Op Amp offset for no reason, is that normal? - Page 1
Forum section: electronics / beginners

author: alexg
author_rank: Regular Contributor
timestamp: November 18, 2016, 04:10:09 pm
content: Hi All, quick question, I have this op amp:http://www.ti.com/lit/ds/symlink/lmv551.pdfAnd I am building simplest inverting voltage follower as you can see in first attached image. Just feeding sine wave, perfectly centered, 680mV amplitude into that op amp in inverting voltage follower configuration, single supply mode so it does not go below zero since there is no negative supply voltage. As you can see, op amps output is 23mV above zero and 32mV less peak than input. What am I doing wrong? Is it suppose to be like that?PS: I also tried this op amp as non inverting voltage follower, same result.

author: Chalcogenide
author_rank: Regular Contributor
timestamp: November 18, 2016, 04:39:03 pm
content: You have built an inverting stage, but since you are feeding the positive input of the opamp with zero volts, the output will want to swing below ground, which the opamp cannot do since ground is its negative supply, so you are clipping.You need to feed the positive pin with a positive voltage (typically half the supply) and make sure that the input sinewave also has a DC component equal to the positive input bias voltage. In order to do so, you might want to add a capacitor to AC couple the signal.

author: dmills
author_rank: Super Contributor
timestamp: November 18, 2016, 04:44:32 pm
content: Looking at the datasheet, with a 100K (not 10K) load the thing is only specified to get within 70mV or the rail, so I would call 23mV quite a bit better then I had a right to expect.Gain seems to be slightly off, but only by about 5% which could be measurement error (Scopes do NOT make good voltmeters).73 Dan.

author: wraper
author_rank: Unknown
timestamp: November 18, 2016, 04:47:29 pm
content: You have both input and output clipping. As opamp is inverting, positive output swing is clipped by input as you go significantly below zero. Negative output swing is clipped by output of the opamp. Although it is rail to rail, it cannot go exactly down to 0, check specs in the datasheet.

author: danadak
author_rank: Super Contributor
timestamp: November 18, 2016, 04:58:59 pm
content: Various methods to handle below ground signals -https://www.dropbox.com/s/k1yn4m7heaixvmg/ADinput%20negative%20inputs.zip?dl=0Regards, Dana.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 18, 2016, 09:21:30 pm
content: How matched are the resistors?Note that, since you can't measure the offset around 0V (because the amp saturates, as you noted), you have to look at a positive signal, or the positive  peaks. But the positive peaks also include differences due to gain, i.e., if the resistors aren't well matched. So, it's inconclusive.Tim

author: Zero999
author_rank: Super Contributor
timestamp: November 18, 2016, 09:44:32 pm
content: As mentioned above, it's not correctly configured as a single supply amplifier. See the schematic in the bottom right of the attachment, for a single supply inverting amplifier.

author: w2aew
author_rank: Super Contributor
timestamp: November 18, 2016, 11:49:19 pm
content: Also, be careful about how you probe your circuit. If using 1X probes, the capacitive loading can be significant (100pF or more) and could load the circuit. If using 10X probes, which is highly recommended due to the capacitive loading issue, make sure that they are properly compensated.

author: alexg
author_rank: Regular Contributor
timestamp: November 19, 2016, 04:35:12 am
content: Thanks everyone for help and suggestions, I built small PWM negative voltage circuit (as shown in one of Danadak's files) to supply this circuit with negative voltage and everything working much better now. Waveforms match almost 1:1 and if I add couple diodes to cut off lower half of the wave, now it clips precisely at zero. Thanks.When this was powered by single supply, the reason why I don't want to offset input by 1/2 supply voltage is because the next step for this is to build full wave rectifier and for that I want output to start at zero.