negative rail for low side current sense?

Take a look at ADE7912 and 13 maybe.

Thanks for reminding me about these; I've used the ADE7753 before. Tempting, but... I already have the ADC part in the MCU, and the ADE7912 doesn't sample fast enough for the software I'm using. Plus, those are big chips compared to a single quad op amp. Also, 3.3v, although that's work-around-able.

Yeah, unspoken requirements, but...

A pretrimmed instrument amp (INA105 for example) can do the translation; that one takes inputs 5V outside the rails so should handle 40 mV easily. You get the accuracy by paying extra, of course.

If you are using one uP ADC, you could make one good amp and mux into it.

St full current, the rails should not be exceeded.

Maybe I am missing what you are doing, but you should not see signals below 0V

Cheers

Klaus

I have several (the RX66T is designed for this), the problem isn't that side of the circuit - the problem is dealing with the original signals, which will go outside the rails.

That one says inputs are limited to +- Vs (+- 15V supply), which leads back to one of my original questions - if I need a negative rail.

The sense resistor gets bidirectional current, so the sense voltage can be negative (not much, maybe 40 mV) and between that, the offsetting resistors for virtual ground, and the amplification needed, it results in a signal at the IN- pin that may go below 0V.

The *output* of the opamp is centered around 2.5v, sure.

On a sunny day (Fri, 18 Mar 2022 13:58:39 -0400) it happened DJ Delorie snipped-for-privacy@delorie.com wrote in snipped-for-privacy@delorie.com:

I replaced some LM with the CMOS TLC274CN quad opamp. Those have a minimum inout voltage range from -.2 to -.3

An RRIO opamp will process signals on both sides of ground. If you mux into one amp that DC offsets up into about the middle of your unipolar ADC range, one of the mux inputs can be ground. Digitize that once in a while and subtract it from the others.

You could do an opamp per channel, and tolerate a modest DC offset error. Depends on the accuracy you want and how much you want to pay for resistors. You could software autozero if you ever know for sure that the current is zero.

I was thinking, if I ended up creating a 2.5v rail, I'd hook that in to an ADC channel too. The RX has its own internal mux (three units of

Yeah, the software had an autozero function. I was never really happy with how well it worked.

Oh, those look promising... thanks!

The datasheet I read does NOT say that at all. It says +20 to -20V, and claims ten volts from the rails is harmless.

Other options are better still:

It also says, under "Absolute Maximum Ratings", that the input volage range is +- Vs. Sigh. "If you can't trust the specs..."

It also requires a +- 5v supply minimum, and if I need to add a negative supply, any opamp would do.

The catch there is that it's relying on the internal resistor network to bring the opamp inputs into range. In my case, the signal is small, but still out of range at the opamp inputs, despite the resistor network.

Yeah, I'm reading input/voltage range/common mode (also a footnote that says it goes to +/- 25 V with +/-15V power and no 'protection').

Just use a +12 supply (and a 2.5V source/sink for "ground") . The symmetric-dual-supply language is merely a convention, not a requirement.

It needs that '2.5V' point to be wired to its 'ground' pin, is the hard-to-communicate part.

The real catch is, the resistor network is (laser-trimmed?) very accurate, and that add-on hikes the price up. It's difficult to search on common-mode range, but duals and quads exist in current-sense offerings, that ought to do the job.

I have a +15v supply and a +5 supply already. I'm offsetting to 2.5V for the ADCs, but the sense resistors are on the power lines for the BLDC motor - which is a 160V 8A nominal system. I can't just offset that by 2.5V ;-)

(well, effectively I could, but that's where the negative rail comes into play)

Now I wonder if I could offset the +15v supply down 2.5v, but I don't think the power chip supports that (IKCM15L60GAXKMA1) - looks like it allows up to +- 5v between the low side pins and Vss. Hmm...

For a unity gain amp, yeah, that would bring it up. I need to also amplify it up to +- 2.5v swing, and doing both in the same stage causes problems. Doing it in separate stages means twice the chips (and twice the cost, twice the pcb area), so... tradeoff. I could use that pcb/cost to add a negative rail instead. Which is better?

Or a negative charge pump.

Sylvia.

You offset both the input and the reference, they should both be around