Part Number: DDS39RFS10
Other Parts Discussed in Thread: DDS39RF10
Hi TI,
I'm using the DDS39RF10 in DDS mode (no JESD204C link) and observing only a DC bias at the DAC output with no signal waveform. I need help identifying the issue.
Setup:
- Device: DDS39RF10
- External clock input (CLK+/CLK-): 3.5 GHz
- Target output frequency: 355 MHz
- Output mode: NRZ (355 MHz is within the 1st Nyquist zone)
- Output measurement: Differential probe on DACOUT+/DACOUT-
- DDS mode: JESD_EN = 0, DDS_EN = 1, DP_EN = 1
Configuration completed (per Section 8.1.2 DDS Mode Startup Procedure):
| Register | Address | Value Written | Readback |
|---|---|---|---|
| JESD_EN | 0x0100 | 0x00 | 0x00 |
| NCO_CTRL | 0x0300 | 0x03 | 0x03 |
| JESD_M | 0x0102 | 0x02 | 0x02 (1 DDS channel) |
| DUC_FORMAT | 0x02E3 | 0x00 | 0x00 |
| DAC_SRC | 0x02E4 | 0x01 | 0x01 |
| MMODE | 0x02E8 | NRZ mode | 0X00 |
| AMP[0] | 0x0318 | 0x7FFF | 0x7FFF |
| FREQ[0] | 0x0320 |
0x19f7390d2a6c4000 (Calculated for 355 MHz) |
0x19f7390d2a6c4000 |
| PHASE[0] | 0x0340 | 0x0000 | 0x0000 |
| DP_EN | 0x02E0 | 0x01 | 0x01 |
| TX_EN | 0x02F9 | 0x03 | 0x03 |
| SPI_SYNC | 0x0304 | 0x00 | 0x00 |
NCO synchronization: Performed SPI_SYNC after writing AMP/FREQ/PHASE. Waited ≥80μs after DP_EN.
Readback verification:
- CHIP_ID (0x0004): 0x003B
- AMP_R (0x0378): 0xFFFF
- FREQ_R (0x0380): 0x19f7390d2a6c4000
- TX_EN (0x02F9): 0x03
- CLK+/-: 3.5GHz（measured via spectrum analyzer ）
Problem: All readback registers confirm correct configuration and NCO synchronization. However, the DAC output (measured differentially on DACOUT+/DACOUT-) shows only a DC bias with no signal waveform.
Questions:
- 1.Is there any additional register or condition required for DDS mode to actually drive the DAC output, beyond DDS_EN, DP_EN, TX_EN, DAC_SRC, and the waveform parameters?
- 2.Is there a specific register sequence dependency I might be missing?
- 3.Are there any known issues with DDS mode output that could result in a DC-only output?
- 4.Is there an internal status register I can check to verify the DDS datapath is actually active and routing data to the DAC?
Any guidance would be greatly appreciated.
