DC Offset after a Voltage Follower  - Page 1
Forum section: electronics / projects

author: aw_rw1
author_rank: Contributor
timestamp: February 01, 2023, 07:01:16 pm
content: I have a MiniGen from SparkFun. I am trying to get 1-100Hz out of it. So far, the MiniGen can produce this, but the output is offset by a Vcc/2, about 1.6VDC. I suppose the offset is due to the power being 3.3VDC. To remove the offset, I put an AC coupling capacitor in series with after the MiniGen.The problem is after the capacitor when I pass the MiniGen signal through a voltage follower. I get another offset. I don't see an offset after the capacitor, but I see an offset after the voltage follower.Oddly, the offset after the follower goes away by increasing the coupling capacitance to 1uF. Another way to get the offset after the voltage follower is to load the cap with a large resistor about 1MOhm.Does anyone know why there is an offset after the voltage follower? The follower should just be 1-to-1 with its input.

author: Slh
author_rank: Regular Contributor
timestamp: February 01, 2023, 07:11:52 pm
content: Sounds like the opamp input bias current. Opamps are sadly not as ideal as all the tutorials pretend.I think I get what your circuit is but if you put your circuit with part numbers then I think I could be more help.

author: aw_rw1
author_rank: Contributor
timestamp: February 01, 2023, 07:17:10 pm
content: The opamp that I am using for the voltage follower is a TL071.

author: wasedadoc
author_rank: Super Contributor
timestamp: February 01, 2023, 07:35:14 pm
content: Quote from: aw_rw1 on February 01, 2023, 07:17:10 pmIs that a typo? LT071 or TL071?

author: magic
author_rank: Super Contributor
timestamp: February 01, 2023, 07:47:28 pm
content: Quote from: aw_rw1 on February 01, 2023, 07:01:16 pmWhat do you mean by "loading the cap with 1MΩ"?Theremustbe a resistor to ground after every AC coupling capacitor if you want the output to be centered around ground.Measuring the output voltage of the capacitor with a typical DMM connects ~10MΩ resistance to ground, which may act as the missing resistor and temporarily bring the voltage towards ground.

author: shapirus
author_rank: Super Contributor
timestamp: February 01, 2023, 07:50:29 pm
content: It would be helpful to see schematics, also indicating the points at which you take measurements.

author: Bud
author_rank: Super Contributor
timestamp: February 01, 2023, 08:10:46 pm
content: Does the buffer follow the gen's signal excursions? Then it *is* a follower. Local DC bias does not account for following action.

author: Bud
author_rank: Super Contributor
timestamp: February 01, 2023, 08:12:05 pm
content: And if you power the follower from a unipolar power supply you cannot expect the follower output to go negative.

author: Bud
author_rank: Super Contributor
timestamp: February 01, 2023, 08:15:58 pm
content: By the way, 1uF is awfully small for 1Hz (even 100Hz) signal, you should put more like 100uF

author: aw_rw1
author_rank: Contributor
timestamp: February 01, 2023, 09:02:56 pm
content: Sorry, there was a typo. The opamp is a TL071.

author: aw_rw1
author_rank: Contributor
timestamp: February 01, 2023, 09:15:59 pm
content: Attached is the schematic of the circuit that I am working with.Again, opps, typo again. Opamp is TL071.

author: aw_rw1
author_rank: Contributor
timestamp: February 01, 2023, 09:40:58 pm
content: QuoteI suppose the capacitance is small. I saw on this site common capacitor sizes:http://www.learningaboutelectronics.com/Articles/What-is-a-coupling-capacitor"For coupling a 100Hz signal, a 10μF capacitor can be used."However, when I used a 10uF without the voltage follower, I saw a huge offset. I tried 10uF with the voltage follower and RHP just now. The offset is nearly 0 VDC, about 0.05VDC.

author: gf
author_rank: Super Contributor
timestamp: February 01, 2023, 09:52:03 pm
content: C1 and RHP form a high-pass filter. Therefore the value of C1 depends on the the value of RHP and the desired cut-off frequency.

author: timenutgoblin
author_rank: Regular Contributor
timestamp: February 01, 2023, 10:02:29 pm
content: Quote from: aw_rw1 on February 01, 2023, 09:15:59 pmI'm not sure I understand exactly what your circuit is doing.I think that your schematic has a typo.https://www.electronics-tutorials.ws/opamp/opamp_2.htmlFor a non-inverting opamp circuit, the input signal goes to the non-inverting input and the output is connected to the inverting input.https://www.electronics-tutorials.ws/opamp/opamp_3.html

author: Bud
author_rank: Super Contributor
timestamp: February 01, 2023, 10:45:50 pm
content: The output of the op amp has to be connected to the Inverting input. And inpug signal should go to non-inverting pin. Your wiring of the op amp is wrong.

author: aw_rw1
author_rank: Contributor
timestamp: February 02, 2023, 04:19:11 pm
content: QuoteYes, the two components do form a high-pass filter. Ideally, RHP shouldn't be there if C1 was an AC coupling capacitor. RHP is there since we are talking about this thread.QuoteQuoteYes, there is a typo in the schematic. I draw it in a rush. Attached a revised version. The device name and wiring of feedback were corrected.

author: magic
author_rank: Super Contributor
timestamp: February 02, 2023, 04:23:47 pm
content: Okay, so what are the values of C1 and RHP?What's the voltage at the input of the opamp?And at the output, preferably measured simultaneously with a second meter?

author: shapirus
author_rank: Super Contributor
timestamp: February 02, 2023, 04:35:46 pm
content: Quote from: aw_rw1 on February 02, 2023, 04:19:11 pmCheck this:https://electronics.stackexchange.com/questions/206275/do-i-need-to-make-a-dc-path-at-the-input-of-an-op-amp...or, a little more fundamentally, this:https://www.analog.com/media/en/technical-documentation/application-notes/an-937.pdf

author: aw_rw1
author_rank: Contributor
timestamp: February 02, 2023, 05:23:41 pm
content: QuoteThis values are in the spec out phase. At the moment, 10uF and 1uF for C1 seems to work, and 1MOhm and 0.5MOhm for RHP seems to work. I think RHP should be lower to lower any Johnson noise.QuoteInput voltage ranges from 0.7 to 1.1 Vpk-pk, and the frequency ranges from 1 to 100Hz.QuoteThis app note is spot-on.

author: shapirus
author_rank: Super Contributor
timestamp: February 02, 2023, 05:35:09 pm
content: Quote from: aw_rw1 on February 02, 2023, 05:23:41 pmKeep in mind your high-pass filter cut-off frequency when you consider 1 Hz. However if you take 10uF and 1MOhm, it shouldn't noticeably affect your input signal even at 1Hz.