Can someone explain to me "PWM Rejection" (Current Sense Amplifiers) - Page 1
Forum section: electronics / beginners

author: doublec4
author_rank: Regular Contributor
timestamp: October 01, 2020, 03:15:37 am
content: Hi all,I asked a question a few days back about a current sense amplifier application and since then I spent many days diving into the theory behind them and how to select them. I now have a much greater understanding than I did a few days ago.One thing I cannot seem to find good layman's information on is the idea of "PWM rejection" as it applies to a current sense amplifier. I'm hoping someone might be able to give me a simple explanation or example.I will have an application where I will be measuring the current consumption of an LED unit being pulsed at 200Hz, I believe the duty cycle is 50% . The LED unit also has an on board controller that flashes / strobes the LED in different patterns. In my case, I would be selecting a pattern that flashes roughly once per second and therefore the current would be going from ~0 to 0.25A repeatedly, and when on it would be PWM'd at 200Hz.Would my application benefit from a current sense amplifier that features PWM rejection, or what would be the implication of having such a feature for this case?Thank you

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 01, 2020, 01:29:33 pm
content: Show where the term has been used.Tim

author: coppice
author_rank: Super Contributor
timestamp: October 01, 2020, 01:34:46 pm
content: Quote from: T3sl4co1l on October 01, 2020, 01:29:33 pmI think he is talking about the use of unfiltered PWM waveforms, where a current sense amp, that is looking for the mean load current, needs to LP filter the waveform (i.e. reject the pulsing) before the slow amp can handle it properly.

author: doublec4
author_rank: Regular Contributor
timestamp: October 01, 2020, 03:36:11 pm
content: https://e2e.ti.com/blogs_/b/industrial_strength/archive/2016/11/08/five-benefits-of-enhanced-pwm-rejection-for-in-line-motor-controlIn this article they use a motor as an example... I'm using an LED so I guess that would be considered a single phase, but at my PWM frequency (relatively slow, 200Hz) or even the flashing rate of the LED unit (ie. once per second) does this even apply to my application at all? I'm just trying to better understand the capabilities of these current sense amplifiers.I'm assuming with my "slow" application I would just continue to see the current rise and fall with the on/off flash rate of the LED unit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 01, 2020, 06:19:53 pm
content: They're just talking about techniques to improve CMRR. How they've actually achieved this internally is a trade secret (or at best, patented). No filtering is done, or perhaps more accurately, filtering is being un-done so as to preserve signal quality and bandwidth.Ya, at 200Hz you probably don't need filtering. But maybe you do, what do you need to measure the current for?Tim

author: doublec4
author_rank: Regular Contributor
timestamp: October 02, 2020, 12:25:38 am
content: Quote from: T3sl4co1l on October 01, 2020, 06:19:53 pmWhat I'm trying to do is prototype a safety light system whereby someone activates a pair of flashing lights from a control panel. The lights flash indicating that a worker is present in the area and would be a warning for incoming equipment operators that personnel is present.The current sensing is to show the person at the control panel if there is a light failure (ie. no current = no lights, half the current = one light has gone out). The lights may or may not be visible from the control panel this is why I wanted to have some indication of a failed state of the LED lights. The control panel would also activate a powered RFID tag as well. The current draw of the RFID tag is only 1.5mA @ 12VDC ... data sheet says 5mA (maybe some current induced when being read?) so I've done some calculations on the current sense amplifier to get a suitable range ~0.5mA to 7ma for the RFID tag.Perhaps there is a better way to detect a failed state of these devices?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 02, 2020, 12:39:29 am
content: Ah. Well, implementation will very much depend on what the standard requires, and it may be worthwhile consulting with a safety/automation company to see how it's done, or use an off the shelf system, etc.Situations like that, are perhaps easier to deal with as part of the state machine. For example, when current is detected, assume it's persistent for some duration; after all, human eyes don't respond to light any faster than the persistence of vision (>10ms), and humans themselves aren't going to respond much faster than 100ms to a stimulus. Further, if a light is flashing for example, you have to wait until it flashes again to tell if it's still flashing, or gone solid on or off; the minimum detection delay is about half the period.The electronic building block you'd use to implement this, is a missing-pulse detector, or other combination of logic with a resettable monostable timer.Which is basically a digital version of an analog filter, but perhaps the different toolkit will get you thinking about it differently.So, a person at the control panel is detected by the power consumption from an RFID transponder? Is that a good idea? Why not communicate with the digital interface? In what possible ways might the transponder malfunction, and how many of them can you detect safely with each method?Tim

author: doublec4
author_rank: Regular Contributor
timestamp: October 02, 2020, 01:25:04 am
content: Quote from: T3sl4co1l on October 02, 2020, 12:39:29 amThanks again for the response, I will have to look into the "missing pulse detector" / analog filter etc ! For the LEDs, I was going to take the approach of trying to detect the "rising edge" of the analog voltage output of the current sensor. This would start a "count" whereby the current sensor output would be monitored for the "falling edge" and the average value over that half period would be calculated and compared to what would be "normal."As for the RFID tag, I should have been more clear. The RFID tag is actually on the ground/wall/entrance way of the room. Fork lift, whatever equipment going into the room would have a tag reader that reads the active tag and a light on the dashboard would flash as a reminder that personnel is present. Sort of a redundancy to the entry lights flashing but something that is a continuous reminder to the operator.Again, this is just a prototype and I'm sort of brown nosing it where I work. I'm a mechanical engineer, not electrical but I enjoy these little projects on my spare time and I'd like to put something together as a demonstration. Eventually it would be professionally implemented as you suggested by an automation / safety company. But for proof of concept I would really like to be able to put something together!