high voltage opamp?

[T3sl4co1l]

A $10 boutique part, that isn't stocked anywhere, just because you don't understand a solution that half a dozen more knowledgeable engineers have proposed?

Well, you know what they say about money and certain types of people...

Tim

[sgt_bear]

Yes i saw the part is unavailabel
Also i have read the datasheet and sadly the device also steals current 500uA

I simulated one of the shown solutions, but they all consume massive current  (like 1.5mA) through the Zener. 1.5mA is way to bad.  I need 100uA precision, so the current has to be WAY under 100uA (10uA) or so.


What solutions do good multimeters use? They have a way more extreme job, they have to measure 10uA to 10A, in  mV to 750V range,

[C]

 sgt_bear
     Think you are missing something very simple.

When you add the  measurement circuit, your high voltage system (150-250V DC) is how supplying two loads.
One that you want to measure current in.
One that supplies power to measurement circuit.

[David Hess]

Yes i saw the part is unavailabel
Also i have read the datasheet and sadly the device also steals current 500uA

Internally it duplicates the other posted solution but with worse accuracy and higher cost.

I simulated one of the shown solutions, but they all consume massive current  (like 1.5mA) through the Zener. 1.5mA is way to bad.  I need 100uA precision, so the current has to be WAY under 100uA (10uA) or so.

The supply current and output current are completely separate circuits although they could be joined and the offset produced by a fixed supply current removed.  The supply current with that high voltage is still annoying so it is advantageous to use a low power operational amplifier which is why I suggested the TL031 which only requires 200 microamps but there are modern more expensive parts which draw much less current if you can live with lower bandwidth.

What solutions do good multimeters use? They have a way more extreme job, they have to measure 10uA to 10A, in  mV to 750V range,

Multimeters measure the voltage across a switched current shunt and are floating so there is none of this high side and low side nonsense.  As far as the multimeter is concerned, it is just another low, typically 100s of milivolts, voltage measurement.  Duplicating this would be more expensive and complex; it means floating the analog to digital converter to the high voltage supply.

[Greg Robinson]

Yes i saw the part is unavailabel
Also i have read the datasheet and sadly the device also steals current 500uA

I simulated one of the shown solutions, but they all consume massive current  (like 1.5mA) through the Zener. 1.5mA is way to bad.  I need 100uA precision, so the current has to be WAY under 100uA (10uA) or so.


What solutions do good multimeters use? They have a way more extreme job, they have to measure 10uA to 10A, in  mV to 750V range,

If your zener has that much current flowing through it, the design needs to be adjusted - those example circuits are just that - not a one-size fits all solution. Typical zeners need about 300-500uA bias current to keep above the negative region of their di/dt slope. Adjust the current limiting resistor to suit the zener you have selected, and the lowest voltage your supply will ever be during normal operation.

And you do realise that those circuits only measure the current in the load right? They do not include the current drawn by the high-side current sense amplifier and it's zener-regulated floating power supply, so depending on which device you choose, you can easily get 100uA precision. Is the power supply for the load not able to spare another couple of milliamps? If so, you should probably re-think the power supply, not ditch the proper current sense solution.

By the way, your comparison to multimeters with a range of 10uA to 10A is not at all fair, as that is over multiple ranges, with different sense resistors, different burden voltages and different accuracies in each range.

Anyway, you should probably tell us what your application is, that way we can give more specific advice or understand why something may or may not be appropriate.

[David Hess]

If your zener has that much current flowing through it, the design needs to be adjusted - those example circuits are just that - not a one-size fits all solution. Typical zeners need about 300-500uA bias current to keep above the negative region of their di/dt slope. Adjust the current limiting resistor to suit the zener you have selected, and the lowest voltage your supply will ever be during normal operation.

I usually figure on at most doubling the total current so a 200uA TL031 allows a reasonable 200uA for the zener diode.  If I used a lower power operational amplifier like a 20uA OPA347 or TLV2451, then I would use a micropower shunt regulator instead of a zener diode.

Another alternative to power a micropower operational amplifier is to use a charge pump and high voltage capacitor for isolation and this would allow using a standard micropower operational amplifier with better precision like an LT1077 because the operational amplifier's positive supply can be pumped to be greater than the high voltage.

[macboy]

...

What solutions do good multimeters use? They have a way more extreme job, they have to measure 10uA to 10A, in  mV to 750V range,

Multimeters measure the voltage across a switched current shunt and are floating so there is none of this high side and low side nonsense.  As far as the multimeter is concerned, it is just another low, typically 100s of milivolts, voltage measurement.  Duplicating this would be more expensive and complex; it means floating the analog to digital converter to the high voltage supply.

Exactly. All multimeters, even the cheapest ones, have an optically-isolated output! You look at the screen with your eyes, so the multimeter can sit happily at hundreds of volts above or below ground (and your body) without issue. You don't need a common ground connection to the multimeter to view its screen 

The usual issue with high-side current measurement is that you need to shift that measurement result back down to ground/common for use by the circuitry which needs the current measurement in the first place. Unless of course that circuitry can reside on the high side as well.

Of course the OP didn't even say if the measured current is going to be used by anything other than human eyes. Maybe the best solution is a simple panel meter with a transformer-isolated floating supply? Too many assumptions.

[David Hess]

The usual issue with high-side current measurement is that you need to shift that measurement result back down to ground/common for use by the circuitry which needs the current measurement in the first place. Unless of course that circuitry can reside on the high side as well.

For current control circuits like in power supplies, my preferred solution is to move the entire error amplifier to the high side and use the level shifter to shift the control signal to the high side.  Reading back the actual current would require a second level shifter or a floating readout.

[f5r5e5d]

https://moosh.im/mooshimeter/ ??

[sgt_bear]

You mean this circuit? Yes, I agree.

I tried this circuit to simulate with LTspice and the LT1672 (2uA Supply Current).
But my simulation does nothing it only has a % counter in the bottom which never finishes

Attached my circuit and simulation.