Electronic – Fast analog switch overshoot


The circuit purpose is to control square signal frequency from 100Hz to 1kHz (generated by MCU GPIO) and be able to control amplitude from 100mV to 2.3V. It is like a very simple signal generator. The goal is to keep edges <100ns.
Switch is SN74LVC2G53. The schematic:

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Problem: I get ringing on rising edge (ringing Frequency 3MHz), and on falling edge going negative about -600mV.
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Layout(i know it's bad, but i had no choice at this time. I will redesign the PCB later ):
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How can i reduce ringing on falling edge?

Best Answer

You can try wiring it dead bug over a ground plane:

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Source of the picture is here: Ugly is Beautiful!

The chip's VCC pin should be decoupled with a very close surface mount ceramic cap, which will give you a nice low inductance supply.

When the switch switches it will inject some charge into the input. Since the DAC is far away, and probably buffered by a very slow opamp, or unbuffered, its output impedance will be high. So, a 1-10nF C0G or film cap should be placed at the input of the switch to soak up the injected charge.

If the DAC is buffered by an opamp, a low value resistor like 22R will prevent it from having an allergic reaction to a capacitor at the output.

Now, the output impedance of the switch is its internal resistance (6.5 ohms) plus 0R if it's set to output GND, or the 10nF cap if it's set to output the DAC voltage. In both cases this is too low to drive a 50R coax or even a trace without some ringing, so source termination is required: you can add a 33R-47R resistor at the output.

Dead bug can yield extreme performance if you do it right. For example this is a 3.3V 100mA DIY voltage regulator. Ain't it cute? I used Kaptan tape (chinese ripoff of the well known Kapton brand) and adhesive copper tape.

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This is the transient response to a 50mA current step with a few ns risetime, generated by a 74AC14 with all gates paralleled. Yellow is output voltage. Blue is current in the pass transistor. The time scale is indeed 20ns/div.

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