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AD9684 Performance Variations Across Soft Resets

Category: Hardware

Hi there,

Summary:

  • AD9684 shows appearing and disappearing signal image after each (soft) datapath or DDC reset.

Setup:

  • AD9684 samples collected using ILA right after input DDR decoders in FPGA.
  • Input: single tone at ~375 MHz (second Nyquist zone)
  • Input amplitude: -10 dBm
  • Sample rate: 491.52 MHz
  • Decimation rate: 4
  • NCO Mode: Fs/4
  • Using DDC0 only using only one ADC channel.
  • Output complex-to-real stage disabled.

Detailed Description:

We're using the Fs/4 mode to bring down a single tone to baseband. Sometimes after a (soft) datapath or DDC reset, we see a mirrored image of the downconverted input tone with a much lower amplitude (images attached).

This image seems to appear and disappear only by resetting either the data path or the DDC in an unpredictable manner. All "resets" are done using SPI.

ADC Bring-Up Sequence:

  1. spi write addr: 0x000, value: 0x81
  2. sleep 0.1 seconds
  3. spi write addr: 0x001, value: 0x02
  4. spi write addr: 0x008, value: 0x02
  5. spi write addr: 0x015, value: 0x01
  6. spi write addr: 0x008, value: 0x01
  7. spi write addr: 0x03F, value: 0x00
  8. spi write addr: 0x040, value: 0x3F
  9. spi write addr: 0x200, value: 0x01
  10. spi write addr: 0x201, value: 0x02
  11. spi write addr: 0x310, value: 0x60
  12. spi write addr: 0x300, value: 0x10
  13. spi write addr: 0x300, value: 0x00
  14. spi write addr: 0x001, value: 0x02

Sample Captures with and without the Unexpected Image:

Each screenshots was captured after running the above bring-up sequence on startup. We will then observe a constant image amplitude across different captures until the next write to the datapath reset. When we do this, the image amplitude varies unpredictably as shown in the figures below.