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DIY oscilloscope (yet again)
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PMA:

--- Quote from: Yansi on November 17, 2019, 03:38:05 pm ---Where exactly do you see it, or what do you mean by low pass? It is a compensated divider.  Plus some resistors to get the response more right.

--- End quote ---

Long story short: there was series of brain farts.
Yansi:

--- Quote from: David Hess on November 17, 2019, 05:16:06 pm ---There are some disadvantages to the DC servo configuration:

1. The input divider multiplies the offset drift and low frequency noise of the operational amplifier above that which would be present in the dual JFET implementation.  I have seen designs which did away with this attenuation however doing so makes point 2 below worse.  Low frequency noise on some modern DSOs is truly atrocious compared to past oscilloscopes despite marketting claims about "low noise".  Their broadband noise is no better.
2. Input overload can cause windup of the DC servo prolonging overload recovery time.

--- End quote ---

ad 1)  noise in what exact quantity? I doubt that would become anywhere close to what a shitty 8bit ADC dynamic range can offer in that application, with a standard lowest 5mV/div step.* Bad PSRR, channel crosstalk etc would be your bigger enemy. But I stay being corrected.

ad 2) I think that is left as is in even many not-low-end scopes.  If the AFE is overloaded, well, then you are using the scope wrong.

*just guessing OP is aiming for the standard 8bit acquisition which is "good enough" for many scope applications, 10, 12 or more bits would definitely require more than just "simple & easy" AFE design.

//EDIT: Not sure what "low-medium compensation" you talk about, but the DC servo's low frequency gain matching (to the JFET buffer medium-high frequency gain) is usually auto-tweaked using a couple of electronic pots within the feedback loop. 
Kleinstein:

--- Quote from: Yansi on November 17, 2019, 10:48:47 am ---That "JFET pairing" and need for "tweak this Rx or this Ry until good" is exactly the reason, why the dc servo was invented and is widely used.

I see absolutely no reason why the servo shouldn't be superior to a pair of paired JFETs. The Opamp used in the servo does not need to be that fast and can be of low Vos type.  In some appnote I have seen OPA2227 suggested.  But sure it will work with almost any decent opamp.

--- End quote ---

The OP2227 is not suitable for the DC servo, as it would add considerable current noise, that the fast part would also see. The suitable choice would be more like a good JFET based OP, like OPA141, maybe LT1012 if it really needs to be BJT based.

In the discrete JFET buffer the offset adjustment with the current source also effects the temperature drift. So one could adjust for minimal temperature drift and compensate the usually small residual offset separately. The advantage of well matched JFETs is that zero offset and zero drift are close together.

Even with a  8 bit ADC, input noise can still be important. It kind of limits the lowest useful sensitivity (e.g. 10 mV/div in a noisy config, and maybe 0.5 mV/div if low noise).  The leveling factor here is more the noise of a typical 1:10 probe - no real need for a very low noise input with the noise of an 1 Meg resistor. It's with a 1:1 probe where the noise really counts. The ADC resolution is more like linked to screen resolution.
PMA:

--- Quote from: Yansi on November 17, 2019, 05:56:21 pm ---*just guessing OP is aiming for the standard 8bit acquisition which is "good enough" for many scope applications, 10, 12 or more bits would definitely require more than just "simple & easy" AFE design.

--- End quote ---

8bit is good enough for me. More is always better, but I think that the project is difficult enough even if I try to keep everything simple.
David Hess:

--- Quote from: Yansi on November 17, 2019, 05:56:21 pm ---
--- Quote from: David Hess on November 17, 2019, 05:16:06 pm ---There are some disadvantages to the DC servo configuration:

1. The input divider multiplies the offset drift and low frequency noise of the operational amplifier above that which would be present in the dual JFET implementation.  I have seen designs which did away with this attenuation however doing so makes point 2 below worse.  Low frequency noise on some modern DSOs is truly atrocious compared to past oscilloscopes despite marketting claims about "low noise".  Their broadband noise is no better.
2. Input overload can cause windup of the DC servo prolonging overload recovery time.
--- End quote ---

ad 1)  noise in what exact quantity? I doubt that would become anywhere close to what a shitty 8bit ADC dynamic range can offer in that application, with a standard lowest 5mV/div step.* Bad PSRR, channel crosstalk etc would be your bigger enemy. But I stay being corrected.
--- End quote ---

At high sensitivities, even 5mV/div, the dynamic range of an 8 bit ADC is plenty to pick up the noise at least in a noisy design.  The servo loop is good for DC drift but low frequency noise levels can be much higher.

For most applications, this is irrelevant.  However if you are expecting to use an FFT to measure low frequency noise, it will limit performance.


--- Quote ---ad 2) I think that is left as is in even many not-low-end scopes.  If the AFE is overloaded, well, then you are using the scope wrong.
--- End quote ---

There is a matter of degrees here.  If the servo loop design is careless, then overload recovery time can be extended by orders of magnitude.  Some oscilloscopes are very resistant to overload giving additional application flexibility.  A few are even immune.  (1)


--- Quote ---*just guessing OP is aiming for the standard 8bit acquisition which is "good enough" for many scope applications, 10, 12 or more bits would definitely require more than just "simple & easy" AFE design.
--- End quote ---

I am not sanguine that 10 and 12 bit designs are anything more than marketing at work.  DSOs have enough difficulty with noise, settling time, and distortion even with 8 bit resolution.  Sampling oscilloscopes are a different matter.

Many low cost commercial designs use the extra resolution provided by a higher resolution converter as a free programmable gain stage.


--- Quote ---//EDIT: Not sure what "low-medium compensation" you talk about, but the DC servo's low frequency gain matching (to the JFET buffer medium-high frequency gain) is usually auto-tweaked using a couple of electronic pots within the feedback loop.
--- End quote ---

The paper by Steve Roach that I mentioned discusses the difficulties with dual loop buffers.  Automatic calibration would require additional complexity to inject the test signals.  Is that desirable in a DIY design?  I would settle for support for ground coupling so that the offset of the following stages can be removed.

(1) Sampling oscilloscopes are immune to overload.  I know how to make a low impedance signal path immune to overload but making a high impedance buffer immune is difficult to impossible and many commercial designs do not even bother to try.
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