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Automatic ranging current sensing

Discussion in 'General Electronics Discussion' started by andcor02, Sep 19, 2016.

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  1. andcor02


    Sep 19, 2016
    I am trying to implement a current shunt sensing circuit. This should be automatic and currently i want it to measure uA to around 200mA, and Change to the mA range at around 200uA. This is to profile sleep and active profiles in micro controllers. It will then finally interface with an ADC.

    The circuit comprises of the following:

    - 2 x LTC6102 - These are current sense amplifiers and are fed with two different size shunt resistors. 10 ohm for the uA range, 10m ohm for the mA range.

    - 1 x LT1016 Comparator active low - I am using this here for switching to the mA range. With a reference of 3v3.

    - 1 x PMOS - Using this to short the connection of the 10ohm resistor when the current changes to mA.

    So the ideal theory of operation is:

    - Measuring low amount of uA. Micro controller turns on and consumes mA this causes the node uA to saturate, turn on the compartor and thus on the PMOS shorting the 10 ohm resistor and now we use the 10m.

    Here is the circuit in LTSpice


    The issue i have with this is it just doesn't work. I don't know if this is due to the fact i would need some kind of delay in the switching and sensing of M2?

    I also have an issue with the common mode voltage input. So when using 5V the LTC6102 will output the correct gain when saturated, this is given by Av=R3/R2 which is 1650 in both amplifiers. However when using 3v3 this same gain does not work seems around a 1/4 of the voltage output?

    I can provide current/voltage profiles if needed
  2. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    What happens to output uA when COMP goes high and turns on M2? What happens to COMP after that? There does not appear to be any provision for hysteresis in the comparator switching point. Just saying...
    CDRIVE likes this.

    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    Ah, always keeping that mind of yours sharply honed. ;)


    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    This will be answered 6 months from now. In the interim you'll have participated in countless threads and long forgotten what the question was! :p

  5. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Ha! Since the OP never responded, maybe we can just hijack this thread for our own personal use, or ask a moderator to close it. The sentence I liked from post #1 was this:
    ROFLMAO! :p Nice LTSPICE model though. :rolleyes:
    Last edited: Oct 5, 2016
  6. TedA


    Sep 26, 2011
    I think we might as well proceed with the hijack!

    We should first thank the OP for his clear schematic. The schematic makes some sort of answers possible even if we have lost the OP for the moment.

    Of course, he might have given us a link to a data sheet for the LTC61102. Here's one to look at:

    This part is a nifty Precision Zero Drift High Side Current Sense Amplifier.

    It would be nice to know the purpose for this circuit. Is it for use on the bench? Will it drive conventional voltmeters? Is the load going to care about the voltage drop across the shunt resistors? A remote sensing voltage regulator might keep the voltage at the load constant.

    I see several problems with the above circuit.

    The inputs of both of the LTC6102s are reversed. As more current is drawn by the load, the positive inputs are driven more negative. The outputs can only source current; the output voltage will not be driven negative. So the outputs are stuck at zero.

    The LTC6102 may not work correctly at 3.3V. It is only specified down to +4V on the power and Vsense+ pins. Several of the "typical" curves seem to be heading south just a bit below 4V.

    The range switching has multiple flaws:

    The lack of hysteresis has already been pointed out. Around the switching point, the range switching circuit will be unstable and noisy.

    Perhaps the biggest flaw is that the range switching scheme makes an oscillator. As the current increases, the output of U2 increases until the comparator threshold is reached. The comparator then switches, and turns on the transistor shorting the shunt resistor Rsense1. This removes most of the input to U2, the U2 output falls, the comparator switches back, and the transistor turns off. Sequence then repeats. The delays around the loop are plenty long to make this a nice oscillator. You might get better results by connecting the comparator input to the output of U1, which should not change as the comparator switches. This will require using a very low threshold, though, so noise and stability may be problems.

    A 1000 to 1 jump in sensitivity between ranges may not be satisfactory. Many errors grow large at the bottom or each range. At least one more range might be a very useful addition.

    The P-Channel transistor must have a very sensitive gate, so it will work at the low voltages involved. If a higher supply voltage is available, more gate drive voltage might be arranged. Even an N-Channel part might be used. In any case, this transistor might be connected so the body diode conducts if the transistor is off while the current is high. This will allow a smaller shunt resistor to survive.

    There may be an advantage in using only one LTC6102. It can be connected across a string of different shunt resistors, with the current switched to different points on the string to set the range.

    I suspect there may be a few more problems that someone might point out.

    CDRIVE and hevans1944 like this.
  7. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    I have managed (so far... it isn't midnight yet!) to resist the offer by Circuits Specialists for a spiffy cell-phone enabled Mooshimeter digital VOM with up to 24-bit resolution and decent accuracy for a measly $120, less a substantial discount because today is IEEE Day. :D

    I wonder if I can roll my own with a 24-bit ADC-equipped microprocessor and a few precision resistors and a Bluetooth interface module and make it all fit in a little box with a couple of AA cells to power it? I wonder if I can do that for a hundred bux and change? Oh, I guess I gotta add a 35 GB removable memory card interface, too, for data logging... (memory card is extra cost item) and write some software for the cell phone interface, in addition to whatever software the meter microprocessor needs to do its thing. <sigh>

    Anyone here used a Mooshimeter yet? My go-to meter is an ancient BK Precision 2890A which has "only" 51000 counts resolution but no data logging capability without an infrared interface that I don't happen to have. And it's getting a little long in the tooth and is also obsolete. Maybe it's time to "move up" to something more modern with more capabilities...hmmm. BK Precision 393 looks decent.

    I didn't see capacitance measuring, or temperature measuring, or frequency measuring with the Mooshimeter, but it is small... and I always have my Galaxy 4 cellphone nearby... Maybe I could add some features and sell my version for big bux. Maybe throw in a software high-resolution oscilloscope display too. Dual channel. 500 MHz bandwidth. 10 GHz sampling. 10 mV per division sensitivity. Up-loadable color screen images! A really deeeep and long memory to store traces in! Oh, wait... I think someone has done that already with a USB link. Maybe I can just add a USB-to-Bluetooth cellphone interface to that. Still gotta write that pesky Android app though. <double sigh>
    Last edited: Oct 5, 2016

    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    Ted, I don't think your reply qualifies as a hijack. Not even even remotely! It was an interesting read and a thoughtful critique.;)

    hevans1944 likes this.
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