Help needed - Siglent TPA10 / Tektronix TekProbe offset not working

[FlexibleMammoth]

Note that this offset feature is separate from the vertical position control. The way the manual describes it, it can prevent the scope from clipping a signal that's out of range, so clearly the offset is applied in the analog input circuit. As far as I know, it controls the offset voltage fed into the vertical amplifier. It might be that when it detects an active probe with offset feature, it supplies a voltage on pin 5 instead, but it's quite hard to measure while a probe is attached, since the case covers the area around the connector.

After watching an excellent video on the topic offset vs. vertical position, I know now why I was so confused: Tek scopes allow to adjust the vertical position on the screen using the knobs at each channel, but also have a menu that converts one of the multipurpose knobs to adjust the analog offset voltage inside the front end.

My siglent sds5000x on the other hand does not seem to differentiate between the two. Without any adapter (e.g. with a passive probe), the "Position" controls allow me to shift the trace by +-15.9 div, so way beyond the screen. I assume that the siglent generates an analog offset voltage and just puts the reference wherever the offset sits?

@tautech is this accurate?

[Performa01]

All Siglent DSOs use a true offset compensation and not a limited screen positioning.

The vertical "Postion" knob controls the offset compensation voltage injected to the frontend buffer; this is used to cancel out the offset voltage from the input.

The maximum offset compensation range for the various vertical gain settings is clearly specified in the data sheet.

[alm]

Do Siglent scopes have no vertical positioning? So if you are at the edge of your offset range, and just want to move around the traces so four fit on the screen, you can't? That seems like an odd limitation, given how simple and cheap a digital screen position feature (add a constant to the vertical position of every point).

Now I understand why the lack of internal offset feature is such a big deal to you. Having no control over vertical position except through the offset controls on the amplifier (ADA400A), which will also affect measurements, and having no control over the vertical position with the P6247, is a serious limitation. But I guess they have no way to allow you control both the offset voltage on pin 5 of the TekProbe interface and the offset voltage to the internal amplifier separately, and controlling both at the same time would mean double the offset in some cases. I guess with this design limitation, it will be hard to get the TPA10 working. Maybe the best compromise would be to always set pin 5 to 0V and use the vertical knob to control the offset inside the scope?

Does pin 5 of the TPA10 give sensible values now? I thought it was stuck to 2.5V?

[FlexibleMammoth]

Do Siglent scopes have no vertical positioning? So if you are at the edge of your offset range, and just want to move around the traces so four fit on the screen, you can't? That seems like an odd limitation, given how simple and cheap a digital screen position feature (add a constant to the vertical position of every point).

I would say that is the case, though I have never ran into that as a limitation until now. The offset range is quite massive.

Now I understand why the lack of internal offset feature is such a big deal to you. Having no control over vertical position except through the offset controls on the amplifier (ADA400A), which will also affect measurements, and having no control over the vertical position with the P6247, is a serious limitation. But I guess they have no way to allow you control both the offset voltage on pin 5 of the TekProbe interface and the offset voltage to the internal amplifier separately, and controlling both at the same time would mean double the offset in some cases. I guess with this design limitation, it will be hard to get the TPA10 working. Maybe the best compromise would be to always set pin 5 to 0V and use the vertical knob to control the offset inside the scope?

Yes exactly - I have no control at all of the vertical position of the P6247, which I assume is due to the 2.5V defect of my TPA10. The ADA400A ignores pin5 and can only be controlled via the offset controls on the probe itself.

I am not sure if it is even possible to use traditional offset with active probes - that would surely change the point where the offset voltage is injected? Maybe probes like the P6247 inject the offset from pin 5 into the probe head, and have an output buffer directly driving the 50Ohm scope interface?

Does pin 5 of the TPA10 give sensible values now? I thought it was stuck to 2.5V?

Technically, pin5 on my TPA10 is 2.5V for the lower 90% of the offset range and then suddenly flips to -2.5V for the last 10%. But both completely overwhelm the circuit in the P6247 and crash it into the positive/negative voltage rail. So regardless of the offset I choose, there is always a +/-1.2V DC on the screen, which corresponds to the maximum offset of a functional TekProbe interface.


EDIT for those who are late to the party: The TPA10 was confirmed defective. The voltage rails (+-15V, +-5V) are fine, but the offset on Pin 5 is stuck at Vmax = +-2.5V depending on the position of the offset knob).