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DC bias (or offset) adjustment availability

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alm:

--- Quote from: taydin on November 25, 2023, 07:52:18 pm ---My understanding is that the DHO4000 DC bias setting removes that amount of DC voltage from the input signal. That way, you can set the DC bias to 9V and then monitor the output of a 9V SMPS using your oscilloscope's input, and only see the ripple/noise and very little DC, whatever is leftover from the adjusted DC bias and the actual output voltage.

--- End quote ---
True.


--- Quote from: taydin on November 25, 2023, 07:52:18 pm ---But the vertical position knob doesn't remove anything. It just moves the image on the screen up and down.

--- End quote ---
On the Rigol scope, true. On Siglent and Lecroy scopes, no. The "vertical position knob" is actually an offset knob (and labeled as such on Lecroy scopes). See the offset range that rf-loop posted.

nctnico:

--- Quote from: taydin on November 25, 2023, 07:52:18 pm ---
--- Quote from: nctnico on November 25, 2023, 05:33:04 pm ---DC bias and vertical offset are the same thing. Only the representation on the display is different. You won't find two different signals to set the DC offset / vertical offset when you are going to look for them in the analog frontend.

--- End quote ---
My understanding is that the DHO4000 DC bias setting removes that amount of DC voltage from the input signal. That way, you can set the DC bias to 9V and then monitor the output of a 9V SMPS using your oscilloscope's input, and only see the ripple/noise and very little DC, whatever is leftover from the adjusted DC bias and the actual output voltage.

 But the vertical position knob doesn't remove anything. It just moves the image on the screen up and down.

--- End quote ---
You may think that but if you set the DC offset without a signal connected, you'll see the signal moving up or down. Just as if you adjusted the offset knob. The only difference is that DC offset adjustment is specified in volts and vertical offset is typically expressed in divisions (in case there are seperate controls). However, the limit of the amount of offset you can apply is limited by the offset range as stated in the datasheet. There are some variations on how this has been implemented in the user interface on various oscilloscopes (which you may like or dislike), but without a DC offset setting, the vertical position provides the same function as a DC offset adjustment.

taydin:
Ok, let's go based on an actual scenario: I have set my signal generator to generate 1 kHz, 200 mVpp sine wave with an offset of 8 V. I apply that to my SDS1104X-E with the input coupling set to DC. I set the vertical offset with the vertical position knob to -8V and reduce the vertical range. And I am hitting the limit where the range is 500 mV/div. So right now, if I measure RMS, it measures about 8 Vrms, And it measures 8 V average.

But, if the front end would have removed the 8V offset, I would be able to go to a much lower vertical range (in case the sine wave amplitude was much lower), and I would be able to measure the ACTUAL RMS and average value of the sine wave that's riding on the DC offset. So I don't understand, how are these two scenarios equivalent?

alm:

--- Quote from: taydin on November 25, 2023, 08:29:30 pm ---But, if the front end would have removed the 8V offset, I would be able to go to a much lower vertical range (in case the sine wave amplitude was much lower), and I would be able to measure the ACTUAL RMS and average value of the sine wave that's riding on the DC offset. So I don't understand, how are these two scenarios equivalent?

--- End quote ---
Read rf-loop's post again. Every scope will have a limited offset range, one that usually goes up with the vertical range. Regardless if DC offset is a separate setting from position or not. What you're looking at, is what the offset range is at the vertical attenuation setting you want to use. For example, for an 8V offset the SDS-1000X-E goes down to 152 mV/div, while the DHO4000 goes down to 65 mV/div, and the SDS-2000X goes down to 10.2 mV/div. Looks to me like the SDS-2000X is the superior scope for this particular measurement.

rf-loop:

--- Quote from: taydin on November 25, 2023, 08:29:30 pm ---Ok, let's go based on an actual scenario: I have set my signal generator to generate 1 kHz, 200 mVpp sine wave with an offset of 8 V. I apply that to my SDS1104X-E with the input coupling set to DC. I set the vertical offset with the vertical position knob to -8V and reduce the vertical range. And I am hitting the limit where the range is 500 mV/div. So right now, if I measure RMS, it measures about 8 Vrms, And it measures 8 V average.

But, if the front end would have removed the 8V offset, I would be able to go to a much lower vertical range (in case the sine wave amplitude was much lower), and I would be able to measure the ACTUAL RMS and average value of the sine wave that's riding on the DC offset. So I don't understand, how are these two scenarios equivalent?

--- End quote ---

Of course. Take first just DC without this riding 200mVpp sine. And it is right. 8V DC  = 8Vrms.

Now if there IS real signal what is example 200mVpp sine and its DC offset is 8V this 8V offset need keep in calculation even if it is added with oscilloscope offset because we are measuring input signal...

But if you want only this 200mVpp sine RMS what have 8V offset... you need take only AC part. You can add -8V and look it on screen.  Now if we talk about AC RMS. Mathematically it is Stdev (some call it AC RMS)  and this Stdev you can find in Siglent as also RMS.
But if you want RMS  it is input signal RMS because it need also count added offset. Because RMS need include also DC part. Only AC RMS do not care this DC offset.

But really if you adjust example in Siglent vertical "Position" it is just same as DC Offset uou perhaps try talk. In front end analog circuits it really add DC offset to signal in summing amplifier. Offset DC come out based to offset DAC.

But oscilloscope really need take count this internally added offset(- or +)  because purpose is measure input signal as perfectly as it is possible. In your example 8V offset 200mVpp sine: 8.0003 Vrms. 
Oscilloscope know it have subtracted 8V if you have set -8V offset.  Original signal what we are measuring have it and it need tell in measurement result.
RMS is RMS  8V DC  if measure RMS result is 8Vrms. But if you measure AC RMS (aka Stdev)  it is 0

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