Why oscilloscopes dont have AC coupling for 50 ohm input - Page 1
Forum section: electronics / beginners

author: fonograph
author_rank: Frequent Contributor
timestamp: September 20, 2017, 06:45:44 am
content: I was looking at videos about Keysight N7020 power rail probe,one of the benefit they claim was that it have large +- 24V DC offset to allow it to measure noise and ripple of 24 volt power supplies.I was thinking,that doesnt sound that attractive,why not just set the 50 ohm oscilloscope input to AC coupling.Then I looked at Lecroy hdo and Keysight S datasheets and I noticed that both of them have AC coupling only for 1 mega ohm input but not 50 ohm.1. Why they didnt put option to switch to AC coupling on 50 ohm?2. Isnt AC coupling just a simple high pass filter? Cant it be simply made in DIY without signal degradation?3. Dont scopes have inbuild hardware offset function as default? Can it be used in place of power rail probe offset?

author: Benta
author_rank: Super Contributor
timestamp: September 20, 2017, 09:07:28 am
content: If the inputs are AC coupled, you can't measure DC signals, which would be a major drawback. Neither 50 ohm nor 1 Mohm inputs are AC coupled on the 'scopes I know. AC coupling is done in the probe.

author: nugglix
author_rank: Regular Contributor
timestamp: September 20, 2017, 09:17:52 am
content: Quote from: Benta on September 20, 2017, 09:07:28 amIs it?My probes don't have a switch for that...

author: grumpydoc
author_rank: Super Contributor
timestamp: September 20, 2017, 09:23:06 am
content: Quote from: Benta on September 20, 2017, 09:07:28 amYou must know some unusual 'scopes (or ones with 50ohm inputs only?)Care to share which models?All the traditional medium bandwidth 'scopes that I have had my hands on have had AC coupling in the 'scope.

author: helius
author_rank: Super Contributor
timestamp: September 20, 2017, 09:27:30 am
content: 1.Why they didnt put option to switch to AC coupling on 50 ohm?The pi network would distort the 50 ohm termination and lead to reflections in the cable.3.Dont scopes have inbuild hardware offset function as default? Can it be used in place of power rail probe offset?Typically DSOs have a vertical position control, but all that does is change where the trace is drawn onscreen. If the signal coming in is outside the current range it will still clip. If you want to use the full precision of the scope between, say, 10-30V (using the -10V to +10V range), you would need to add an offset at the probe.In an analog scope, the vertical position was in some sense changed by adding an offset voltage, but that happened post-attenuation, so it still wouldn't be a solution to the problem of using the whole range.

author: grumpydoc
author_rank: Super Contributor
timestamp: September 20, 2017, 09:39:53 am
content: Quote from: helius on September 20, 2017, 09:27:30 amI don't know about more modern DSO's but my LeCroy's apply a true offset to the input.In fact sometimes I would like a control just to change the position (on the LeCroy you can cheat and use the maths traces which do have a "position" setting).

author: grumpydoc
author_rank: Super Contributor
timestamp: September 20, 2017, 10:51:14 am
content: Quote from: Benta on September 20, 2017, 10:36:22 amEmphasis added - AC coupling in the 'scope.QuoteIt would, but I did not say that.QuoteYes, and I'm certainly not confused about what 1 Mohm / 1.5 pF means on a 'scope input.I'm kind of struggling to see the point you are making with your last post, unless it is to agree with me that the AC coupling is implemented in the 'scope, not the probe.

author: Benta
author_rank: Super Contributor
timestamp: September 20, 2017, 10:55:39 am
content: On rereading the original post, I realize I've misunderstood it. It read it as if talk was about a scope with only AC coupling, but actually it's about lack of AC coupling. My apologies.

author: David Hess
author_rank: Super Contributor
timestamp: September 20, 2017, 04:18:48 pm
content: Some 50 ohms inputs support AC coupling but most do not. 50 ohm circuits often expect and require a DC termination to ground. It is easy enough to add a DC block if AC coupling is required.The Tektronix 7A19 vertical amplifier is an example of what is required to safely support AC coupling with a 50 ohm termination.Quote from: fonograph on September 20, 2017, 06:45:44 amIt adds complication and most users do not require it. If needed, a 50 ohm DC block may be added externally.QuoteSure, but large high frequency capacitors are not trivial and make maintaining a 50 ohm environment difficult.QuoteThe internal 50 ohm DC termination comes before any offset capability (1) so the safe input voltage range is not increased. Measuring voltages above about 5 volts will require AC coupling or an offset probe.Quote from: grumpydoc on September 20, 2017, 09:39:53 amHelius is wrong about this; only the cheapest DSOs operate like he describes. Most DSOs which only have a position control including modern ones implement it as a combined position and offset control just like analog oscilloscopes do. The combined position and offset signal is injected after the input attenuators and buffer stage and before the final attenuator or gain control stages. This is reflected in the specifications which show different position/offset ranges for only some attenuator settings.For example, a Rigol DS1000Z series has a position/offset range of +/-2 volts at 1mV/div and 8 vertical divisions (2) so its position/offset range exceeds the ADC resolution by up to 500 times because of the gain after the point where the position/offset signal is inserted. It has two separate offset ranges because it has one switchable attenuator before the point where the position/offset signal is injected.(1) This does not need to be the case but I do not know of any counter examples. The 50 ohm termination could be offset just like how a low impedance offset probe does it but this would be an invitation to lots of blown out terminations and front end circuits.(2) As far as I know, Rigol shows the entire ADC range on their 8 vertical division display. Other DSOs often clip at 10 or 12 divisions while showing 8 divisions which has some processing advantages.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 20, 2017, 04:25:25 pm
content: What do you mean? Mine does AC coupling just fine.The cutoff frequency (where "AC" becomes "DC") is around a MHz in the 50 ohm mode.It's there, doesn't mean it's useful. I suppose newer machines might be configured to disallow that combination.Tim

author: David Hess
author_rank: Super Contributor
timestamp: September 20, 2017, 05:40:36 pm
content: Quote from: T3sl4co1l on September 20, 2017, 04:25:25 pmThis varies a lot. The Tektronix 7A19 that I used as an example has an unusually low cutoff of 1 KHz. My DC blocks all produce an low frequency cutoff of about 30 kHz.QuoteEven in the past, AC coupling in 50 ohm mode was rarely supported. Offhand the only example I know of is the Tektronix 7A19 and it went to an extreme to make it work reliably.