LTSpice not looking like the actual measurement - Page 1
Forum section: electronics / beginners

author: Enron
author_rank: Contributor
timestamp: December 28, 2012, 09:19:05 am
content: I have the hardware made for this chorus guitar pedal schematic:http://www.geofex.com/PCB_layouts/Layouts/zombie.pdfThe pedal sounds perfect when I hook up my guitar through it, to my amp. During the making of this, I used LTSpice for each stage of the circuit. The first stage measured on the actual hardware looks very different than what LTSpice shows.I have an image of the actual reading from the hardware. I set both my function gen and my LTSpice original signal to 1V p-p at 440Hz.The LTSpice file signal references are:Blue: Original signal out of V1Green: Signal after the capThe first photo is being measured after the cap in LTSpice (about 1Vpp in the +5.4V area), and the second photo is being measured after the cap on the actual hardware (about 1Vpp in the +1.5V area).Is this a problem that most people have using spice programs? Or can someone spot out something that I might doing wrong.Also for the Vb reference on the actual hard, I am measuring 5.3V. The output of the opamp appears to be the same on both LTSpice and the hardware.EDIT: Just added LTSpice files, so if anyone wants to look at them.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 28, 2012, 10:39:18 am
content: Your scope input impedance is not infinite, so what you are doing while measuring is adding an extra (10M?) resistor from the + input of the op-amp to ground. My guess is when you do that in LT Spice it will show you the same.

author: Enron
author_rank: Contributor
timestamp: December 28, 2012, 05:22:05 pm
content: Both setups are exactly the same. There is a 10M to ground on the + of the opamp on both LTSpice and circuit board.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 28, 2012, 06:00:34 pm
content: Yes, but you are not poking a physical scope probe in your simulation do you? Please read my first reply again...

author: Enron
author_rank: Contributor
timestamp: December 28, 2012, 06:34:11 pm
content: I added a 1M ohm from + opamp to GND. My Rigol has a input impedance of 1M. Now I am getting what the Rigol is in LTSPice.Do I have to do this at every point now (add 1M to GND) that I want to measure to get what the actual hardware reads?

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 28, 2012, 07:31:44 pm
content: Quote from: Enron on December 28, 2012, 06:34:11 pmYou don't have x10 probes? They should give you a 10M input impedance.QuoteNo, only on high impedance points. You can imagine that having 10M impedance in your circuit and then adding 1M to ground will do something to the result, normally you will not encounter this. If in doubt just temporary add a 1M/10M resistor and see what happens.

author: Enron
author_rank: Contributor
timestamp: December 28, 2012, 07:50:06 pm
content: Thanks for all the help. My main problem was that I was using x1 on my probe.I switched it to x10 on the probe and scope, added a 10M to the +opamp and the number match perfectly. Thanks you so much PA0PBZ for your observations. kudos

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 28, 2012, 08:07:59 pm
content: No problem, it's always good to see another mystery solved!

author: w2aew
author_rank: Super Contributor
timestamp: December 28, 2012, 08:17:54 pm
content: Quote from: Enron on December 28, 2012, 07:50:06 pmThe thing you didn't take into account in the simulation was the capacitance associated with the 1x probe - probably on the order of 60-100pF (scope input capacitance and the coax capacitance).