Clarification on Energy metering IC - Page 1
Forum section: electronics / beginners

author: Freshman
author_rank: Regular Contributor
timestamp: May 09, 2025, 10:37:44 am
content: I am currently working on a project that involves the use of the M90E32AS energy metering IC. As I am relatively new to this domain, I am planning to base my design on the official reference design:Metering IC:https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ProductDocuments/DataSheets/Atmel-46003-SE-M90E32AS-Datasheet.pdfReference design provided over here -https://ww1.microchip.com/downloads/aemDocuments/documents/SE/ProductDocuments/BoardDesignFiles/ATM90E3x_V1.05.zipI would appreciate your guidance on the following technical points:1. Current Sensing Method:From the documentation, it appears that the IC supports current sensing via current transformers (CTs). Could someone clarify if this IC is limited to CT-based measurements only? Additionally, I would like to understand the circuit-level differences between a CT-based current measurement and a shunt resistor-based measurement.2. Component Value Selection (Refer to attached image):How are the resistor and capacitor values in the current sensing circuit determined?Specifically, the design uses two 2.4Ω resistors as the burden resistor. Is there any reason I shouldn't replace them with a single 4.8Ω resistor? What are the design or safety considerations for this choice?3. Grounding Configuration:The circuit includes what seems to be a center-tap ground between two resistors and capacitors.What is the purpose of this center-tap connection?Why is it necessary to connect this midpoint to ground, and what function does it serve in the measurement or signal conditioning?Any insights or references you can share would be greatly appreciated.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: May 09, 2025, 01:30:21 pm
content: The largest advantage of using a CT is isolation, and in the case of a severe overload like a short circuit, the likelihood of the circuit surviving is greater.Having said that, low cost CTs aren’t as linear and may introduce a phase shift, important if you’re planning in making very accurate power readings or if you’re calculating power factor.Always use the lowest manufacturer-recommended burden resistor.

author: Freshman
author_rank: Regular Contributor
timestamp: May 09, 2025, 01:41:43 pm
content: @UnijunctionTransistor, thanks.Could you provide some clarification for all my questions for the benefit of all. I checked the datasheet. It doesn't mention anything about shunt type of current measurement? Is shunt measurement not possible with this IC.Could you also let me know what's the logic behind or the calculation of the burden resistor value? And what is the RC network there for

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: May 09, 2025, 02:34:29 pm
content: I am away from home and reading this on my phone. It is difficult to read a whole complex datasheet on a small screen.I will answer based on my experience using other power meter ICs.-The resistor-capacitor configuration is a low pass filter to attenuate high frequency noise which is always present on power lines. This high frequency noise could cause aliasing artifacts. Additionally, for powerline measurements the bandwidth of interest is limited to 2 Khz. 33rd harmonic on 60 Hz, or 39th harmonic on 50 Hz.-For improved performance and to determine the polarity of AC signals, it is advisable to employ a differential ADC. These differential levels require to be referenced to something, in this case ground.I would build a prototype as indicated by the datasheet. Measure the performance, only then attempt any cost reduction efforts and evaluate performance again.EDIT: 2kHz was the bandwidth used back in the early 1990s when I was involved. Nowadays a 4kHz is recommended. I calculated the filter’s frequency as 8 kHz, which is sufficiently removed to prevent phase-shifting artifacts.

author: Freshman
author_rank: Regular Contributor
timestamp: May 09, 2025, 02:56:33 pm
content: @UnijunctionTransistor, thanks a lot.Just a couple of questions.1. The datasheet doesn't mention anything about shunt type of current measurement? Is shunt measurement not possible with this IC.2. Why cannot I use a single burden resistor instead of two? Can the center tap ground between the two burden resistors be removed?

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: May 09, 2025, 03:35:24 pm
content: Take paper and pencil and draw a current waveform.Then sketch what the voltage waveforms, referred to ground, would look like at the terminals IxP and IxN with the two burden resistors as indicated in the datasheet, and with a single burden resistor as you propose.