ADG1434 VOUT "ramp" behavior

I'm operating an ADG1434 at a relatively slow pace (~40 kHz) to produce an RC shaped square wave. I am curious about how the output of the switch should behave once the S and D pins are nearly the same voltage. I have attached an image of the basic setup -- my "output" is on the drain and is simply an RC low pass filter (200 ohm, 2.2 nF) to shape the pulses I want to create. Each source is held at a DC voltage by an op amp -- currently I have one source set to ~200 mV and the other set to 13V. Scope shots are attached of the source and "vout" points from the setup drawing. The behavior that I see is that when the output is switched it follows the typical RC time constant until the voltage at the output is near that of the source. After the initial edge one can see that the output voltage is about 400-600 mV different than the source voltage and what should be the "flat" portion of a pulse instead has a very slow time constant that almost looks like a linear ramp approaching the source voltage (and the source voltage remains constant -- it has not drooped or changed to affect this output). If not constantly switched the output does reach the source voltage after sufficient time is allowed to pass. The switch digital input is being driven by an FPGA with a 3.3V IO standard. I see this behavior on all channels of the switch.

Four images are attached: one describes the rough test setup (although I've mentioned that each source is driven by an op amp circuit) [adg1434.png]. Two images show the static source voltage measures near the source pin along with the output after the RC on either edge[tek0027.png and tek0029.png]. This mainly demonstrates the "gap" between the source voltage and the output voltage along with the slow ramp approaching either source voltage. Another scope shot shows the digital input pulse vs the output pulse [tek0028.png].

Is this the expected behavior of the switch in a scenario like this? Does anyone know the source of the slow "linear-ish ramp" time constant after switching?

  • One more note that I forgot: Supplies are set with VDD = 15V and bypassed with a ceramic 0.1 uF cap. VSS == GND == 0V.

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    •  Analog Employees 
    on Oct 28, 2020 1:46 PM 1 month ago in reply to bogomips

    Hi,

    Thank you for getting in touch. The ADG1434 can pass signals from Vss to Vdd so it should not be causing any drop as you are showing. Have you tried this setup without the op-amp? Certain op-amps have a headroom restriction on their output voltages.

    What op-amp are you using to supply the voltage for SA & SB?

    Regards,

    David

  • Hello,

    Thanks for replying. The op amp is an LT1499 also powered with 15V on V+ and 0V on V-. The voltages being output by the LT1499 meet its specs. I also included the output voltage from the op amp in the scope traces attached (blue traces in two of the images). It's very stable and at the desired voltage -- I don't see the input to the switch having issues when switching or at the wrong voltage or drooping as can be seen in the scope snapshots.

    I have not tried the setup without the op amp -- I would need to build a separate setup in order to do that, I may end up building a test setup to sort this out...

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    •  Analog Employees 
    on Nov 2, 2020 12:20 PM 29 days ago in reply to bogomips

    Sorry, I see now you mentioned the blue traces were the op-amp outputs. That should rule out that issue.

    Is there any other load on the switch, do you know the specifications are of the scope probe you are using?

    David

  • >Is there any other load on the switch, do you know the specifications are of the scope probe you are using?

    There is no other load on the switch other than the RC load shown (each channel on the ADG1434 has its own independent RC load -- all the same values) in my drawing (and the probe of course). The scope probes are new Tektronix 10x probes that came with an MDO4000 series Tektronix scope (looks like 10 Mohm, 12 pF or better -- I'm not currently in the same location as the probes).

    >Sorry, I see now you mentioned the blue traces were the op-amp outputs.

    And those traces were probed right at the input to the ADG1434 just in case some kind of PCB trace issue was causing a problem.