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DIY oscilloscope (yet again)
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gf:

--- Quote from: PMA on November 17, 2019, 09:50:27 am ---I have been looking different attenuator designs and found out that they are relatively complicated even in low end scopes.
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Besides the compensated divider, yet another design goal for a scope's attenuator is IMO constant input capacitance - independent of the selected attenuation - otherwise you need to re-adjust a connected 10:1 probe whenever you switch to a different attenuation stop :( [I'd not be not sure, though, that this is really granted for all low-end scopes].

gf
Yansi:
Constant impedance?  That is not happening, even with high end scopes. Unless you use a 50ohm termination mode.

You probably meant that the impedance curves shall be similar, both with and without the attenuator selected. That is probably true. But probably not that critical in a 10-50MHz setup. I'd just make sure the input capacitance stays the same.
PMA:
Why there is basically low pass filter in the Rigol? It cannot be just for bandwidth limiting for the next stage because the attenuator is selectable. Something to do with capacitive coupling of the circuits on higher frequencies?

Yansi:
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.
David Hess:

--- 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.
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Actually if you match the JFETs then the current setting resistor does not need to be adjusted however the offset of the high impedance buffer stage needs to be adjusted at some point to trim the step attenuator balance.  Sometimes this is done in the high impedance buffer, either by adjusting the JFET drain current or adjusting the offset of the DC servo, and sometimes it is done in the following stage so there is always an offset adjustment somewhere.

The DC servo method has its own adjustment for the low to medium frequency compensation whether they show it or not.  This is why some modern implementations have a low to middle frequency step in the amplitude response that older designs did not suffer from.


--- Quote ---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.
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Practically any low input bias current operational amplifier is suitable for the DC servo.  Tektronix first used the TLC271 programmable operational amplifier set for high bias probably for minimum input noise.

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.

However for a simple design, I would ignore all of these performance considerations and implement whatever is simplest or the designer is most comfortable with except for a shunt feedback design which just leads to troublesome transient response.


--- Quote from: Yansi on November 17, 2019, 01:45:45 pm ---Constant impedance?  That is not happening, even with high end scopes. Unless you use a 50ohm termination mode.

You probably meant that the impedance curves shall be similar, both with and without the attenuator selected. That is probably true. But probably not that critical in a 10-50MHz setup. I'd just make sure the input capacitance stays the same.
--- End quote ---

He means constant input capacitance and resistance with different input attenuator settings.  The former requires a trimmable shunt capacitance for each attenuator stage in addition to the timmable capacitor for frequency compensation.  These adjustment have a *major* effect on the low to medium frequency compensation.


--- Quote from: PMA on November 17, 2019, 09:50:27 am ---I have been looking different attenuator designs and found out that they are relatively complicated even in low end scopes. Not that voltage divider is complicated, but there is lots of art in compensation circuits. Is there good literature available about the subject?
--- End quote ---

The Tektronix Circuit Concepts Books have a detailed discussion of high impedance input attenuators in the Vertical Amplifier Circuits book available at that link.

The Tektronix 7A15 (not 7A15A) service manual shows details for discrete attenuator sections from 1x to 1000x.  Older designs are easier to implement with discrete parts and printed circuit board assemblies.

Steve Roach who worked for Hewlet Packard designing digital storage oscilloscope front ends wrote a chapter included in Jim William's "The Art and Science of Analog Circuit Design" called "Signal Conditioning in Oscilloscopes" which covers how more modern instruments do it.


--- Quote ---Higher end designs tends to use  multiple selectable attenuators in parallel or in series. Is it worth of effort and pain or is it better to keep things simple and use just one?
--- End quote ---

You can almost certainly get away with just a single switchable high impedance attenuator stage but better modern oscilloscopes use at least two x10 stages in series.

But do not forget about the switched AC/DC coupling stage.  Older oscilloscopes implemented this before the first high impedance attenuator but Steve Roach's paper mentioned above discusses how to implement this as part of the DC servo controlled high impedance buffer.
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