Problem with osciloscope and signal - Page 1
Forum section: electronics / beginners

author: spintinas
author_rank: Contributor
timestamp: February 28, 2018, 11:11:27 am
content: Hello, I have circuit with this amplifier, and with this circuit I amplify my pressure transducer signal from 30mV to 700mV. Amplifier works correctly, when applied pressure is increasing, multimeter shows higher voltage (range 700mV-1.3V). Both, transducer and amplifier are powered from Arduino 5V. Problem is that, when I connect scope cables to circuit output, voltage is decreasing to 500mV from 700mV (700 mV in multimeter) and when i applied more pressure to transducer, signal in scope still is about 500mV and there is no changes in signal amplitude, while there should be some increases in signal amplitude. (i attach photo with scope screen and my schematics).

author: danadak
author_rank: Super Contributor
timestamp: February 28, 2018, 11:51:35 am
content: Is the scope used with probes or straight cable, if latter is it set at 50ohms ? If so turn off 50 ohms, thats too low a Z for OpAmp to drive.What are the R and C values used in the circuit ? What is the bridge armnominal R value ?Regards, Dana.

author: spintinas
author_rank: Contributor
timestamp: February 28, 2018, 12:45:39 pm
content: Scope is used with probes, R1 and R3 are 10 k'ohms.

author: TK
author_rank: Super Contributor
timestamp: February 28, 2018, 01:25:16 pm
content: Are you using a 1X or 10X probe?

author: spintinas
author_rank: Contributor
timestamp: February 28, 2018, 01:31:53 pm
content: 10X but on scope settings also 10x, all circuit connected on breadboad and multimeter in output shows 700mV, just touch osciloscope probe to the ground pin and voltage is decreased to 300-400mV.

author: TK
author_rank: Super Contributor
timestamp: February 28, 2018, 01:40:36 pm
content: I think picoscope is a USB instrument, probably GND of your circuit is not at the same GND level on the oscilloscope.

author: spintinas
author_rank: Contributor
timestamp: February 28, 2018, 01:45:21 pm
content: So, what solution can be to make them similar?

author: StillTrying
author_rank: Super Contributor
timestamp: February 28, 2018, 01:54:05 pm
content: I'd connectjust the GNDof the scope, and then check with the multimeter if it's the scope's GND connection that's causing the problem.

author: TK
author_rank: Super Contributor
timestamp: February 28, 2018, 01:59:55 pm
content: How are you powering the amplifier? Picoscope is connected to a PC or laptop? You can try powering the circuit from the PC/laptop USB port so both are connected to the same GND.

author: kg4arn
author_rank: Unknown
timestamp: February 28, 2018, 02:07:58 pm
content: Consider that placing the probe on the output is causing oscillation. I have found that weird dc readings often end up meaning the circuit is oscillating.

author: spintinas
author_rank: Contributor
timestamp: February 28, 2018, 07:25:04 pm
content: Picoscope connected to the laptop and arduino connected to the laptop via usb, then 5V from laptop is supplied Arduino. From arduino, one 5V pin supply transducer, and other 5V pin is connected to the breadboard to supply amplifier.

author: spintinas
author_rank: Contributor
timestamp: February 28, 2018, 07:25:55 pm
content: But signal drops when i connect ground of the scope probe to the ground of amplifier (in breadboard). When I connect only scope tip, signal don't drop.

author: Old Printer
author_rank: Frequent Contributor
timestamp: February 28, 2018, 11:03:04 pm
content: Curious to see how this works out, sure sounds like scope loading the circuit, otherwise I would have guessed the USB was running out of current. Oscillating circuit well out of my experience level, but I will watch keenly.

author: StillTrying
author_rank: Super Contributor
timestamp: February 28, 2018, 11:47:26 pm
content: Any 'spare' opamps in the lmc6484 should have their inputs connected to something, not left floating, ground is probably easiest.Can't think of anything else to try!

author: danadak
author_rank: Super Contributor
timestamp: March 01, 2018, 10:20:17 am
content: Picoscope allow you to do differential measurements ?http://download.tek.com/document/51W_10640_1.pdfhttps://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&ved=0ahUKEwjpsumL9MrZAhVIdt8KHQFrBe8QFgh_MAU&url=http%3A%2F%2Fwww.newark.com%2Fpdfs%2Ftecharticles%2Ftektronix%2FFFM.pdf&usg=AOvVaw2udBMlAswXXiYwjquqYBRQhttps://www.picotech.com/library/oscilloscopes/differential-and-floating-voltage-measurementsRegards, Dana.

author: rhb
author_rank: Super Contributor
timestamp: March 01, 2018, 04:03:20 pm
content: That scope trace surely looks as if you have an oscillation problem. Post a trace with a faster sweep and tell us where the probe is connected.What kind of caps are you using? Electrolytics have considerable parasitic inductance. I built a 120 dB gain DC coupled AF amplifier that oscillated like mad at 200-300 KHz until I bypassed the electrolytics with some small ceramics. Also check your input power source.

author: Audioguru
author_rank: Super Contributor
timestamp: March 01, 2018, 05:02:12 pm
content: On another website a photo of the solderless breadboard shows the ground pin 11 of the opamp not connected to anything and another kid in this guy's class said it fixed it when the 'scope and it was connected to ground.