Weird Scope Waveform - Page 1
Forum section: electronics / beginners

author: advark
author_rank: Contributor
timestamp: September 07, 2016, 04:25:57 am
content: Hello everyone,Just got my new equipment lately, I was playing around with a simple TL972 opamp (See image : note R1 is 200 ohms, not 1K). So R1 +R2 is 10.2K and R3 +R4 is 14.7K which should give me a gain of 2.4411...So if Vin is 900 mV, the scope should on CH1 2.197.... while CH2 should read 900 mV. As you can see on the screen caputre of the scope, this is quite close (CH1 = 2.2V and CH2 = 860-870 mV). Since I'm using 5% resistors, no surprise here.What bugs me, are those spikes in the middle. What the...! I just don't quite get it. The circuit is powered has follow using Rigol's DP-832 PSU (3CH 30V-30V-5V). CH1 is set to 12V, which power the opamp. Opamp's V- is GND on CH1. CH3 is set to 900 mV which is my Vin. CH3's ground is connected to CH1's ground (it has to be connected somewhere, right?). The circuit is built on a standard solderless breadboard. I think its the culprit, but I'm not so sure.Any ideas?Thanks

author: Ammar
author_rank: Regular Contributor
timestamp: September 07, 2016, 06:05:02 am
content: Try some capacitors on your supply rails as close to the IC as possible. Try 0.1uF in parallel with 10uF.

author: advark
author_rank: Contributor
timestamp: September 07, 2016, 06:08:16 am
content: Well, after some dickin' around, I manage to find that the spikes come from the DP-832 itself. On all 3 channels. Around 20 mV peak-to-peak every 8.2-8.4 ms. Sounds just like a 60Hz main line.If you think I'm wrong, please feel free to correct me.

author: advark
author_rank: Contributor
timestamp: September 07, 2016, 06:12:42 am
content: Quote from: Ammar on September 07, 2016, 06:05:02 amDid try it, no luck.

author: ali6x944
author_rank: Frequent Contributor
timestamp: September 07, 2016, 06:49:46 am
content: is it continues? if it is than it is maybe common noise from the ac line...see this video by Dave, i hope it will be helpful:but if it happened only once - when u drive a load - maybe it is a current spike is what resposable for this to happen...is there any inductive elements on ur supply rail?or any thing that might create current spikes like a IC with totem pole output or something like that,anyway i wish u good luck in finding the problem and solving it and don't stop trying

author: tatus1969
author_rank: Super Contributor
timestamp: September 07, 2016, 09:44:50 am
content: Watch this

author: forrestc
author_rank: Unknown
timestamp: September 07, 2016, 10:40:46 am
content: Quote from: advark on September 07, 2016, 04:25:57 amExactly how do you have the scope attached to the device under test?Specifically, how do you have the ground leads on the scope probes attached?

author: danadak
author_rank: Super Contributor
timestamp: September 07, 2016, 11:34:55 am
content: The pulses indicate they occur at ~ 60 nS = ~ 1.6 Mhz, a clue ?What is the delay time from the first pulse in the 2 pulse burst to the first pulsein the next 2 pulse burst ? That frequency ?Regards, Dana.

author: advark
author_rank: Contributor
timestamp: September 07, 2016, 05:08:52 pm
content: After more dickin' around, sound more like it is some noise on the main's AC line. Spikes occur every 8.5ms (give or take) and last for about 8us. The things is, the room I'm in is on the same circuit as the next room (living room) where ther is a bunch of stuff connected (TV, game console, Home Theater, Internet Router, etc). Chances are that one of these is equiped with SMPS that is causing some noise on the line (60Hz, full bridge = 8.3333ms). I even try a AA battery (1.2V) at Vin. Although the input is now clean, the output still has the spike (probably due to the fact that the opamp is powered at 12V from my DP832). I'll try to get a hand on a 12V battery and see what difference it makes.Thank you all for good advices. Greatly appreciated.

author: StillTrying
author_rank: Super Contributor
timestamp: September 07, 2016, 05:46:11 pm
content: If it's decoupled, low frequency noise on the 12V supply shouldn't get through to the output.Any measurements taken while there's a scope probe on the opamp's -ve input don't count for much, it will upset both the DC and AC feedback.On the data sheet it looks like the TL972's inputs can't be used closer than 1.2V to the rails, and your 900mv input is below that.

author: advark
author_rank: Contributor
timestamp: September 07, 2016, 08:48:19 pm
content: Finally got it to work. Just had to add a 0.1uF decoupling at IN+ pin (Note that CH4 of the scope is directly connected to PSU terminals).Quote from: StillTrying on September 07, 2016, 05:46:11 pmI couldn't find that info. The only thing I found was on Figure 18 of the datasheet where the input voltage seem to go slightly below 1V. Actually, it worked down to around 950mV on my breadboard.

author: StillTrying
author_rank: Super Contributor
timestamp: September 07, 2016, 09:25:02 pm
content: Just had to add a 0.1uF decoupling at IN+ pinI forgot to mention the input needs a pull down of 1k ish.It might be a good idea not to use these op amps with inputs of 0.9V - 1V. LOL

author: tatus1969
author_rank: Super Contributor
timestamp: September 08, 2016, 06:49:26 am
content: Quote from: advark on September 07, 2016, 08:48:19 pmlook for Vicm. For a +-2.5v supply this is +-1.35v. this means for unipolar supply of 5v you can use it with input signals from 1.15v to 3.85v. your 900mV is out of spec, may or may not work a second time, at lower or higher temperature, etc. Your 100n cap is not decoupling but instead low pass filtering your input signal. This is a hint to look there for your problem.