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Double transformer ADC input

Question asked by gavint on Jul 6, 2015
Latest reply on Jul 8, 2015 by gavint

Hello

 

I have an ADC front-end that consists of the following components:

4:1 RF transformer (Coilcraft WBC4-1TLB, rated 250kHz to 750MHz)

ADC driver (Analog Devices ADL5565)

ADC (Analog Devices AD9467)

 

The ADC is clocked at 204.8MHz.

 

The input frequency range of interest is 1MHz to 200MHz.

 

The required SFDR over this range is 80dBc.

 

I am trying to optimise the performance of the ADC input stage. Sometimes the produced boards achieve 80dBc SFDR, sometimes they don't. Specifically, it seems to struggle at 1MHz and at 200MHz (the results in between are typically OK).

 

I have read about the advantages of a double transformer configuration to reduce second order harmonics. However, I have only ever seen it being used with 1:1 transformers (with the secondary of the first transformer connected to the secondary of the second transformer in a back to back configuration).

 

Does the double transformer configuration work if the impedance ratio is not 1:1? In my case, I require the transformer to provide gain so I cannot use a 1:1 transformer. Although the ADL5565 can provide the additional 6dB of gain instead of the transformer, the performance of the ADL5565 will not meet 80dBc SFDR at 200MHz when configured for 12 dB of gain (according to the datasheet).

 

Any advice would be greatly appreciated!

 

Many thanks

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