4-20mA to 0-3.3V

David Wang60

Other Parts Discussed in Thread: ISO122, ISO124, INA226, ISO1540, ADC121C021

We currently have 4-20mA sensors converting into a 0-5V analog signal from a shunt resistor. The signal is then fed into an isolated amplifier (ISO122) of which the output is connected to our COTS processing modules. We are now interfacing the analog signal to a TI microcontroller that is 3.3V IO tolerant. What is the best way of converting the 4-20mA signal into a 0-3.3V?

Can it still be done efficiency with the ISO122 or ISO124 but with a resistor divider? Would you suggest any other ways? An isolation barrier is required for us so please keep that in mind.

Thanks

-David

over 9 years ago

0 Javier Contreras4 over 9 years ago

TI__Mastermind 32110 points

David,

Could you lower the Rshunt resistance to give you a lower voltage required? This will then only require a resistor change from what you currently have.
Regards,
Javier

0 David Wang60 over 9 years ago in reply to Javier Contreras4

Intellectual 980 points

Yeah that is possible to use a 165 to make the range from 0.66V - 3.3V. Looking at the ISO122/4 are there more cost effective solutions that you would recommend? The part is ~9 dollars for 1ku. I assume that it's that expensive due to the isolation barrier specification?

0 Javier Contreras4 over 9 years ago in reply to David Wang60

TI__Mastermind 32110 points

David,

What is the voltage level you are attempting to isolate? What is the maximum difference in voltage between the current of interest and the low voltage side?

Have you considered using one of our digital out current sense amplifiers such as the INA226 with a digital isolator such as the ISO1540?

Regards,
Javier

0 David Wang60 over 9 years ago in reply to Javier Contreras4

Intellectual 980 points

The voltage level to isolate is 0-5V. After the isolation, we have a sallen-key 2-pole low pass butterworth, @5Hz cutoff and gain of 1. We want to do the scaling after the filtering to be able to preserve SNR. 4mA would be mapped to 1V while 20mA would be mapped to 5V, but if we scale it down to a 3.3V range, 4mA would be mapped to 0.66V. So the max difference would be 0.66V from GND.

I have thought about an external ADC rather than use the uC ADC, but the INA226 is not isolated, which is a must for our system due to high EMI/noise environment. We would like to isolated the signal first, and then filter it to reduce the interaction and noise on the sensor side.

I think we have decided to use a the ADC121C021 on the non-isolated side.

Thanks
-David

0 Javier Contreras4 over 9 years ago in reply to David Wang60

TI__Mastermind 32110 points

David,

See link below and let me know if it helps.

e2e.ti.com/.../4-20-ma-industrial-loops-made-easy

Regards,
Javier

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4-20mA to 0-3.3V