Signal Amplitude drop due to TVS diodes, Impedance Matching issue

[Kamran786]

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.

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.

 

[FvM]

TVS issue sounds like you are using unipolar TVS diode, but it's just a guess unless we see a meaningful schematic.

Not sure why you want to increase generator impedance? In any case, it will involve voltage drop.

Ferrite bead can be viewed as a lossy inductor. Impedance specifications are usually given for 100 MHz, impedance at 5 MHz is only a small fraction.

 

[D.A.(Tony)Stewart]

When impedances are matched the amplitude reduction is exactly 50%.

With 100 uH inductor in series at 5 MHz the impedance is near the midpoint of the sensor range or 5.1k. Z(L)= 2 pi f L

 

[BradtheRad]

It sounds as though you want your signal through low impedance to match to a high impedance? If this is the case then you can obtain voltage increase but Ampere decrease. Power dissipation appears unavoidably several percent. The goal is to take the initial combination of voltage and current, and convert it to a new combination which is more useful to the following device.

LC networks are commonly used for impedance matching although your optimum arrangement is not necessarily the same as my simple theoretical simulation below. LC ratio is adjusted to accommodate available Ampere level.

 

[D.A.(Tony)Stewart]

What electrical parameter do you wish to measure using the Microfluidic sensor, considering impedance is a function of R,L,C,f or is it phase, surface velocity, Q, delay or ?
Your choice of Op Amp is current feedback, very high BW and near 0-ohm output impedance with negative feedback.
Why do you need a TVS? If it is polar type, you had it backwards and will add small capacitance in the correct orientation with cathode bar to +ve voltage and anode to ground.