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ADS124S08: HART MODEM Burden Resistor with 4-20mA analog input

mansur hidayat
Prodigy 170 points
Part Number: ADS124S08
Other Parts Discussed in Thread: ADS114S08,

Dear TI engineer,

I want to design 4-20 analog input with 12 ch inputs using ADS124s08 or ads114s08. our planning  want to combine the analog input with one HART modem (1 channel only). We use Burden resistor as small as possible to make more efficient. My Question:

1. is it ADC ADS124s08 or ADS114s08 compatible with HART modem?

2. How many ohm of Burden resistance value can accepted by HART Modem.

Thanks & Regards

Mansur. H

  • 0
    TI__Guru** 113765 points

    Hi Mansur,

    I think there is some confusion as to all the pieces required to make 4-20mA loop.  The ADS124S08 (and ADS114S08) are analog to digital converters (ADC).  The ADC output is in a digital format transmitted via SPI communication to a microcontroller.  The HART modem, receives and transmits data on the current loop.  The modem would not directly connect to the ADC but rather the microcontroller.  When the current loop is used as a transmitter, the current must be controlled via a current output DAC.  The current DAC would also be connected to the microcontroller.

    Although the ADS124S08 has current DACs, they are not designed to be used as part of the 4-20mA loop.  The ADS124S08 cannot provide enough current to even operate at the minimum of 4mA. 

    I would suggest looking at the following TI Design:

    http://www.ti.com/tool/TIDA-01504

    The ADS124S08 can be used to measure the loop current.  Typical resistances used range from 100 to 250 ohms.  Smaller resistance could be used, but at some point you will lose some resolution due to noise.

    Best regards,

    Bob B