AD9644 - driving DC-coupled signle ended analong signal

Hi,

I have AD9644 evaluation board, which I need to use with DC-coupled analog signal.

At this moment I am not interested in ADC performance, so e.g. increased noise floor is not a problem.

The evaluation board is built with transformers/capacitors, blocking any DC component in analog signal.

I'd like to connect my analog DC-coupled signal straight to the ADC analog input , as single ended signal. There are two options:

a) Utilizing the AD8376 (U401) of the evaluation board. This requires changing C402/C403/C404/C406 => 0R, and activating active path (R303 => DNI, R309 => 0R etc; C305 => DNI, R311 => 0R etc), and populating anti-alias filters (between AD8376 and AD9644 (with DC capable topology).

And then feeding the single-ended signal to J301 & J303, and grounding J302/J304.

b) Feeding the single-ended signal straight to AD9644 VIN_POS_A/B and fixing the voltage of VIN_NEG_A/B.

This can be done by:

removing T302/T307

C305/C306/C311/C312 => 0R

**

The datasheet of AD8376 isn't precise enough in input => output transfer function. E.g. it doesn't tell how single-ended input to differential output transformation output common mode voltage is generated. A realistic functional SPICE model would be nice too.

Please provide some help so that I can decide which path to take, and what problems I might face.

BR, -Topi

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  • 0
    •  Analog Employees 
    on Mar 8, 2013 11:44 PM

    Hello Topi,

    1. The open collector output mode implies that the outputs need to be pulled high to supply.  The AD8376 is a 5V device.  Therefore, to answer your question, the output common mode voltage will be 5V in the case where you replace the output bias inductors with bias resistors and remove all DC-blocking capacitors.  The AD9644 requires a common mode voltage of 0.9 V.  As a result of this, you will need to do something in between the AD8376 and the AD9644 to go from 5V common mode to 0.9V common mode while preserving your DC signal. 

    Attached are the S-parameter files for your simulation

    Qui

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  • 0
    •  Analog Employees 
    on Mar 8, 2013 11:44 PM

    Hello Topi,

    1. The open collector output mode implies that the outputs need to be pulled high to supply.  The AD8376 is a 5V device.  Therefore, to answer your question, the output common mode voltage will be 5V in the case where you replace the output bias inductors with bias resistors and remove all DC-blocking capacitors.  The AD9644 requires a common mode voltage of 0.9 V.  As a result of this, you will need to do something in between the AD8376 and the AD9644 to go from 5V common mode to 0.9V common mode while preserving your DC signal. 

    Attached are the S-parameter files for your simulation

    Qui

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