Analog input protection - LittleFuse SP720 - lots of cross-talk

Hi Everybody,

I've got 16 analog input channels coming into my device, that need to be sampled using an ADC.
These inputs come from the Piezo of an electronic drum sensor.

For input protection I naively chose LittleFuse SP720 (14 channel TVS array - SCR) however much to my amazement feeding a perfectly "legal" (within 0-5v volts which are my supplies) I see significant - ~50mV - sine wave on the other channels (the closest the pin to the "aggressor" the bigger
the cross-talk).

This surprised me as I was not aware that such an IC would introduce cross-talk.

Being a newbie regarding input protection I guess my questions are :

- has anybody seen this, with the SP720 or with a similar device? are such devices meant to protect only digital stuff which might not care about 50mV?

- anybody has a good suggestion for alternatives? Actual tried+tested IC/diode names would be great


Many thanks in advance !!

BT.

I'm using SP720 for analog circuit protection in a number of instruments since about 10 years. I must confess, that I didn't yet measure the devices crosstalk capacitance (input pin to input capacitance), but I won't expect a value above 0.1 pF or so. In other words, the external circuit crosstalk capacitance will be most likely larger.If you are dealing with high impedance input signals, it may be unsuitable to bring the signals close together at all. In this case, you won't want them to share a common IC, whatever it is.

SP720 is particularly good for low capacitance protection of high speed signals. For a drum sensor, regular silicon diodes seem sufficient in my view. Or just a series resistor, utilizing the internal substrate diodes of the connected ADC or amplifier chip.

If the drum pads are home made and have no added resistor you may want to consider adding a load resistor of maybe 100k. That would probably help with the crosstalk and stop the drums being too "hot".

Keith

Hi Guys,

Thanks for your replies & apologies for my late response !

FvM, can you please elaborate on "high impedance" inputs what is meant by that and how will that have an effect?

keith1200rs, yes I do plan to use resistive attenuation on the pads for sure, as they can go up to even 80volts or so if I remember correctly from when I measured it.

BTW the issues happens with both DIP and SOIC packages.

I've also asked LittleFuse support on this and they said they'll check. I'll post updates when I have them.

Checking the cross-cap of inputs 1 and 2, I actually don't remember the value but it was very very low indeed. However the problem persists.

So right now looks like plain diodes + waiting for little fuse's response is the best options I have.

I was referring to high source impedance. Obviously the succeptibility to crosstalk is proportional to the source impedance.