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  • Author Author: Zainii
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Signal Amplitude problem due to TVS diodes, Impedance matching problem

Zainii

Hi everyone, I hope you are fine.

I have a question.

I have generated a 4Vpp signal with a 5MHz frequency using the following sequence: AD9851 (DDS) → AD8001 (Op Amp) → Ferrite bead (2KΩ) → Microfluidic sensor

This signal is connected to a microfluidic device whose impedance varies from 3KΩ to 10KΩ.

When I attach any through-hole TVS diode with a Reverse Stand-Off Voltage (V_R) rating of 6V to 30V across the signal, with one pin grounded and the other pin across the signal for circuit protection, it drops the signal to less than 1Vpp.

My questions are:

  1. Why does my signal amplitude drop significantly, even though the Reverse Stand-Off Voltage (V_R) is much higher than the signal amplitude?
  2. I want to match the impedance between my microfluidic device, which has an impedance ranging from 3KΩ to 10KΩ, and my signal, which currently has an impedance of 50Ω without the ferrite bead (2KΩ).
    a)     How can I increase the impedance of the signal?
    b)      When I add some series resistance across the signal, it greatly reduces the amplitude. Please suggest a solution to match the signal impedance with the microfluidic impedance without dropping the signal amplitude.
  1. C) does connecting a ferrite bead of 2KΩ in series to the signal add to the overall impedance of the signal?

The AD8001 has high input impedance and low output impedance, making it perfect for this application.

Your response would be highly appreciated.

  • Just out of curiosity, Why do you want to match the impedance? Is 5MHz the maximum frequency your circuit going to deal with?
    Can you share the part number for 2KOhm ferrite bead? 2KOhm impedance is usually rated at 100MHz. If your signal is maxed at 5MHz the ferrite bead's impedance will be quite low. If you really want to match the resistance, better of using a resistance in series! ( I don't know why it's required )


    Like has mentioned the voltage drop is probably due to the junction capacitance. At high frequencies it provides a low impedance path to ground.

  • in reply to jc2048

    I was just checking some part capacitances to make the same point !

    These might do PESD3V3U1UA  (from NXP).

    But there are many other candidates if the breakdown voltage is too low.

    Google: low capacitance ESD protection diodes

    MK

  • Reverse biased, the TVS is a capacitor (thin insulating layer - the junction - with charge either side). This is what ST give in a datasheet for a 400W SMA part

    image
    So you're loading your output with anything from a few hundred pF to 1.5nF, depending on the voltage of the part.

    You need to look for a part designed for protecting signal lines (generally, they'll incorporate a second low-capacitance signal diode that 'shields' the capacitance of the TVS part until it avalanches, or sit the TVS in a bridge for the bidirectional one). Perhaps look at what's available for protecting RS485 lines, or possibly USB if the 5V isn't too restrictive for you.