remove dc offset from signal going into an op amp - Page 1
Forum section: electronics / projects

author: beandigital
author_rank: Contributor
timestamp: May 16, 2019, 06:03:45 pm
content: I have a signal that varies between 19V and 17V @ 250kHz. The signal operates in a burst pattern where it outputs 2048 values then stops for a few ms. I want to input this signal into an op amp and remove the dc level. I have a circuit that has a dc blocking capacitor between the signal and the op amp input. The op amp is set as a buffer, and also has a resistor to ground. So I have a high pass filter on the input of the buffer. The issue I have is that I am finding that the circuit takes a long time to settle if I select values that maintain the signal integrity. If I then change the values to speed up the settling time the signal starts to degrade. Are there any other options that will maintain the signal integrity whilst giving a fast settling time? I am open to using a completely different circuit. Thanks

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 16, 2019, 06:11:33 pm
content: What is the time constant of your R-C coupling circuit?Could you share some scope images of what you see as signal degradation?During idle, what is the DC voltage value? Is it steady?Without knowing the answers to the above questions, and with many actual details missing, my first idea would be to have a circuit which would get the average DC voltage integrated for a long time, and apply that voltage to a difference amp which also processes your signal.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 16, 2019, 06:25:02 pm
content: This probably means that your pulse sequences are not DC balanced, or have substantial signal content below the cutoff of your highpass filter. This causes the capacitor in your RC to charge up and then bleed off slowly.Is the output during the "idle" periods the DC offset? If so you can use a sample-and-hold architecture to sample the DC offset only during the idle periods. Then use an op-amp difference circuit to subtract that from the input signal.

author: Marco
author_rank: Super Contributor
timestamp: May 16, 2019, 06:43:31 pm
content: How long does it send and how long does it not? Unless it's not sending for say 10× .more than it is, a capacitor is not the right solution.The signal presumably has DC even after you remove 18v. So if the steady state of the signal is 17v, it will always try to pull the signal up during the burst.

author: strawberry
author_rank: Super Contributor
timestamp: May 16, 2019, 06:46:58 pm
content: offset OP reference input 0V to 17V and amplifie differencecapacitor give error due to PWM average value

author: beandigital
author_rank: Contributor
timestamp: May 16, 2019, 07:34:22 pm
content: The signal looks like a square wave when its outputting. High (19V) for 2us then low (17V) for 2us. This occurs 2048 times then the output is 17V for about 10ms. I have a 100n capacitor and have tried various resistors, but as I mentioned improving one thing makes the other worse. So I am not sure that this is the right way to go. I need the signal integrity to remain but also have a fast transient response.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: May 16, 2019, 07:43:34 pm
content: You should try a second order bessel filter.Googling that will give a lot of informationSent from my iPhone using Tapatalk

author: Marco
author_rank: Super Contributor
timestamp: May 16, 2019, 09:43:51 pm
content: Quote from: beandigital on May 16, 2019, 07:34:22 pmDoesn't matter what kind of linear filter you use, that's not going to work. The 10ms it's not sending aren't enough to overpower the 8ms of signal with a variable DC offset.Assuming the 17V is stable enough, just subtract 17V.

author: StillTrying
author_rank: Super Contributor
timestamp: May 16, 2019, 10:53:43 pm
content: Quote from: beandigital on May 16, 2019, 07:34:22 pmCould you apply just the 2Vedgesto a comparator with about 0.5V hysteresis instead of an op amp.

author: mikerj
author_rank: Super Contributor
timestamp: May 17, 2019, 07:07:39 am
content: What exactly do you want to extract from the signal? e.g. are you just interested in the edges, or do you need to measure the amplitude of the pulses etc?

author: sanwal209
author_rank: Regular Contributor
timestamp: May 17, 2019, 08:14:02 am
content: You can try DC servo Offset removal circuit or phase lock in amplifier to remove offset.

author: StillTrying
author_rank: Super Contributor
timestamp: May 17, 2019, 11:18:58 am
content: What exactly do you want to extract from the signal? e.g. are you just interested in the edges, or do you need to measure the amplitude of the pulses etc?That's the vital question, analogue or just digital. If it's just the high/low switching that's needed a 555 run on 5V can be used as the comparator.