How to DC-offset to obtain A/C - Page 1
Forum section: electronics / projects

author: rodcastler
author_rank: Regular Contributor
timestamp: October 13, 2013, 07:55:00 pm
content: Hi all,I am new to EE and I found a nice signal generating software for my smartphone.I wanted to amplify the output signal and managed to do it with an op-amp and a 12V power supply (schematics attached).Now this would provide a sinewave from 0 to 12V centered at about 6v.How do I DC offset the signal to center it at 0v and obtain a -6v to +6v signal ?I want this resulting A/C signal for signature component testing using the X-Y mode on the scope.

author: penfold
author_rank: Frequent Contributor
timestamp: October 13, 2013, 08:11:02 pm
content: Based on that schematic it should be a sine-wave centred about zero, that DC blocking capacitor in series with the pot at the output should take care of that,Just to clarify something, is the capacitor symbol supposed to have a line going right through it? (just checking there's not a rookie mistake going on somewhere)

author: Sander
author_rank: Contributor
timestamp: October 13, 2013, 08:17:34 pm
content: It can't go below zero. I would rig an opamp to output half the Vcc and use that voltage to be your output "ground".

author: rodcastler
author_rank: Regular Contributor
timestamp: October 13, 2013, 08:34:15 pm
content: Thanks !If this circuit won't work, then what kind of circuit should I build in order to amplify and hopefully isolate a smartphone or soundcard generated sinewave that results in a sinewave that centers its oscillation at 0V in order to test components (component signature)?I can always obtain a 12V A/C signal from a small transformer but I was hoping to get it from this DIY signal generator.Any other thoughts?

author: Christopher
author_rank: Frequent Contributor
timestamp: October 13, 2013, 08:54:24 pm
content: You can get isolation amplifiers which you can setup as followers but I find them a bit mehQuite expensive and high offsets, not a lot of choice.For adding DC offset, how about using an opamp configured as an adder?

author: Sander
author_rank: Contributor
timestamp: October 13, 2013, 09:54:01 pm
content: Something like this? And for isolation you could use a 1:1 signal transformer or an analog optocoupler, but I doubt if you really need that.

author: rodcastler
author_rank: Regular Contributor
timestamp: October 14, 2013, 03:57:53 am
content: Hey, I think that may work !...Although I may be confusing that 1/2Vcc "GND" with real GND at some point and blowing something up.Q1: Where do I get 1/2Vcc? Can I just build a voltage divider for that?Q2: any way of centering it at "real" GND?

author: Sander
author_rank: Contributor
timestamp: October 14, 2013, 12:04:58 pm
content: You don't need to supply 1/2Vcc, the opamp with the 50/50 voltage divider on the positive input takes care of that.But C3 should block the DC-offset you experience, do you have a load on the output?

author: madshaman
author_rank: Frequent Contributor
timestamp: October 14, 2013, 01:05:51 pm
content: Quote from: rodcastler on October 14, 2013, 03:57:53 amThere's no such thing as "real" ground, it's an arbitrary point *you* define in your circuit; you can even have multiple grounds.Although it's an oversimplification, when you connect two circuits together you can connect their ground points to each other and you can also connect them to the actual earth. Connecting your ground to the earth simply pulls that circuit node to the same relative voltage as any another node connected to the earth.A better way to put it: unless you're dealing with electron devices (and even there you're unlikely to need to think of absolute "charge"; read: never) all you ever use in circuit calculations is the "relative potential difference", that is, all voltage is relative, there is no absolute "0" unless you assign "0" to some node.So, for Q1:Yes, a common technique is to use a voltage divider with its output connected to a unity gain buffer (opamp with 100% -ve feedback).for Q2:You can connect that ground point to the ground of another circuit so that all voltages in both circuits operate and can be analysed relative to that ground.You can also take that ground point and tie it to your metal plumbing or to the ground prong on an electrical outlet (which may or may not be tied to the actual earth; best to know if that's the case or not)edit: additions and spellingness

author: rodcastler
author_rank: Regular Contributor
timestamp: October 15, 2013, 03:11:14 pm
content: Thank you for that nice clarification. And yes, with "real" ground I just meant to match the GND of the amplifier just to be on the safe side.I'll give this circuit a shot over the weekend and I'll let you know if it solves the problem. Thanks !!!