Split Supply Integrator Scope Question - Page 1
Forum section: electronics / beginners

author: eev_carl
author_rank: Regular Contributor
timestamp: January 03, 2018, 03:26:14 pm
content: Hi,I implemented the circuit in the attached schematic and am getting the expected +-1.3V triangle wave. However, it looks like there is a lot of noise or ringing on the triangle. When I zoom in to the attached scope screenshot, it looks like there are little sine waves moving along the ramp.Is this a problem with my equipment or implementation? I'm using the ground leads that came with the probes. Besides that, I'm using a +-9V power supply and a function generator for the square wave input.Thanks,Carl

author: Audioguru
author_rank: Super Contributor
timestamp: January 03, 2018, 03:33:23 pm
content: It looks like the 'scope is a digital one and it shows digital artifacts. An analog 'scope wont doo dat.Also if the circuit was built on a solderless breadboard then the many rows of contacts and wires all over the place pickup interference.The 741 opamp design is 49 years old (!) and should be kissed goodbye and buried.

author: eev_carl
author_rank: Regular Contributor
timestamp: January 03, 2018, 03:46:02 pm
content: Thanks! I have an LM6171 on hand that has the same pinout. Here's what the scope looks like with the 741 upgraded.

author: Zero999
author_rank: Super Contributor
timestamp: January 03, 2018, 04:11:28 pm
content: The 741 appears to be oscillating but it's strange it's only doing it on the rising and not the falling edge.I'm surprised it doesn't oscillate with the LM6171, which is a much faster op-amp and generally, higher speed op-amps are more likely to oscillate, than lower speed op-amps, especially when a less than optimal layout is used, such as a solderless breadboard.

author: eev_carl
author_rank: Regular Contributor
timestamp: January 03, 2018, 04:14:47 pm
content: I did a few more rounds of A/B testing and got the same results. Long leads and solderless breadboard, but they're the same through all the trials.

author: Damianos
author_rank: Frequent Contributor
timestamp: January 04, 2018, 06:58:05 am
content: Maybe the input/output ranges of the "741" are violated. To verify that, increase the power supply voltage or decrease the input amplitude and check again.

author: eev_carl
author_rank: Regular Contributor
timestamp: January 04, 2018, 01:04:35 pm
content: The datasheet says the 741 is good with VCC=+-18V and VIN=+=15. My values are VCC=+-9V and VIN += 5.Increasing the power supply from +-9V to +-12V (split) made the effect worse. The thick band of the rising triangle wave edge became thicker. However, decreasing the amplitude of VIN to +-3V tightened the output up.Is there something I'm missing in reading the datasheet?

author: capt bullshot
author_rank: Super Contributor
timestamp: January 04, 2018, 03:21:29 pm
content: Try adding decoupling capacitors from +V to GND and GND to -V, as the most recent datasheet recommends this (I wouldn't have expected that for such an old and slow device, but one never knows). Many oscillations result from missing decoupling capacitors.

author: Damianos
author_rank: Frequent Contributor
timestamp: January 04, 2018, 08:54:33 pm
content: Quote from: eev_carl on January 04, 2018, 01:04:35 pmYes, you are reading the datasheet with the wrong way! The datasheet has some more things than the "Absolute Maximum Ratings" table...The one from NS/TI says that the operating input range is less than ±12V with ±15V supply. The output, with the same supply, is restricted to ±10V. Note that it is used with a power supply level that is not included in its characteristics...Another matter may be a partial damage to the device that happens when experimenting with loose connections.Also the power supply decoupling may help a lot.

author: David Hess
author_rank: Super Contributor
timestamp: January 05, 2018, 12:59:53 am
content: There is nothing wrong with the 741 but it should not be doing that. Decouple R3 or short it; maybe positive feedback is causing the oscillation.