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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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  • Hello Topi,

    Here is the paragraph in the datasheet you should be focusing on. 

    Datasheet page 13. 

    "The outputs of the AD8376 are open collectors that need to be pulled up to the positive supply with 1 μH RF chokes. The differential outputs are biased to the positive supply and require ac-coupling capacitors, preferably 0.1 μF. Similarly, the input pins are at bias voltages of about 2 V above ground and should be ac-coupled as well. The ac-coupling capacitors and the RF chokes are the principle limitations for operation at low frequencies. "

    Since the AD8376 outputs are biased to positive supply and given your application is DC coupled, you would remove the ac couplinge capacitors and use a voltage divider to step down the voltage before it is inputed to the ADC.  This will result in substantial signal loss.  Additionally, you would replace the 1uH choke inductors with resistors to get to low frequencies.   

    What is your operating frequency?  The AD8376 may be on the ADC evaluation board but it may not be the best option for your DC coupled application.  I am not sure of the performance spec you're hoping to achieve and the operating frequency but the ADL5561/62/65 and ADL5566 are more DC coupled friendly amplifiers and they have very good linearity performance.  If your operating frequency is from DC to a couple of hundred MHz, I can also recommend other amplifiers from another group within Analog Devices.

    Qui

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  • Hello Topi,

    Here is the paragraph in the datasheet you should be focusing on. 

    Datasheet page 13. 

    "The outputs of the AD8376 are open collectors that need to be pulled up to the positive supply with 1 μH RF chokes. The differential outputs are biased to the positive supply and require ac-coupling capacitors, preferably 0.1 μF. Similarly, the input pins are at bias voltages of about 2 V above ground and should be ac-coupled as well. The ac-coupling capacitors and the RF chokes are the principle limitations for operation at low frequencies. "

    Since the AD8376 outputs are biased to positive supply and given your application is DC coupled, you would remove the ac couplinge capacitors and use a voltage divider to step down the voltage before it is inputed to the ADC.  This will result in substantial signal loss.  Additionally, you would replace the 1uH choke inductors with resistors to get to low frequencies.   

    What is your operating frequency?  The AD8376 may be on the ADC evaluation board but it may not be the best option for your DC coupled application.  I am not sure of the performance spec you're hoping to achieve and the operating frequency but the ADL5561/62/65 and ADL5566 are more DC coupled friendly amplifiers and they have very good linearity performance.  If your operating frequency is from DC to a couple of hundred MHz, I can also recommend other amplifiers from another group within Analog Devices.

    Qui

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