Reduce voltage - Page 1
Forum section: electronics / beginners

author: funkimunky
author_rank: Contributor
timestamp: October 22, 2012, 09:33:39 pm
content: I have a signal out of an op amp with Vavg of 5v but the signal I want is only 100mA voltage peak to peak. I want to reduce the signal by 3 volts but keep the signal riding on it. How would I do this?

author: LEECH666
author_rank: Frequent Contributor
timestamp: October 22, 2012, 10:06:05 pm
content: First: 100mA Voltage ... Ampere is the unit for current, not for Voltage, so that kind of doesn't make sense.Second: A schematic/circuit diagram will make it a lot easier to help you. Please include one.It's easy to adjust the gain of an op amp with the change/additon of 2 resitors, but without a circuit diagram it's hard to tell what you're trying to achieve or what your circuit actually looks like. And that makes it not easy to recommend anything.Cheers,Florian

author: LEECH666
author_rank: Frequent Contributor
timestamp: October 22, 2012, 10:17:47 pm
content: Take a look here to see how the ouput voltage is calculated for the inverting and the non inverting amplifier (I just assume it will be one of those two basic circuits).http://en.wikipedia.org/wiki/Operational_amplifier_applications#Non-inverting_amplifierJust select one of the resistors (like R1 = 20K or so) and then calculate the other one (R2) from the formula.

author: TriodeTiger
author_rank: Regular Contributor
timestamp: October 23, 2012, 05:05:46 am
content: I can't imagine it isn't 100mV, given the context. You say Vavg, so it is not a 5V with AC component where a decoupling capacitor can be used?If they are different frequencies maybe a high pass filter with a cutoff around the lower frequency could result in the higher frequency signal to look at.I do not know where to start however myself on this, so maybe someone else can chime in :-)

author: funkimunky
author_rank: Contributor
timestamp: October 23, 2012, 09:18:43 am
content: yeah your right its 100mV I wrote this post last night so was a little tired. I used the op amp to amplify the signal from a condenser mic that needs 1.5v to run. This gives peak to peak signal of 40mV. I wanted to amplify the peak to peak signal that is super imposed on the 1.5v. after amplification the peak to peak is amplified to 250mV as well as the 1.5V base voltage to 5V. I now want to keep the peak to peak information at 250mV but reduce the voltage to 2v or less for processing with an arduino. Is there a way to reduce the voltage but keep the peak to peak signal as is. I tried using voltage divider but this also reduced the peak to peak signal. I have also tried adding -3v to the signal but cannot get them to combine. please see drawing attached.

author: Rerouter
author_rank: Super Contributor
timestamp: October 23, 2012, 09:21:24 am
content: use a capacitor to remove DC offset (series) and bias your signal with some high value resistors so it floats around the point you want,

author: funkimunky
author_rank: Contributor
timestamp: October 23, 2012, 04:35:20 pm
content: Thanks for helping me out. I going to have a look at how bias works. I have found some info about using capacitors to remove offset. Your reply has really helped me as I had reached the end of my knowledge on how to do this.