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Measuring 4-20mA using MSP430 ADC

Prad1
Genius 3825 points
Other Parts Discussed in Thread: MSP430F5438A

Hi,

Our customer needs to measure a 4-20 mA current signal using an ADC port of MSP430F5438A.

Using 100ohm resistors, the voltage need to convert is going to be 0-2V in theory.
But in practice, because of resistor tolerances and the 4-20mA transmitter at the other
can output slightly more than 20mA at full scale.
We have to make sure that any surge in the current doesnt affect the microntroller
or the interface circuit.
It seems our customer has already burnt few MSP's to this test this application.

We would know
1) Is it ok to use MSP's ADC directly to measure the 4-20mA Current(through 0-2V voltage)
(In this case will it be necessary to add a current-limiting resistor,
which may influence the sampling time of the MSP430 ADC?)

2) or else use additional 4-20mA Current Loop Receiver before the MSP430's ADC?

We would like to know what could be the best solution to measure the
4-20mA current signal through an ADC port of MSP430.

Best Regards.
Prad

  • 0
    Guru 46710 points

    I recall answering this question. Did you delete thread including answer(s)? Why?

  • 0
    Guru 227245 points

    @Ilmars: I don't recall reading this or a similar thread in the last days. So if it was there, then for less than a aday.

    Prad1 said:
    1) Is it ok to use MSP's ADC directly to measure the 4-20mA Current(through 0-2V voltage)

    Yes. Works fine for us.

    Prad1 said:
    (In this case will it be necessary to add a current-limiting resistor,  which may influence the sampling time of the MSP430 ADC?)

    It is a good idea to have a resistor in series with the MSP input. YOu have two choices: Either you use a sufficiently large one so that even on a surge, the resulting voltage won't exceed VCC, or you put a ztener diode parallel to the 100Ohms resistor. Then the series resistor can be small, as the diode will short the resistor on overcurrent. While being more expensive, this will limit the influence on the sampling time.
    Alternatively, you may use a larger resistor and a capacitor, so small transients on the 4-20mA loop will be smoothed and you have a low-impedance source for the ADC input. It will limit the bandwidth but allow short sampling time.
    Which way to go depends on your application.

    Prad1 said:
    2) or else use additional 4-20mA Current Loop Receiver before the MSP430's ADC?

    This sure is a way. But a rather expensive one.

    BTW: you may calibrate your device by applying a well-known current (e.g. 10.000µA). This eliminates the resistor tolerance.

  • 0 in reply to Jens-Michael Gross
    Guru 46710 points

    Jens-Michael Gross said:
    ztener diode parallel to the 100Ohms resistor.

    If this is to be used in various types of installations which are not installed/controlled by hardware vendor then I suggest to use surge protection devices. Copper Ethernet surge protector like TVS3V3L4U (but not limited to) can do the job. Series resistor between 100 ohm and ADC input is highly recommended because high current surge events can cause clamping voltage jump above VCC of the uC. You don't want 4V & 20A spike into clamping diode of ADC pin, right?

  • 0 in reply to Ilmars
    Genius 4015 points

    Hello, all

    On this inquiry, I have discussed on other thread below; 

    http://e2e.ti.com/support/microcontrollers/msp430/f/166/p/278677/972516.aspx#972516

    On this discussion, I got information to add 330ohm resistor for diode protection.

    I understood that this 330ohm resistor is aimed for the case No.1.

    Please let me clarify whether you have some suggested resistor value for case No.2 and No.3 as well.

    1.Either you use a sufficiently large one so that even on a surge, the resulting voltage won't exceed VCC, or you put a zener diode parallel to the 100Ohms resistor. Then the series resistor can be small, as the diode will short the resistor on overcurrent. While being more expensive, this will limit the influence on the sampling time.

    2.Alternatively, you may use a larger resistor and a capacitor, so small transients on the 4-20mA loop will be smoothed and you have a low-impedance source for the ADC input. It will limit the bandwidth but allow short sampling time.Which way to go depends on your application.

    3.If this is to be used in various types of installations which are not installed/controlled by hardware vendor then I suggest to use surge protection devices. Copper Ethernet surge protector like TVS3V3L4U (but not limited to) can do the job. Series resistor between 100 ohm and ADC input is highly recommended because high current surge events can cause clamping voltage jump above VCC of the uC.

    Thank you in advance for your information.

    Best regards,

  • 0 in reply to Atsushi Okui
    Guru 46710 points

    Atsushi Okui said:
    I understood that this 330ohm resistor is aimed for the case No.1.

    Well... 330ohm is for case when you have 1V overvoltage compared to VCC considering clamping diode forward 0.5V drop and max 2mA current. In case of higher overvoltages you need larger resistor.

    Atsushi Okui said:
    2.Alternatively, you may use a larger resistor and a capacitor, so small transients on the 4-20mA loop will be smoothed and you have a low-impedance source for the ADC input. It will limit the bandwidth but allow short sampling time.Which way to go depends on your application.

    Consider that lightning induced surges on longer (outdoor) cable runs can reach 1000 volts or even more, thus vaporising 100ohm and series resistors, microcontroller including. Reliable solution which will run anyway needs surge/transient protection device.

    What's your application? Outdoor cables or not? Signal source isolated or not? Do you require for signal source to be on low resistance common ground with receiver or not? If in doubt you can say it here - drop me private message.

  • 0 in reply to Ilmars
    Genius 4015 points

    Hello, thank you for your prompt reply.

    This application is communication board, which is input from sensor module as 4~20mA and has processing in MSP430.

    This application is used inside of factory, no outdoor cable is used.

    Signal source is not isolated. 4~20mA and GND 2wire are directly connected to this board.

    The resistance value is not much important than hardware protection on MSP430's GPIO or ADC core, but I understood that sampling time on MSP430 should be taken in consideration if using external resistor. 

    I hope above information could help us which method would be best solution.

    Best regards, 

  • 0 in reply to Atsushi Okui
    Guru 46710 points

    Atsushi Okui said:
    This application is used inside of factory, no outdoor cable is used.

    So no lightning. However such device still can be suspect of earth potential rise or power induction. Which for instance per ITU-T K.20 clause 2.2 means your device shall continue operation after 230VAC source with internal resistance of 10ohms was connected to input for 15 minutes :) Do you need such level - I don't know. This is device specification question: against what kind of surges/transients you want to protect your device, conformance to which standards you will claim.

  • 0 in reply to Ilmars
    Genius 4015 points

    Hello, thank you for your continuous support.

    As the result of our internal discussion, this protect is enough to avoid input voltage while MSP430 is powered off.

    i.e. On page 39, the description refers as "Voltage applied to any pin　”-0.3 V to VCC + 0.3 V”.

    Anyway, we appreciate for your clarification.

    Best regards,

  • 0 in reply to Atsushi Okui
    Guru 46710 points

    Atsushi Okui said:
    As the result of our internal discussion, this protect is enough to avoid input voltage while MSP430 is powered off.

    Please be more specific what kind of protect you are talking about here.

  • 0 in reply to Atsushi Okui
    Guru 227245 points

    Atsushi Okui said:
    As the result of our internal discussion, this protect is enough to avoid input voltage while MSP430 is powered off.
    i.e. On page 39, the description refers as "Voltage applied to any pin　”-0.3 V to VCC + 0.3 V”.


    If you exceed Vcc+0.3V on th epin, current will flow through the clamp diodes. THis has two effects. First, the current will break down input voltage (that's why a series resistor will help - the sudden current on the series resistor will cause a voltage drop on it. 2mA are th emaximum rated current, so for each volt of overvotlage you'll need 500Ohms or more).
    But since this current goes from input to Vcc, this in turn injects a current into Vcc. In case of no supply voltage (or if the circuit draws less than 2mA from Vcc), this will cause Vcc to rise. This also rises the limit for the input voltage - and powers the MSP.
    From time to time people are wondering why their MSP runs even though the supply is off. They power the MSP indirectly through the I/O pins. Well, above 2mA rated current, there's a risk of permanent pin damage.

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