Post Go back to editing

Opam configuration for differential input

Good day ADI community,

I am designing an amplifier for a magnetic sensor with a differential output, this sensor is intended to measure fast pulses (<1ns rise time); CM signals is not an issue. My requirements is to have at least 300MHz BW @gain>10  and a low noise output; the instrumentation amplifiers in the catalog have a low BW, so does not work for my case. I am thinking about using three Op amps to build an instrumental amplifier, I saw that the AD8000 has nice BW and a high slew rate, is there any recommendation about using another op amp for this configuration? Or do you recommend another configuration, perhaps a balun and then a single RF amplifier. I attached a simulation of my measuring system.AD8000_diferencial.asc

Thanks for your answer.

Kind regards. 

Top Replies

    •   
    •   
    Aug 5, 2021 +1 verified

    Hi mier,

    With the high speed that you need, if you're planning on a low impedance (50ohm) input impedance, you may be better off using a high speed differential amplifier like LT6401-20 (1.3GHz) or LT6402…

  • Hi mier,

    With the high speed that you need, if you're planning on a low impedance (50ohm) input impedance, you may be better off using a high speed differential amplifier like LT6401-20 (1.3GHz) or LT6402-20 (300MHz) with differential input, but use only one of the two differential outputs (with half the gain). Some differential amplifiers are not very good with even order harmonics if only one of the two outputs is used. However, since "differential to single ended conversion" is listed on LT6402-20's front page, I'd guess it's worth a try in your application. I did not characterize that device myself so I'm just making some assumptions here.

    I think your proposed 3-amplifier instrumentation setup may end up suffering from resistor matching issues and also the heavy interconnect done with external components may cause settling time issues or instability or at least bandwidth loss due to parasitic capacitance. So, I'd recommend you build it first and not rely on LTspice alone cause there could be subtle differences.

    Regards,

    Hooman

  • Dear Hooman

    Thanks for your answer, I will follow your recommendation, hopefully it works.