Of course. That’s why a reasonably high power op.amp should be used. It should not then be necessary to separately filter the bias to each channel, doing so largely negates the purpose of the op.amp, which is to remove n-1 sets of resistor and large capacitor.
Not if it’s accounted for in software, which historically has always been the case.
I think I remember your tests and a comment a while back. I know a little bit more about how this circuit is behaving now, and it is definitely the case that track width and opamp spec’s will influence how much a signal on one channel effects the other.
I tried to gauge this impact on the most recent revision of the board, by @dBC’s suggestion of putting a high signal on all channels except for the middle one (channel 5).
There was a very small influence on the bias line, which came through as a reading on channel 5, about 1400 times smaller than the original signal across the 8 other channels.
So I won’t necessarily need an RC filter to isolate given such a small influence, and I think isolating the bias from the CT is a bad idea, intuitively, but it is interesting to note.
There is also an idea we’re testing out here… as a side-effect it further isolates the CT channels and provides an extra feature which we’ll be happy to promote once it’s tested.
Dan hi,
What’s the status of the development? When do you expect the hardware to be finalised, i.e. released as opposed to ‘in development’.
We’re building a new house next year and I thought I’d put one of these in rather than the emonbase we’ve had in the existing house.
If I bought one of the current boards what’s the likelihood that I’d need to upgrade when the release version is finalised? And would you guys cover that?
Also why is the price in the shop with the enclosure over £100 when you say in this post that the enclosure will cost £35?
Simon
Hi Simon,
CE testing is happening in the new year. We have a revision arriving this week which has a new feature on the CT inputs which, once tested, I’ll post the details of.
Sorry the shop price + enclosure has been set at +£40 to account for handling, then the assembled PCB and tax takes it over £100. I pay the printer £35 and originally thought to skip any extra costs.
I could arrange for a replacement of an upgrade sure. I’d wait until this board has arrived this week (I’ve had a notification of a delivery today…).
I’ll then update the shop page and post here.
Thanks Dan, I’m assuming you’ll post here when it’s arrived and has been tested?
In the meantime I’ll start looking at the firmware.
Simon
I’ll update this thread yes.
‘Sombrero Working7’ is the latest within the firmware folder.
Cheers
Sorry Dan, maybe I’m being thick but the latest version on Github under STM32 is V5.
Am I looking in the wrong place? I followed the link from the Shop page
Cheers Dan
Simon
Hi everyone!
I’ve been asked to make another revision on the stm32 board.
We’ve taken the unit for a pre-CE check at the CE lab we use.
A conversation with the director of the lab introduced me to seriously considering instrumentation amplifiers on the inputs instead of single opamps. Mainly, this is reduce cable to board noise by increasing CMRR.
Anyone have any recommendations when using instrumentation amps? Any suggested reading?
Any decent textbook on operational amplifiers will cover the topic. Of course, any improvement is a good thing, that’s indisputable. What I would question is, will there be, in most installations, a significant improvement in performance? Is the cost worth it? How many users will see a genuine benefit?
The argument runs like this: you cannot have a balanced input with a single op.amp, it is inevitable that the impedances at the + and - inputs are different, hence although you can have balanced voltages presented at the inputs, that doesn’t help and it is balanced impedances that are crucial to rejecting common-mode interference.
The book I have is “Operational Amplifiers”, edited by Tobey, Graeme & Huelsman, Published by Burr-Brown, ISBN 07-064917-0