputs. e.g.:
Timing of DAC inputs related to CLK seems to be ok:
purple depicts differential CLK input @ AD9707, yellow depicts input of LSB @ AD9707
At the DAC output respectively after the differential buffered output the curves are a way of rippling in a periodic way of 1/32 of the (full-scale) period:
sawtooth as full scale range input - depicted is differentially measured strange behaviour @ DAC output
sinusoidal as full scale range input - depicted is differentially measured strange behaviour @ DAC output
sawtooth only the 5 MSBs as input - depicted is differentially measured behaviour @ DAC output
sawtooth as input _ only the 9 LSBs as input - depicted is differentially measured strange behaviour @ DAC output
What could I have missed? Can this be smoothed without calibration in the first step?
Your help is greatly appreciated :-)
regards Matthias