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Current Output Current Sense Improves Remote Accuracy

In applications where the readout boards are remotely located, a voltage output current sense amplifier such as the ON Semiconductor NCS21xR series can be converted to a precision current output. The precision output current measurements are read more accurately as it overcomes the errors due to ground drops between the boards or systems.

Figure 1

Remote current sensing

Remote current sensing

As shown in Figure 1, the RIOUT resistor is added between the OUT pin and the REF pin to convert the voltage output to a current output which is taken from the REF pin to the readout board (this creates an Improved Howland Current Pump). This circuit is intended to function with low potentials between the boards due to ground drops or noise. The current output is simply the relationship of the normal output voltage of the Current Sense Amplifier:

A resistor value of 1 kΩ for RIOUT is a convenient value as it provides 1 mA/V scaling.

On the readout board, for simplicity, RITOV can be equal to RIOUT to provide identical voltage drops across both. It is important to take into consideration that RITOV and RIOUT add additional voltage drops in the current measurement path. The current source can provide enough compliance to overcome most ground voltage drops, stray voltages, and noise. However, accuracy will degrade if noise or ground drops exceed 1 V.

2 comments on “Current Output Current Sense Improves Remote Accuracy

  1. D Feucht
    February 27, 2018

    Jerry,

    Not to detract from the current article, but one of the best applications of a Howland current-source circuit came from you in an article long ago whereby the 50 ohm output resistor found in the output amplifiers of test equipment for terminating cables between equipment (such as pulse and function generators) was replaced (along with power loss in the 50 ohm series resistor) by a Howland current source having an output impedance adjusted through resistor value design to be 50 ohms.

    Do you know of any measurement instruments where this bright idea has actually been used?

  2. Pesky Varmint
    February 28, 2018

    From Jerry Steele aka Pesky Varmint

    Dennis,

     

    Thanks, I am flattered that you remember and mention that work. As far as what it may have been used for the following links are patent sthat referenced the article (it was actually published twice, again in 2000) as well as an application note. Looks like DSL line drivers benefit from the idea:

    Google for, or otherwise find TI app note slyt108.pdf

    Patents:

    6373277B1

    6507223

    I could tell stories related to that article, and the Improved Howland Current pump for hours. You may be interested in my answer to the Improved Howland, google this: “Simple Precise Bi-Directional Current Source” to find that article I recently published on EDN.

     

     

     

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