High-commonmode differencing amp design

[Zero999]

Are you sure you need such a large bandwidth?

What's the maximum expected amplitude of the current ripple?

How about processing the AC and DC signals separately?

Set the cut-off frequency to the high side current amplifier to something low, preferably under the mains frequency in your area. This will give you the DC part of the signal.

Now, AC couple the high frequency part and amplify it separately. If you like, you could add it back to the DC signal but if it's going to be viewed on an oscilloscope it would be pointless because you won't have dynamic range to  be able to see it superimposed on a large DC signal.

[leonerd]

An update:

I tried an LT1999-10 with a 0.1R shunt resistor; this seems quite accurate. I then stuck an AD8271 after it in G=1 configuration, to level-shift the LT's differential output back down to ground-reference (with the AD powered by my generated split +5/-5 supply).

Turns out: this is terrible. There's a significant current flow between the LT's REF pin and the feedback network of the AD, causing about a 12% error in the indicated output. Remove the AD chip, and watching just the LT chip on its own, that chip remains accurate.

So my next plan involves using a full inamp after the LT chip - perhaps the AD8253. This chip has the advantage of having digitally-controlled gain, switchable between G=1, 10, 100 or 1000. That could be handy indeed.

Except, at least on the data sheet, there's another problem. According to the sheet on the LT1999, that has a possible input offset voltage anything up to 1.5mV (over full temperature range), or 750µV at 25C. Using my 0.1ohm shunt resistor that's an implied reading error of up to 15mA full range, or 7.5mA at 25C. A much larger error than I was hoping to have. I suspect I may have got lucky with my breadboarded chip, having a much smaller error on it than that.

So i'm still somewhat stuck for ideas here. Should I:

• Try to trim out the Vos of this first stage chip. Indeed; is this even possible, to work over a range of commonmode input?
• Find a different chip with much better Vos. It's rare to find difference amps that can cope with extended commonmode input range anyway; this was so far the only one I've found with a bandwidth as fast as I'd like
• Adopt a new approach. For example, the FET-based current mirror to level-shift the high-side shunt down to a load-side, before using an in-amp to detect that.

This latter idea might not be so bad actually, if I used a DC-DC converter to power the upper opamp, the one sensing the shunt resistor. That would allow it to cope with any bus voltage. A high bus voltage would just level shift the sensing opamp up but it will still function just fine, the FET just has to drop a larger voltage. A low bus voltage (maybe even below 5V) would mean that the sensing opamp still has 5V to play with, because its lower end is now negative with respect to output ground. The only trick then is to ensure that the FET still has positive headroom.

Probably the most demanding circumstance I could foresee in this scenario is about 500mA through the shunt, at 2.5V bus voltage. That's a 0.05V shunt drop, so at a 10:1 ratio on the mirror line, we're at 0.5V leaving still 1.95V to drop over the FET. At that ratio, it does mean at the low-current end, the bottom mirror resistor might only be at 500µV when indicating 500µA through the shunt. The inamp I have in mind (AD8253) has a Vos of 150µV max, so it's within ability, but not very accurate here. But it still might be in the realm of doable.

Does this sound like a good bet?

[Siwastaja]

Except, at least on the data sheet, there's another problem. According to the sheet on the LT1999, that has a possible input offset voltage anything up to 1.5mV (over full temperature range), or 750µV at 25C. Using my 0.1ohm shunt resistor that's an implied reading error of up to 15mA full range, or 7.5mA at 25C. A much larger error than I was hoping to have. I suspect I may have got lucky with my breadboarded chip, having a much smaller error on it than that.

LT1999 indeed has a rather poor offset voltage spec. Still, the spec includes process variation between the chips manufactured. If you could keep the temperature stable (i.e., it's in the room temperature, the shunt is not heating up too much, too quickly), you could still detect order of magnitude smaller currents if you simply zero it before reading.

[leonerd]

LT1999 indeed has a rather poor offset voltage spec. Still, the spec includes process variation between the chips manufactured.

I expect given the range and the common-mode support, the chip is probably designed for much higher current flows and larger powered systems than I'm aiming it at.

If you could keep the temperature stable (i.e., it's in the room temperature, the shunt is not heating up too much, too quickly), you could still detect order of magnitude smaller currents if you simply zero it before reading.

Yes; I did consider some sort of offset-trimming ability. How would that work here? I don't quite understand where in the circuit I'd attach some sort of nulling trimmer. It's not just an offset in the output voltage, as it would vary with the common-mode too. I guess you'd have to trim out the zero for some given common-mode voltage, which would then need resetting again if the common-mode changes.

[mk_]

TLDR summary: Can I buy a pre-made op-amp chip with laser-trimmed resistors at G=100 with 3:1 commonmode range over supply, or must I make it myself? If I make it myself which resistor ratios are considered "critical"?


Anything else I've missed?

Take a look for something like Avago ACPL-C78A or similar from Analog or TI. With those Opamps you can measure your 100uV even on very high sides...

Maybe the bandwith isn`t high enough.

Michael

[Siwastaja]

Yes; I did consider some sort of offset-trimming ability. How would that work here? I don't quite understand where in the circuit I'd attach some sort of nulling trimmer. It's not just an offset in the output voltage, as it would vary with the common-mode too. I guess you'd have to trim out the zero for some given common-mode voltage, which would then need resetting again if the common-mode changes.

Is a potentiometer hooked up to the REF pin too crude? The user would adjust it to read zero, which would take a few seconds, and needs to be done every time the common mode or temperature condition has changed significantly.