Confused About Siglent Input and Output Impedance Settings - Page 1
Forum section: electronics / beginners

author: gyropilot
author_rank: Contributor
timestamp: June 01, 2017, 04:00:43 pm
content: Greetings,I'm brand new to oscilloscopes and signal generators. I recently purchased a Siglent SDS1202X-E scope and a Siglent SDG1032X signal generator. I'm gradually learning how to use and understand them. I've poured through the manuals and watched hours of videos.I'm still confused about how I should set the input and output impedances of these specific devices when connecting them together directly using a short 50 Ohm BNC jumper coax, and whether or not I should be using a 50 Ohm terminator at the scope's input.The SDG1032X signal generator let's me select output impedances of either 50 Ohm, Hi-Z... or I can manually enter another impedance number. I understand this setting only affects the signal generator's display accuracy and not the actual signal amplitude it's sending out.On the SDS1202X-E scope, it let's me select a channel input impedance of 50 Ohm or 1 MOhm. I'm not 100% clear on what effect this has. The scope user manual says:"The channel input impedance matching gives you the most accurate measurements because reflections are minimized along the signal path. Impedance setting to 1M? is for use with many passive probes and for general- purpose measurements. The higher impedance minimizes the loading effect of the oscilloscope on the device under test."When I do the following:Connect the signal generator to the scope using the 50 Ohm BNC jumper coax (no 50 Ohm terminator at the scope input)Set the signal generator output impedance to 50 OhmsSet the scope input impedance to 50 OhmsSet scope probe compensation to 1XThe scope displays only 1/2 the waveform amplitude that the signal generator says is sending out.To correct this, I think I need a 50 Ohm terminator at the scope input to correct. I don't have one yet but it's on the way.Switching the input impedance at the scope from 50 Ohms to 1 MOhms has no effect on the amplitude error. However, switching the scope's probe compensation to 0.5X corrects the error.If I change the signal generator's output impedance to "Hi-Z" the scope display stays the same but the signal generator's amplitude display doubles thus matching the scope.So what exactly what is the purpose of the input impedance setting on the scope input if it doesn't actually present a 50 Ohm termination?Thanks in advance,JohnAA7US

author: Zero999
author_rank: Super Contributor
timestamp: June 01, 2017, 05:34:28 pm
content: To understand a lot of this you need to read about transmission lines.Signal generators generally have a 50 Ohm output impedance because they normally get connected to a piece of 50 Ohm co-axial cable with a high impedance at the other end and the 50 Ohm impedance absorbs the reflections.An oscilloscope may have a 50 Ohm input impedance setting for two reasons: to connect to RF circuits which need to be terminated with a 50 Ohm load to work properly and more commonly, for use with a low-Z probe.The signal level will be halved when the 'scope is connected to the 50 Ohm output of the signal generator and has its input impedance set to 50R because of Ohm's law. The oscilloscope's 50 Ohm input resistor forms the lower end of a potential divider, with the signal generator's output impedance.

author: gyropilot
author_rank: Contributor
timestamp: June 01, 2017, 06:26:17 pm
content: Quote from: Hero999 on June 01, 2017, 05:34:28 pmThanks for the reply.That makes sense, but then how come changing the scope's input impedance setting from 50 Ohm to 1 MOhm doesn't change the scope display? In my tests changing it did nothing... the scope's display still stayed at half amplitude of the signal generator.JohnAA7US

author: w2aew
author_rank: Super Contributor
timestamp: June 01, 2017, 06:30:04 pm
content: Quote from: gyropilot on June 01, 2017, 06:26:17 pmYou can validate whether changing the scope's input impedance setting using a DMM. The datasheet suggests that the only internal termination is the 1Mohm termination. In that case, I don't really see why they include a setting for input termination in the scope, *unless* they're assuming that you'd used a feedthrough termination, *and* they're doing some calculations (power, etc.) in the Math or Measurement systems that includes the external termination.

author: gyropilot
author_rank: Contributor
timestamp: June 01, 2017, 08:44:11 pm
content: Quote from: w2aew on June 01, 2017, 06:30:04 pmAlan,Just checked and yes, the SDS1202X-E has 1 Mohm input impedance no matter what the input impedance setting is. Don't know why I didn't think to make this simple check before.Well at least now I know I need an input termination with this scope whenever using a coax!QuoteOnce I get my pass-through terminator I'll do some experimenting to see if I notice any differences in measurements or math calculations between the two input impedance settings.73