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    Single supply low-side shunt amplifier

    Hello,

    I have a 250A/60mV shunt to monitor a power inverter operation. The shunt is located on the low-side (that's it, the inverter ground is floating - max 60mV).

    As I only have a single supply available (+12VDC, the inverter housekeeping supply), is there any inamp/opamp (or any other discrete method) available to amplify (x100) that small ground referenced signal?

    As far as I have read, all INA instrumental opamps need a negative supply (but I must admit I have filtered my search on this further condition: I need a through-hole package or a discrete BJT/MOS solution).

    Any help is highly appreciated!

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    Re: Single supply low-side shunt amplifier

    Actually, as a challenge, I'm pretty much interested in a discrete (BJT) solution. What about the circuit bellow?

    Click image for larger version. 

Name:	bjtamp.png 
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ID:	155123



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    Re: Single supply low-side shunt amplifier

    A single BJT amplifier has an offset drift of 2 mV/K, so it's surely inappropriate for a 60 mV shunt.

    Before discussing reasonable amplifier option, we should know your accuracy requirements, acceptable zero and full scale error. If I understand right, your are asking for an unipolar measurement?


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    Re: Single supply low-side shunt amplifier

    I am using that shunt to monitor/detect an overcurrent (shortcircuit) condition hence I don't need a very good accuracy nor a high precision zero/full scale (that issue could be further adjusted on the comparator stage). For now, I just need to amplify (x100) the shunt voltage.

    In theory, the BJT offset drift could be (somehow) compensated, like in the diagram bellow ("1.2V" being a temp compensated voltage reference):

    Click image for larger version. 

Name:	bjtamp1.png 
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    Now that you mentioned it, there are other temperature dependent BJT parameters (like the current gain) so I guess you're right, a single stage BJT amplifier might not be suitable for this situation.



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    Re: Single supply low-side shunt amplifier

    Your schematic is common-base operation of a transistor. Its characteristics are:
    * low input impedance
    * high output impedance
    * high gain.

    By that reckoning it fits your purpose. A semblance of linearity is possible although it requires careful adjustment. Output voltage is prone to drift at low readings. The chief cause is sensitivity to temperature change. When experimenting with hardware, by pinching the transistor between my fingers I've seen slight change in output.

    My simulation resembles your schematic.

    Click image for larger version. 

Name:	common-base NPN measure 250A 12V supply.png 
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ID:	155127

    The sense resistor calculates to 240 micro-Ohm (.06V divided by 250 A). As voltage on it changes, it changes the effective base-to-emitter voltage. This causes the transistor to conduct greater or lesser current.

    Bias voltage needs to be adjusted so it turns on the transistor just the right amount. That is the method to obtain a reading which is proportional to sensed Amperes. You may need to reset response periodically. Or else design automatic calibration of some kind.


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    Re: Single supply low-side shunt amplifier

    An LM324 / LM358 can be used to measure a neg shunt voltage - to 300mV below 0v on a single supply, because of offset etc need to add an offset pot running off a stable voltage then can trim to < 100uV Vios


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    Re: Single supply low-side shunt amplifier

    @BradtheRad:

    Thank you very much for your time (to simulate the circuit above). I'm going to give it a try for sure (in the name of science!).

    @Easy peasy:

    Unfortunately, LM358 has quite a big input offset voltage (5mV) and I have no means (negative supply) to compensate it. On the other hand, I just found some ICs in my drawer (MCP6021, Vof = 0.5mV) which I completely forgot about.


    PS:

    Man, am I getting old or what?! At first, I thought it was just a deja-vu but.. take a look at this: Shunt-current-monitor-in-DIP-package. At least I know for sure that my memory retention is _exactly_ two years long! ;)

    (disclaimer: I must admit that I had to stay away from electronics lately hence I completely forgot about my past/unfinished projects)



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    Re: Single supply low-side shunt amplifier

    neg supply not needed for offset nulling ckt on LM358/324 ...


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    Re: Single supply low-side shunt amplifier

    You can buy single supply or rail-rail op amps with a few tens of microvolts input offset.
    Zapper
    Curmudgeon Elektroniker


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    Re: Single supply low-side shunt amplifier

    Thank you very much for your reply! Actually, you did help me to solve a similar problem (a high-side shunt) two years ago!



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