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.

 

It is not the reverse standoff voltage that is causing the drop, it is the forward conduction voltage, which for most diodes is less than one volt. So if you put the two TVS diodes in series, opposing it will increase the protection threshold by a bit less than one volt. That may solve the problem.
It might possibly be that the capacitance of the diodes provides a low impedance shunt at a frequency of 5 MHZ. Putting the diodes in series would reduce that effect as well.

 

I want to match the impedance between my microfluidic device, which has an impedance ranging from 3KΩ to 10KΩ

Increasing source impedance NEVER produces a higher signal level. Do the math and it will be obvious.

This is a very common misunderstanding of the max power theorem. It applies ONLY when you match the load impedance to a fixed source impedance. And it results in a voltage drop of exactly half the unloaded signal voltage. It is max power, not max voltage.

Draw a voltage divider, source on top and load on bottom driven by a fixed voltage, and you will see that the max voltage on the load is when the top resistor is zero Ohms.

 

OK, ignore the explanation in post #2 and connect your signal to the microfluidic device, and read the AC voltage with a DVMM and see what you get without the diodes, since a meter like that does not need protection. That will clarify the required input protection for your instrumentation system. Then you can connect the TVS diodes and observe the effect of the forward voltage clamp.

 

OK, ignore the explanation in post #2 and connect your signal to the microfluidic device, and read the AC voltage with a DVMM and see what you get without the diodes, since a meter like that does not need protection. That will clarify the required input protection for your instrumentation system. Then you can connect the TVS diodes and observe the effect of the forward voltage clamp.

When i connected the signal with microfluidic device without the TVS diode, The amplitude of signal drop very less, like it drop from 4Vpp to 3.86 Vpp, which is also acceptable for me, but i am scare, in case of increase of load or something other happen, it will it damage my whole circuit.
I need to provide some protection to my circuit as well.

 

i also applied the signal with 5Vpp with 6MHz frequency to microfluidic device without the TVS diodes, it didn't drop the amplitude of signal.
Does my Ferrite bead resistance that was 2000K ohm also added in my signal impedance or not?
Previous impedance without TVS diode: 50 ohm.
ferrite bead own resistance: 2000 k ohm ?
total impedance: 2000+50= 2050 ohm

 

Probably it made some slight effect.
You can still use the diodes for protection, but put in series, not in parallel. Read my post#2 for the description about how to do that.

 

Probably it made some slight effect.
You can still use the diodes for protection, but put in series, not in parallel. Read my post#2 for the description about how to do that.

Sure sir, i will use them in series the way you told me,
thanks a lot for the help.
but it important to add the diodes for the protection purpose

 

Sure sir, i will use them in series the way you told me,
thanks a lot for the help.
but it important to add the diodes for the protection purpose

It is reasonable to add the protection diodes, and so that is why I told you the correct way to connect them. That way they will provide the specified protection.