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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

Parents
  • Hi Qui,

    1.

    I did understand the open-collector output mode. But giving input voltages Vin+ and Vin- what shall be the output currents of Vout+ and Vout- ??  The best I could get from the datasheet is that positive and negative inputs are amplified separately to positive output and negative output, with very well matched gain. But then the "single-ended => differential" function isn't what I should be looking for.

    If the Vout+ is somehow dependent on Vin- (and vice versa), then this is not clearly stated on the datasheet (or I didn't find it). And from your answer I sitll cannot calculate (without trying with the actual device) what shall the output common mode voltage be in the case I replace output bias inductors with bias resistors (and remove all DC-blocking capacitors).

    2. My application requirement is down to DC, but other ADC performance is not critical at this moment. I just want to avoid doing another single-ended => differential circuit, if possible at all. The AD8376 just happens to be in the convenient place and I'd like to use it.

    If it is definitely impossible, then I shall build operational amplifier based adapter.

    BR, -Topi

Reply
  • Hi Qui,

    1.

    I did understand the open-collector output mode. But giving input voltages Vin+ and Vin- what shall be the output currents of Vout+ and Vout- ??  The best I could get from the datasheet is that positive and negative inputs are amplified separately to positive output and negative output, with very well matched gain. But then the "single-ended => differential" function isn't what I should be looking for.

    If the Vout+ is somehow dependent on Vin- (and vice versa), then this is not clearly stated on the datasheet (or I didn't find it). And from your answer I sitll cannot calculate (without trying with the actual device) what shall the output common mode voltage be in the case I replace output bias inductors with bias resistors (and remove all DC-blocking capacitors).

    2. My application requirement is down to DC, but other ADC performance is not critical at this moment. I just want to avoid doing another single-ended => differential circuit, if possible at all. The AD8376 just happens to be in the convenient place and I'd like to use it.

    If it is definitely impossible, then I shall build operational amplifier based adapter.

    BR, -Topi

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