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4-20mA loop current measurement.

user4499359
Intellectual 450 points
Part Number: INA219
Other Parts Discussed in Thread: INA226, , INA237-Q1, INA238-Q1, INA229-Q1, INA228-Q1

Dear team,
We are designing a 4-20mA loop current measurement circuit. In TI E2E, we see IN219 can be better used for 4-20mA loop current measurement (2-wire system). Need to understand the following questions
1. The measurement can reach 12bit resolution.
2. What is the maximum measurement error?
3. Rshunt need not connect diode to prevent reverse connection. Still not needed.

4. What is the value of Rshunt, and the current measurement has high accuracy.

5. Whether the circuit needs to be connected with TVS diode to prevent the surge current from damaging the circuit. Need, then how to pick up. If you can recommend one. The loop power supply is 24V. Chip power supply is 5V

  • 0
    TI__Mastermind 26730 points

    Hello,

    Thanks for posting your questions.

    1. The ADC is 12-bit so yes measurement resolution will meet 12-bit.

    2. Maximum error is dependent upon the shunt resistor and the device gain you choose. So for example lets say you choose a 1-Ω shunt and set device gain to PGA=/1 (Vos = ±100uV max for INA219A). Error is at its maximum when you are sensing the lowest current because the input offset voltage (Vos) will become comparable to shunt voltage (Vshunt). So at 4mA, Vshunt = 4 mV and thus error due to Vos is 100*100µV/4mV = 2.5%. Total RSS error is SQRT(2.5^2 + 0.5%) = 2.55%. Note that I am not including any offsets from temperature, CMRR, or PSRR, which will occur as temperature, common-mode voltage, and supply voltage vary from datasheet testing conditions of 25C, 12V, and 3.3V respectively.

    To reduce this error, you can pick a lower offset device like INA219B or INA226. Or you could increase Rshunt, although 2-Ω will be max since 20mA*2Ω = 40mV and this is at the full-scale limit for Vshunt of INA219 when PGA=/1.

    Another way to reduce error is to calibrate the system by running a known current level (usually 0-A/no load), and then recording the device measurement which should just be the device's offset. Record this value into memory and then subtract this value from all other measurements.

    3. As long as the input pins of INA219 do not see a voltage outside of the Absolute Maximum ratings then device will not damage or break.

    4. See #2.

    5. If there is a specific IEC standard you want to adhere to, then choose a TVS diode that promotes complying to that standard. It will be difficult to choose a TVS diode since the operating voltage (24V) is so close to the Absolute Maximum rating of 26V. You need the TVS to clamp input pins to 26V during any voltage surge. This document tests protecting one of our current sense amplifiers with TVS diodes so this could help you.

    https://www.ti.com/lit/ug/tidu473/tidu473.pdf?ts=1595023050538&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FTIDA-00302

    https://www.ti.com/tool/TIDA-00302

    Input resistor to limit any ESD conduction current into input pins can help but will reduce accuracy. If you must go this route, then I recommend calibration. You could also look into our higher rated voltage devices like the INA237-Q1, INA238-Q1, INA228-Q1, INA229-Q1 which can be searched by VCM rating here:

    https://www.ti.com/amplifier-circuit/current-sense/power-current-monitors/products.html#sort=p726max;desc

    Best,

    Peter