Rigol 2072a 50 ohm input - Page 1
Forum section: electronics / testgear

author: JCK
author_rank: Contributor
timestamp: July 31, 2015, 04:49:03 am
content: I've only had my Rigol 2072a for about a week now and just noticed on the front panel by the probe BNC inputs it says that the max input voltage is 5V pp when the input impedance is 50 ohms. Why is this so low? I was going to build an avalanche pulse generator for TDR use but the avalanche break down for common transistors can be close to 80V or more. So now I'm afraid to use it with this scope. I know I can use fast logic (or other ICs) to get close to 1ns rise times, but I was just wondering how you guys get around the Rigol 5V pp limit @ 50 ohms?John

author: Bud
author_rank: Super Contributor
timestamp: July 31, 2015, 05:55:19 am
content: There is a 49.9 Ohm smt resistor connecting in parallel in 50 Ohm mode (bottom right in the attached picture), so you can burn the resistor if the voltage too high. The attenuators are behind that resistor, so it always takes the first hit regardless of attenuator setting.EDIT: and in case you have not figured out yet - this piece of shi!t does not remember its 50 Ohm setting - you enable 50 Ohm input but next morning you power the scope up it may not be on 50 Ohm, even the channel setting says it is. So if you willing to take an advice - always cycle the channel's 50 Ohm setting when you need it to avoid nasty surprises.

author: dadler
author_rank: Unknown
timestamp: July 31, 2015, 07:15:32 am
content: Maybe I'm misunderstanding your use case or TDR circuit, but can't you just put the scope in 1M input impedance mode and use a 50ohm feed-through terminator? Might want an attenuator in there as well.

author: tautech
author_rank: Super Contributor
timestamp: July 31, 2015, 07:54:56 am
content: Quote from: JCK on July 31, 2015, 04:49:03 amYes, this is the industry standard and care must always be taken to not overload 50 Ohm inputs when they're used.

author: Mark
author_rank: Frequent Contributor
timestamp: July 31, 2015, 08:42:46 am
content: W = V^2 / R = 80 x 80 /50 = 128W. It wouldn't be long before your scope was melting with that amount of power being dissipated insideYou will need to use an external 50 ohm load / attenuator.

author: tautech
author_rank: Super Contributor
timestamp: July 31, 2015, 08:46:30 am
content: Quote from: Mark on July 31, 2015, 08:42:46 amYou need also consider duty cycle.

author: Mark
author_rank: Frequent Contributor
timestamp: July 31, 2015, 10:03:38 am
content: Quote from: tautech on July 31, 2015, 08:46:30 amYes, but when Rigol print something on the front of their scope they have to assume that someone will attempt to put that voltage across the input with 100% duty cycle.

author: McBryce
author_rank: Super Contributor
timestamp: July 31, 2015, 10:37:31 am
content: Quote from: Mark on July 31, 2015, 10:03:38 amI think he was referring to your Watt calculation, not the input voltage.McBryce.

author: Mark
author_rank: Frequent Contributor
timestamp: July 31, 2015, 11:02:51 am
content: Quote from: McBryce on July 31, 2015, 10:37:31 amMy point is that Rigol specify a voltage not a wattage, so unless they start adding duty cycle spec beside the input, they must assume that the user will assume it is capable of 100%.

author: pascal_sweden
author_rank: Super Contributor
timestamp: July 31, 2015, 11:03:46 am
content: Why not just always skip the internal 50 ohm input termination, and always use an external 50 ohm feed-through terminator? Wouldn't this be the best practise?Then you don't have to worry about memory settings or accidently burning things.Worst thing that can happen is that you burn your external terminator?

author: tautech
author_rank: Super Contributor
timestamp: July 31, 2015, 01:31:03 pm
content: Quote from: pascal_sweden on July 31, 2015, 11:03:46 amEven the Tek 1x and 10x 50 Ohm feed through terminators I've got are only 2 W rated.50 Ohm inputs are not normally include with entry level scopes, for good reason, guess why?

author: JCK
author_rank: Contributor
timestamp: July 31, 2015, 03:10:08 pm
content: Thanks everyone, good info which makes perfect sense now that I think about it. This is such a great forum!