Scope AC/DC coupling at 60Hz - Page 1
Forum section: electronics / general

author: StillTrying
author_rank: Super Contributor
timestamp: June 30, 2019, 02:15:32 pm
content: I've known about this AC/DC coupling effect for years, scope probe is on X1.1st is the original 60HzDC coupledsquare wave. - looks fine.2nd is a 22n 50V ceramic on the probe used as the AC coupling cap. - looks fine.3rd is thescope's own AC coupling. - looks terrible.

author: David Hess
author_rank: Super Contributor
timestamp: July 01, 2019, 01:22:08 am
content: You oscilloscope's AC coupling is broken.The input coupling capacitor is typically about 0.022 microfarads.

author: StillTrying
author_rank: Super Contributor
timestamp: July 01, 2019, 09:16:34 am
content: "Your oscilloscope's AC coupling is broken."This CML+ has always had a horrible AC coupling response, both channels are identical.If I eliminate the probes with a short direct connection to the BNC it's identical, if I stick a 1M in the BNC I don't detect any change of input impedance during the curve. I wonder how they managed to make just the AC/DC coupling so bad.20Hz direct connection (no probe) 1V/div overlayed with direct through 1M 0.5V/div.

author: David Hess
author_rank: Super Contributor
timestamp: July 01, 2019, 11:52:57 am
content: Quote from: StillTrying on July 01, 2019, 09:16:34 amI suspect this is a design problem. See below.QuoteThe x1 probe is basically a low value resistor, maybe a couple hundred ohms, so except for shunt capacitance it should produce the same results as a direct connection.I suspect what is going on is that the input buffer is the now common two path design with a high frequency AC coupled JFET buffer and a low frequency DC path through an operational amplifier. The advantage of this design as currently implemented is that AC coupling can be electronically switched without a relay very easily but if the frequency breakpoints between the two parallel stages are not matched, then there will not be a clean transition in the low frequency response.Early oscilloscopes which used a similar configuration had a LF/MF adjustment to matching the response of the two paths but the modern ones I have seen leave this out for some reason. A low frequency swept measurement should reveal what is going on.

author: StillTrying
author_rank: Super Contributor
timestamp: July 06, 2019, 05:02:31 pm
content: Yep, there's no relay clicks or clinks switching between AC & DC coupling.There's quite a peak @ 70Hz usingthe scope's AC couplingcompared withthe scope on DC and using a 22nF used as an AC coupling cap.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 06, 2019, 05:55:20 pm
content: Quote from: David Hess on July 01, 2019, 11:52:57 amGiven the low cost of low frequency ADCs, couldn't the low and high frequency parts be digitized individually and the crossover fine tuned in DSP?

author: David Hess
author_rank: Super Contributor
timestamp: July 08, 2019, 04:18:05 am
content: Quote from: NiHaoMike on July 06, 2019, 05:55:20 pmI suppose so but that is an awful lot of cost and complexity to replace a single trimmer potentiometer or using a design which does not suffer from this problem.