op amp help. - Page 1
Forum section: electronics / beginners

author: electronics man
author_rank: Frequent Contributor
timestamp: July 30, 2015, 02:16:07 pm
content: hi, i need an op amp circuit which that will give me a voltage offset of a voltage that will go into an ADC. here is what i want:when Vin = 0V Vout = 2.5Vwhen Vin = 2.5V Vout = 5Vwhen Vin = -2.5V Vout will = 0vany ideas?

author: PSR B1257
author_rank: Frequent Contributor
timestamp: July 30, 2015, 02:56:51 pm
content: Not so difficult, as you might thinkIf the signal source is high impedance you need an additional voltage follower.

author: electronics man
author_rank: Frequent Contributor
timestamp: July 30, 2015, 03:04:51 pm
content: Quote from: PSR B1257 on July 30, 2015, 02:56:51 pmwill this work with the tl072?

author: electronics man
author_rank: Frequent Contributor
timestamp: July 30, 2015, 03:12:18 pm
content: yup it works thanks a lot

author: PSR B1257
author_rank: Frequent Contributor
timestamp: July 30, 2015, 03:17:23 pm
content: QuoteNo, this is not an rail to rail OPAMP. But it will work with a bipolar supply (minimum -2V to 8V)Than you need a voltage regulator, to generate the 2.5V auxiliary voltage.

author: Zero999
author_rank: Super Contributor
timestamp: July 30, 2015, 05:20:40 pm
content: Quote from: PSR B1257 on July 30, 2015, 02:56:51 pmIt can actually be made using only one op-amp.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: July 30, 2015, 06:06:33 pm
content: Clever. Now we got one spare OPAMP to use as a buffer for the input signal.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: July 30, 2015, 08:21:06 pm
content: Forgive my naivete... Still learning about OpAmps...Would something simpler like this work? What is the purpose of the resistors to ground?Basically, we want a summing amplifier with unity gain, right?Note I have not tested this, and I am a newbie. Just trying to learn more.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: July 30, 2015, 08:29:18 pm
content: Oh, wait... I see the errors in my diagram, and the inverting vs. non-inverting issue.But I would like to hear about the resistors to ground.Thanks.

author: leblanc
author_rank: Regular Contributor
timestamp: July 31, 2015, 12:39:43 pm
content: Or even a bit simpler.

author: leblanc
author_rank: Regular Contributor
timestamp: July 31, 2015, 01:11:24 pm
content: I've done the calculations for your circuit to show you what it does.You cannot blindly use a voltage divider to feed a voltage summer. If you have a 5V to 2.5V divider made of two 20k resistors, it is equivalent to a 2.5V source with a 10k in series (thevenin equivalence).So in your summer circuit, you need to account for this resistance. The typical summer formulas assume that the sources have zero output impedance. So let's say you have a summer with 3 sources, V1, V2, V3, the summer has R1, R2, and R3 for each of these sources. The formulas use R1, R2, and R3 to calculate the relation between input and output. But in reality, you need to use (R1+Rout_V1), (R2+Rout_V2), (R3+Rout_V3).Your rails (5V and 0V) have near-zero output impedance (especially when compared to resistor values in the kohm range).Note that there is also the issue of input impedance of the amplifier circuit (which we assume is infinite). For non-inverting configurations, this is almost true. For inverting configurations, it can have an effect.