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OPA350: Unity gain OPA350 oscillates when driving 10 ft. Coax Cable

David Fishman
Intellectual 485 points
Part Number: OPA350

Hi,

A unity gain OPA350 is driving a 10 ft RG-58 cable terminated in an AC-Coupled 50 Ohm input spectrum analyzer.

A circuit schematic is pasted below. The OPA350 is powered by a +5V single supply. ALA_MID is at 2.5V.

The two inputs that are summed are outputs of a high-gain 200 kHz and 300 kHz band-pass filter.

In addition to the desired signals, the spectrum analyzer is showing an oscillation at 109-112 kHz of significant amplitude.

When the output is connected to a nearby scope (2 ft of coax), there is no oscillation.

Another clue is that when the inputs to the summing amp are reduced, there is no oscillation.

Placing 100 ohms in series with the OPA350 output eliminated to oscillation, but 50 ohms had no effect.

Any insights would be appreciated.

Thanks

  • 0
    TI__Guru 68121 points

    David,

    OPA350 is able to drive directly up to 10uF loads in follower configuration so there is something else at work here (10 ft coax cable ~1000pF) but you failed to attach your schematic.  You cannot paste schematic into this forum post - please click on the Use Rich Formating on the lower right and then use Insert File above to attach your schematic.

  • 0 in reply to Marek Lis
    Intellectual 485 points

    Hi,

    I'm attaching the complete schematic, showing from left to right, the TIA stage, attenuation/gain stage and BPF sections, followed by the summing OpAmp, which drives the cable. When the attenuation stage has the transistor turned on, the oscillation goes away.

    As mentioned earlier, the problem does not appear when viewing the output on a high-impedance scope. Could there be a problem with drive current? The SA has a 50 Ohm input.

    Thanks

    OPA350 Application TINA Schematic.pdf

  • 0 in reply to David Fishman
    TI__Guru 68121 points

    David,

    OPA350 should have no problem driving 10ft of coax cable (~1000pF) - see below.

    However, depending on the magnitude of the output signal, OPA350 may or may NOT be able to drive 50ohm load.  The datasheet specifies the AOL only under 1kohm load (meaning Iout=+/-5mA) and the maximum output current is specified as +/-40mA - see datasheet table below.