Signal Amplitude drop due to TVS diodes

Question

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.

Answer 1

5MHz is a relatively high frequency. Consider the capacitance of a typical 1500W TVS:

The capacitance can be several nF at low reverse bias. This is to be expected, since TVS diodes are designed to handle high currents.

A 2nF capacitance has an impedance of only about 16Ω at 5MHz.

One way to deal with this is to use a diode (a small switching diode, preferably, if the specs work for you) in series with the TVS and bias the TVS with a voltage source through a resistor.

Eg.

^{simulate this circuit – Schematic created using CircuitLab}

That way the signal sees only the 4pF capacitance of the diode provided the reverse bias is 0V or greater, which is almost 3 orders of magnitude better.

There are also low capacitance diode + TVS devices designed for USB lines and similar applications. Here is one I designed in recently. Naturally they are limited in maximum voltage since high voltages are not required for those mass market applications.

To use this kind of device as intended, a voltage such as 5V must be applied to VBUS so that diodes are doing the blocking, not the TVS.

In either case, the diodes can be quite a bit smaller than the TVS because they have a relatively low voltage across them when conducting so the junction capacitance will tend to be lower.