# Maximum Input voltage of the DSO shell oscilloscope and TL084

In jyetech forum FAQ they mentioned that DSO shell is capable of measuring up to 50V directly:

DSO Shell was designed for low voltage measurement. The maximum input voltage was specified as 50Vpk (peak value). So it can not be used to directly measure any voltage higher than 50V.

But by looking at the schematic:

We can see there's no division at U1D, 50V * 5.1M / (510K + 5.1M) = 45.455V. While the TL084 Input voltage is only 15V.

Am I missing something? What's the maximum input voltage for this scope?

• wow, an oscilloscope that uses a 1977 JFET OPamp? an ... interesting choice. – Marcus Müller Nov 4 at 22:13
• @MarcusMüller some analog oscilloscopes have even older technologies but they get the job done... – Hamed Nov 4 at 22:21
• "Get the job done" is very relative. What they get is higher-than-necessary measurement error. Would I have chosen an opamp with multiple millivolt input offset voltage as the voltage follower input stage of an oscilloscope, in the 2010s? nope. Was that oscilloscope board designed with cost and sourcability very much in mind? Yes. – Marcus Müller Nov 4 at 22:32
• @MarcusMüller I didn't buy it yet, while researching for a budget scope I end up with this one. but after reading your comment I think I should keep looking for another one... Thank you. – Hamed Nov 4 at 22:44

If you put 50V directly on the pin of the BNC connector, it would violate the absolute maximum ratings of the TL084.

The magnitude of the input voltage must never exceed the magnitude of the supply voltage or 15 V, whichever is less

At that point the ESD diodes may be turning on (they don't give any details on this) or the FET spec is violated. Although the current would be low (in 10uA range) still probably not a good idea.

So why they kept it to 50V is beyond me. I would say it's 15V.

After 63V is considered High Voltage for some regulatory standards so maybe that plays a part in it.

• The information is just misleading users, who knows how many burned their device with voltages higher than 15V... – Hamed Nov 4 at 22:45
• It's not that it will 'burn up' but it could fail. I think the designers might be counting on the low current into the part, but you can't always count on that with fets, as they can be damaged by voltage also. It's also unclear if the 15V rule is for the FET or protection diodes, but still, not a good design – Voltage Spike Nov 4 at 23:24

It would be possible to blow this scope up at 50V. Here is one way .Say you have a buck converter that runs an input of say 48VDC and it runs a hardswitched squarewave of say 1 MHz .The slewrate could be 48V/20ns .If you did this bench test with a times one probe setting U1d will die probably before R20 burns out .Remember that a high slew rate square wave will cut through C2 like a knife through butter leaving U2d to defend itself.

This could also be a faulty schematic. Looking at the two voltage deviders in front of the OP-amps, the divider at U1A causes a voltage drop of ~5V at R4 which is in perfect working range for the OP-amp. Divider at U1D causes a voltage drop of ~45V at R2 which like others said would harm the amp. If we now swap R1 and R2 we have the same divider ratio as seen at the other amp. This then would also lead to a ~5V drop which is way within working range and also within supply voltage range.

Looking at this it seems like the two OP-amp channels may be intended as "low" resistance and "high" resistance input.

• I found This image of the PCB and it seems that schematic matches the actual PCB. – Hamed Nov 5 at 7:08
• Yes, it does match the PCB. I have also compared this, but looking at several images in the soldering manual it seems like they used different PCBs. This schematic being wrong isn't more than an educated guess but I don't see any other possibilities for a 50V range. – Alexander Nov 5 at 7:15