How does a DSO control vertical sensitivity? - Page 1
Forum section: electronics / projects

author: monkeysuncle
author_rank: Contributor
timestamp: March 10, 2016, 07:52:44 pm
content: I am going to be teaching a class in a few weeks on the use of an oscilloscope, and wanted to add a few slides about the scope's internals. While I know pretty well how an analog CRO works (and have access to countless schematics), I am a bit less certain of what happens inside a DSO.I've watched Dave's reverse-engineering of the Rigol DS1054z front-end (several times!), but I have a question about what happens _after_ the part of the circuit that Dave describes. Specifically, how is the vertical sensitivity controlled.I can see three ways that it might be handled. There might be a digitally controlled attenuator (i.e., a digital pot). There might be some kind of digitally controlled variable-gain amplifier. Or they might simply change the voltage reference for the ADC so that it reads "full scale" at the desired value.Can anyone tell me which of these strategies (or what other strategy if there is one that I haven't listed) is actually used in a modern basic DSO like the aforementioned Rigol?

author: Fungus
author_rank: Super Contributor
timestamp: March 10, 2016, 08:18:48 pm
content: Quote from: monkeysuncle on March 10, 2016, 07:52:44 pmThe Rigol uses a variable-gain amplifier made with discrete transistors. It's mentioned (briefly) in the video at 30:30Quote from: monkeysuncle on March 10, 2016, 07:52:44 pmIt also uses relays to switch between completely different signal paths (that's why you hear internal clicks when you turn the vertical control knob).

author: monkeysuncle
author_rank: Contributor
timestamp: March 10, 2016, 09:58:56 pm
content: I know that there is an attenuator that kicks in with a mechanical relay for 500 mV/div and above. That still leaves a range of 200:1 in sensitivity that I'm trying to account for, to cover the 1 mV to 200 mV ranges.Well, I've learned a little more.One thing I missed in my previous research is that the 1054z has a highly integrated ADC, which has a VGA built in. That does a lot of the work of the expensive VGA that apparently was used in the older 1052e.That doesn't seem to get me all the way there, however. The ADC's built-in VGA gives a 50:1 adjustable range, and only 32:1 with guaranteed no-missing-codes. Since I'm trying to account for 200:1 gain range, there must be another 4:1, at least, from something else.So maybe you are right, there may be some discrete gain finagling going on in there as well.

author: nctnico
author_rank: Super Contributor
timestamp: March 10, 2016, 10:12:14 pm
content: Usually it is a combination of an attenuator and gain control. If you have 1:10 and 1:100 attenuation with a relay and *2 and *5 gain you can make a lot of combinations already. Changing the ADC reference voltage is not done because the reference is often shared and the control range is poor.You might want to look at service manuals for various DSOs for ideas on how they work.I don't know how much in depth you want to discuss oscilloscopes but the real fun part is creating an input circuit with a 1M input impedance capable of working up to several hundred MHz.