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Original question:

Ghz differential to single-ended conversion

OPA855: Question about circuit design

YS Yap
Prodigy 10 points
Part Number: OPA855


Hi,

I am interested to do the same i.e. to convert up to 8 GHz differential (sinewaves) to single-ended.

I am interested to use the OPA855 with a single power supply, connected to 50 Ohm inputs and 50 Ohm outputs.

1. Can I use the OPA855 directly, with the + and - inputs connected to each P and N of the differential, without any feedbacks?

2. For the single voltage supply, the recommended circuit has Vs/2 on all -ve and +ve inputs as well at the output. Does that mean i must supply Vs/2 or does it mean that the output has an offset/bias of Vs/2?

 

Thank you very much in advance to all

  • 0
    TI__Mastermind 18280 points

    Hi,

    For your first question could you clarify what you mean by the configuration you described. Would it be possible to create a TINA schematic with the layout you are interested in implementing. For your second question, for the amplifier to work in any supply configuration, the input common-mode and output range must not be violated. This is the difficulty with using a single supply, as it limits your input and output range somewhere around Vs+ and ground. To avoid violating these ranges, the datasheet highlights the figure in question. The Vs/2 biases both input nodes and output to mid supply, this will allow a wider input signal range than if no biasing was used in the circuit. In order to implement this configuration, the idea is that you do connect a voltage to the suggested nodes in the figure, this is how you bias the inputs/output. I would also like to mention that the amplifier is highlighted to have a GBW of 8GHz, however the part is decompensated which means it requires a gain configuration of 7 V/V or greater to be stable. The small signal bandwidth of the device at a gain of 7 V/V is around 2.5 GHz and a gain higher than 7 V/V would only decrease the small signal bandwidth further. Therefore, 8 GHz input signals would not work for this amplifier, is this the expected frequency of the input signal?

    Best Regards,

    Ignacio