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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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  • Qui,

    This goes into academic now (I opted to use another full-differential operational amplifier, which has a suitable evaluation board available).

    I understood that AD8376 is a transconductance amplifier, thus there is Vin => Iout gain. And the open collector output constraints the output current to 0 <= Iout <= Imax; negative output current is not possible.

    If the open-collector transconductance output (i.e. current sink) is connected to biasing inductor; AND the signal is free from DC component; THEN the DC-component of the output VOLTAGE shall be biasing point (5 V in this case).

    But this is a feature of the biasing inductor only. It is not possible to build an open collector amplifier which works differently, when biased with an inductor.

    But when biased with resistor, the output common mode depends on the output current at Vin_diff=0. And also how the two paths interact together. But definitely the output common mode, with resistor biasing, can not be the biasing voltage (this could be possible only if the output current is zero, when the Vin_diff=0, which wouldn't work as there weren't any room to decrease output current below zero (as the output is open collector)).

    But anyway, I believe AD8376 is a great part when properly used.

    BR, -Topi

Reply
  • Qui,

    This goes into academic now (I opted to use another full-differential operational amplifier, which has a suitable evaluation board available).

    I understood that AD8376 is a transconductance amplifier, thus there is Vin => Iout gain. And the open collector output constraints the output current to 0 <= Iout <= Imax; negative output current is not possible.

    If the open-collector transconductance output (i.e. current sink) is connected to biasing inductor; AND the signal is free from DC component; THEN the DC-component of the output VOLTAGE shall be biasing point (5 V in this case).

    But this is a feature of the biasing inductor only. It is not possible to build an open collector amplifier which works differently, when biased with an inductor.

    But when biased with resistor, the output common mode depends on the output current at Vin_diff=0. And also how the two paths interact together. But definitely the output common mode, with resistor biasing, can not be the biasing voltage (this could be possible only if the output current is zero, when the Vin_diff=0, which wouldn't work as there weren't any room to decrease output current below zero (as the output is open collector)).

    But anyway, I believe AD8376 is a great part when properly used.

    BR, -Topi

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