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INA138: INA138

Ariel Pickholz26
Intellectual 1295 points

Part Number: INA138

Hi Team ,

A cat 5 customer is using a prototype of a current sensor for 4-10 mA output stage uses INA138 :

The Current that flows trough R45, giving 300 mV at 20 mA, with 50K R46, this should give 3 Volts to ADC. ( and 0.6 volts at 4 mA)

They already noticed that they would need to change Vcc to +5V instead of 3.3V

Their problem is an RTFM one, 

Whne they designed their board they  did not read the data sheet details :-( , and found now the Vout max is:

so when using a low impedance control loop, the voltage on pin 4 will drop very low (in fact there is no limitation and user can connect to GND as this is a current source.

In such case (even with 100 Ohm resistor), the voltage on pin 4 can be 0.4 V, which means Vout max is 0.1V and feedback cannot be used.

They can connect a resistor, in the current path after R45, but this seems like a bad solution as this needs to be 275 Ohm resistor:

Their needed vout for 4 mA is 0.6 V which means voltage on pin 4 (VIN-) need to be at least 1.1 V /4ma = 275 Ohm.

This will limit the current loop resistance to a max of ~ 300 Ohm ( if current source max voltage is 12 Volts)

Would we have any better ideas how to solve this issue ?

BR ,

Arik.P.

 

  • 0
    TI__Genius 15770 points
    Hi Ariel,

    For some reason the pictures you tried to upload are not visible. Please try to attach the schematic and all other images.
  • 0 in reply to Mayrim Verdejo
    TI__Intellectual 1295 points

    Hi Myriam ,

    A CAT5 based a new board on the below scheme :

    A Prototype circuit of a current sensor for 4-10 mA output stage uses INA138 :

    The current flows trough R45, giving 300 mV at 20 mA, with 50K R46, this should give 3 Volts to ADC. ( and 0.6 volts at 4 mA)

    The customer have already noticed that he would need to change Vcc to +5V instead of 3.3V

    His problem is an RTFM one,  They did not read the data sheet details :-( , and found now the Vout max is:

    so when using a low impedance control loop, the voltage on pin 4 will drop very low (in fact there is no limitation and user can connect to GND as this is a current source. In such case (even with 100 Ohm resistor, the voltage on pin 4 can be 0.4 V, which means Vout max is 0.1V and feedback cannot be used).

    They can connect a resistor, in the current path after R45, but this would be a bad solution as it needs to be 275 Ohm resistor:

    Their needed vout for 4 mA is 0.6 V which means  voltage on pin 4 (VIN-) need to be at least 1.1 V /4ma = 275 Ohm.

    This will limit the current loop resistance to a max of ~ 300 Ohm ( if current source max voltage is 12 Volts)

    Do we have anybetter idea to offer thias customer ?

    BR ,

    Arik.P.

    AFAE

  • 0 in reply to Ariel Pickholz26
    TI__Genius 15770 points
    Hi Arik,

    Again, the pictures are not loading. Please email me at mayrim.verdejo@ti.com and attach the pictures. I will review tomorrow.

    Mayrim