Scope input coupling capacitor discharging & pre-charging?

[Stephen Durr]

Another newbie oscilloscope question.  I recently purchased a used Tek 2236 (works great!) and the operators manual recommends discharging and pre-charging the input coupling capacitor before taking measurements or between probing different points that have significant difference in potential.  The manual says this is to protect the input amplifier circuit from sudden voltage transients and also to protect the DUT from a sudden high capacitive load.

In all the videos I have watched of people using oscilloscopes I don't recall seeing anyone bother to discharge or pre-charge the input coupling capacitor before or between measurements.  I've not seen this topic mentioned in any of the oscilloscope video tutorials that I have watched.  How often do you perform this procedure?  Do you think about this before each time you take a measurement?  Is this only something to do on older analog scopes or is this procedure not required on newer digital scopes?

[w2aew]

Another newbie oscilloscope question.  I recently purchased a used Tek 2236 (works great!) and the operators manual recommends discharging and pre-charging the input coupling capacitor before taking measurements or between probing different points that have significant difference in potential.  The manual says this is to protect the input amplifier circuit from sudden voltage transients and also to protect the DUT from a sudden high capacitive load.

In all the videos I have watched of people using oscilloscopes I don't recall seeing anyone bother to discharge or pre-charge the input coupling capacitor before or between measurements.  I've not seen this topic mentioned in any of the oscilloscope video tutorials that I have watched.  How often do you perform this procedure?  Do you think about this before each time you take a measurement?  Is this only something to do on older analog scopes or is this procedure not required on newer digital scopes?

It is good practice to follow, especially when using higher sensitivity settings (lower V/div settings) on the scope and large DC voltages on the DUT.  On the older analog scopes, this was accomplished by setting the input coupling switch to GND, connecting the probe to precharge (or discharge) the input coupling cap, then switch to AC coupling.  This prevents the input amplifier from seeing a large charge/discharge from the input coupling cap.  As mentioned, it is (was) good practice to do this, but probably rarely done religiously.  Many (most?) modern scopes don't have a GND input coupling position - so the point is somewhat moot on these scopes.

[Stephen Durr]

Many (most?) modern scopes don't have a GND input coupling position - so the point is somewhat moot on these scopes.

Thanks w2aew.  I wonder why DSO's don't have that GND input?  Could they have somehow redesigned the input circuitry so as to eliminate the need for manual capacitor discharge/pre-charge?  If not, then how would one go about doing this with a DSO (presuming a situation involving legitimate cause for concern)?

Btw, I have watched many of your videos and I think they are great!     Your hand drawn illustrations are superb and it looks like you do most of them in ink pen?!  It would take me twenty tries with pencil and eraser.

[w2aew]

Many (most?) modern scopes don't have a GND input coupling position - so the point is somewhat moot on these scopes.

Thanks w2aew.  I wonder why DSO's don't have that GND input?  Could they have somehow redesigned the input circuitry so as to eliminate the need for manual capacitor discharge/pre-charge?  If not, then how would one go about doing this with a DSO (presuming a situation involving legitimate cause for concern)?

Btw, I have watched many of your videos and I think they are great!     Your hand drawn illustrations are superb and it looks like you do most of them in ink pen?!  It would take me twenty tries with pencil and eraser.

With the modern scopes, there is presumably sufficient protection on the front end to prevent damage from the DC charge on the input coupling cap. Damage was highly unlikely with the old scopes too. The main reason for precharging was to make the trace immediately visible and stable on screen, rather than waiting fir the DC to settle out. I guess modern scope makers figured that users would rather wait instead of flipping a switch.

FYI - most of my drawings are done with 0.5mm mechanical pencil, but they just happen too look like ink on camera.

[robrenz]

To clarify for myself and the OP. 

This only applies to AC coupling on a AC signal with a DC offset. Or a DC signal on AC coupling.

Assuming no charge on the coupling cap, no harm can be done to the initial connection other than it taking a while for the coupling cap to charge up to the DC offset level.

Once the coupling cap is charged to whatever the DC offset voltage is, When moving to another point in the circuit it will have that charged cap voltage available to possibly discharge into the new measurement point.

Is this correct?

[rf-loop]

To clarify for myself and the OP. 

This only applies to AC coupling on a AC signal with a DC offset. Or a DC signal on AC coupling.

Assuming no charge on the coupling cap, no harm can be done to the initial connection other than it taking a while for the coupling cap to charge up to the DC offset level.

Once the coupling cap is charged to whatever the DC offset voltage is, When moving to another point in the circuit it will have that charged cap voltage available to possibly discharge into the new measurement point.

Is this correct?

My "opinion" is that yes.

Old oscilloscopes have practice that input coupling selector middle position is GND. So always you swiths DC to AC and opposite, you short input to GND. It was not accidentally designed this way.

It may do some not so nice things if first charge it to 400V and after then discharge it to some sensitive component in device under test what can not handle this Voltage and/or amount of energy.

[David Hess]

If anything, I suspect oscilloscope input protection has gotten worse with integrated designs but measurement applications have become less demanding.  Not as many people are working with high voltage vacuum tube designs.  You also no longer see input ranges greater than 5 volts/div.

Commonly used x10 passive probes add a lot of protection to both the oscilloscope input and the circuit under test unless the probe itself suffers breakdown.

Oscilloscopes with dedicated 50 ohm inputs that supported AC coupling were a lot more susceptible to damage and had special warnings and provisions for precharging.