AC component remove - Page 1
Forum section: electronics / general

author: MONO29
author_rank: Newbie
timestamp: July 12, 2018, 03:33:51 pm
content: Hi.I want remove AC component, i need to remove it to measure only the DC component in a shunt resistor. I have tried a low pass filter but it is not enough, I could use a choke coil or something similar to eliminate it or create a very high impedance for the AC component.thank

author: Benta
author_rank: Super Contributor
timestamp: July 12, 2018, 03:49:37 pm
content: I'd suggest an op amp integrator, unless you have demands on how fast the circuit should react.

author: MONO29
author_rank: Newbie
timestamp: July 12, 2018, 04:28:28 pm
content: I'm not sure that an integrator can help me. I'm doing an isolation monitor, I'm going to try to explain (my English is bad), i try to measure the ground leakage resistance of a system, i found an application note, but it's not exactly what I'm doing.http://www.ti.com/tool/tida-00440?keyMatch=leakage%20current%20measurement&tisearch=Search-EN-Everythingto measure the leakage resistance I inject a 12 V DC and I measure in shuntThe problem is that I must perform the monitoring of an isolation transformer while it is working and the AC component passes through the shunt resistance.As you can see, in the simplified circuit, the ac component passes through the shunt resistance. I need to divert or delete the ac component to measure correctly.thank

author: MONO29
author_rank: Newbie
timestamp: July 12, 2018, 08:21:59 pm
content: In this image you can see better the AC component that I want to eliminate. I want to measure in the shunt resistance only the DC component

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 12, 2018, 10:17:23 pm
content: Use a high order low pass filter after the first RC stage. There are chips that pack as many as 8 into a very easy to use package.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: July 13, 2018, 04:08:32 am
content: It sounds like you want to subtract the corrupting AC component from the signal on the shunt resistor and measure the result. Try that. First AC-couple and attenuate V1 to obtain a small amplitude sample of the AC voltage. The AC time constant can be slow enough to include only the fundamental and a few harmonics. Then use a summer to add in this inverse version of this AC sample so that it completely cancels the fundamental and low-order harmonics of the power frequency from the sample at the shunt resistor. The summer will need to be calibrated to get best rejection using zero leakage but that can be done with a small trim pot to get you 60 dB of rejection. Then low-pass filter the result to below the power frequency (1-2 Hz?) to get the desired DC result without having to wait a long time for a massive low-pass filter time-constant to settle.

author: MONO29
author_rank: Newbie
timestamp: July 13, 2018, 09:55:59 am
content: Quote from: NiHaoMike on July 12, 2018, 10:17:23 pmHi thanksCould you give me an example of these chips please?Quote from: basinstreetdesign on July 13, 2018, 04:08:32 amok I tried it and these are the resultsI used a subtracter (U3), with a high pass filter of 0.5Hz (C1-R4) to subtract only the AC component, and on the output I used a low pass filter of 0.5Hz (R9-C2).The AC component is attenuated but the CC component is 3V, that is not correctthe AC component in XXM4 is 22V, it may be because it does not have a "ground""Ground" is different than digital ground.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 13, 2018, 12:33:40 pm
content: Quote from: MONO29 on July 13, 2018, 09:55:59 amhttps://www.maximintegrated.com/en/products/analog/analog-filters/MAX7400.html

author: ogden
author_rank: Super Contributor
timestamp: July 13, 2018, 12:43:39 pm
content: You could use ADC withmains rejection filter.Synchronous detector is even better solution:http://www.analog.com/en/analog-dialogue/articles/synchronous-detectors-facilitate-precision.htmlother name:lock-in amplifier

author: MONO29
author_rank: Newbie
timestamp: July 13, 2018, 03:09:12 pm
content: Quote from: NiHaoMike on July 13, 2018, 12:33:40 pmthank NiHaoMike, the problem is that this component is not the SPICE model in mulstisim or proteus, I can not simulate it.Quote from: ogden on July 13, 2018, 12:43:39 pmHi ogden, if i use ADA2200 would it solve my problem? I have never used that type of system and I do not know, I can not simulate it because it does not appear in the simulators, I would have to test it directly on the PCB and that is very expensive, how could I make calculations for my system or test it?with what you have commented, I have tried a filter of a higher order (I am not an expert in filters), the problem of the AC component is solved but the time constant is very large. I need a filter (or something) that has a lower time constant.PS: I will study better the "lock-in amplifier" (ADA2200) and the MAX7400

author: MONO29
author_rank: Newbie
timestamp: July 13, 2018, 05:33:19 pm
content: another more satisfactory test, I used a filter LINKWITZ 24DB / 4th ORDER and 24DB.http://hispavila.com/total/3ds/tutores/filtrosactivos.html#fapb4ordenand I have managed to eliminate the AC component completely, leaving a noise of the order of nA versus mA. but it's still a bit slow, about 2 or 3 seconds. Any ideas to improve this? this could be improved with a ADA2200 or MAX7400.

author: ogden
author_rank: Super Contributor
timestamp: July 13, 2018, 09:39:43 pm
content: Quote from: MONO29 on July 13, 2018, 03:09:12 pmWell.. how to say.. In case you ask then I am not sure. Chip alone will not solve anything. You have to understand synchronousrectificationdetection up-to the level to answer your question yourself.