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Single-supply high-resolution current sense amplifier needed for +24VDC 4-20mA current loop receiver design.

Philabuster
Prodigy 20 points
Other Parts Discussed in Thread: INA326, INA219, RCV420, INA148, INA117, INA333

Hi,

Can someone please help me design a low-cost, minimal component, 3.3V single supply, 0 - 21mA current sense amplifier to be used in a 4-20mA current loop receiver. This CSA will feed a 0-3.3V 16-bit ADC.

I have been looking at possible 3.3V single supply current sense amplifier designs and I've checked out the usual RCV420 (incl. Stitt & Kunst's single supply PDF), high-side shunt current amplifiers for supplies, etc. but each has problems.  The RCV420 can barely support 10-bit resolution and most of the integrated current sense amplifiers/monitors (INA219, etc.) are intended for other high current applications and need very expensive sense resistors (<10 ohms) - they also have less than the desired resolution. I have tried a couple of INA326 based designs but it isn't clear that the designs can deliver the desired resolution, noise immunity, and stability.

Any help is appreciated.

Thanks

  • 0
    TI__Mastermind 28265 points

    i will forward your request to our amplifier department which is covering these type of applications.

    regards. Tom Kugelstadt

  • 0
    TI__Intellectual 2940 points

    Hi,

    I have seen the schematic that you provided and have some comments.  To begin with, it looks like you are attempting to design the current sense amplifier section to be powered off of the 3.3V supply.  Given the environment that these designs are typically implemented in, this can potentially be a problem.  Specifically, if the signals at the current loop transmitter or even at the inputs of the current loop receiver block were to get shorted or reversed, a current sense amplifier circuit would not be able to withstand this amount of over-votage and would be damaged.  This is essentially the reason why the RCV420 was designed to withstand up to a +/-40V common-mode input without being damaged.  The necessary amount of protection at the current sense amplifier's input likely needs to be decided upon in order to determine how best to proceed from here.

    Some of the limitations present with the RCV420 have led us in the past to recommend discrete versions of the RCV420.  Keep in mind that the RCV420 converts the 4-20mA signal to a 1.25V to 6.25V signal and then levels shifts this down by 1.25V to achieve the 0 to 5V output so this would add to the complexity of this block.  Using a single supply approach, a level-shifted output will not easily be accomplished.  To accommodate the common-mode voltage requirements the INA117 and INA148 difference amplifiers have been recommended in the past.

    Another part you may want to look at is the INA333.  This is a precision instrumentation amplifier that can be operated off of a 3.3V supply.  It doesn't have the protection to be able to withstand the fault conditions these designs typically require so that aspect of the design will still need to be addressed.

  • 0 in reply to Scott Hill
    Prodigy 20 points

    Scott,

    Thanks for taking a look at the design requirements.  I have been looking at several instrumentation amplifier configurations that may work out. Do you have any suggestions for heavy duty input protection (protection to say around +/-1000V) that won't cause signal problems?  The RCV420 +/-40V common-mode protection wouldn't be adequate for most real scenarios where the electrician wires AC power directly into the current loop receiver. I just had this happen yesterday from a user on one of our older controllers.

    Thanks,

    Phil