AD9208 not filtering out DC Tone?

Hi all,

I am using an AD9208 in two DDC mode. Decimation ratio is set to 2. My fs is equal to 2.5 GHz.

The following registers hold the values for initial configuration:

0x0310 - DDC0 Control = 0x63

---> Should be Real mixer, 6dB Gain, Fs Hz IF Mode, Complex to real, HB1 Filter selection.

0x0311 - DDC0 Input select = 0x00

0x0327 - DDC0 Test enable = 0x00

0x0330 - DDC1 Control = 0x63

---> Should be Real mixer, 6dB Gain, Fs Hz IF Mode, Complex to real, HB1 Filter selection.

0x0331 - DDC1 Input select = 0x00

0x0347 - DDC1 Test enable = 0x00

When I use this configuration my output signal on both DDCs results in a signal with a DC bias. Which I assume would have been eliminated due to the mixing with fs/4 and filtering with HB1.

I changed register 0x0310 to be 0x43 to be in variable IF and changed the DDC to be fs/4. In variable IF mode and fs Hz IF Mode I received the same result and the DC bias was still there. Additionally I tried changing the DC Offset calibration register 0x0701 to 0x86 instead of 0x06 and the results were the same. Being that the DC was still there.

Why does changing the DC Offset Calibration register result in no change? Is there an additional register I need to set to enable this?

Thanks

Needed to change to be more specific on my question regarding the DC Offset Calibration register.
[edited by: mkutz at 4:37 PM (GMT 0) on 12 Feb 2019]