4~20mA Single-Supply, Low-Side, Current Sensing

Hi Chen,

I will let the SAR ADC and OpAmp teams comment on the details of you proposed circuit.
I don't think the circuit around the OPA4348 is working the way you drew it (you have positive feedback!).
If you only need to measure uni-directional currents and you don't need to measure currents close to 0mA then a simple non-inverting amplifier configuration should work for you.

Is the 25Ohm shunt a requirement of your customer or could they also use a larger shunt value? Many industrial customers use 100Ohm to 250Ohm shunts.

From the delta-sigma ADC portfolio the ADS1120 would be a real simple solution. You could connect the ADC inputs directly to the shunt without the need for an external amplifier. You might only need an RC filter in front of the ADC.

Regards,

Chen,


In addition to changing the connection to the OPA to get the proper feedback, you're missing the connection from the shunt resistor to ground. However, note that the OPA will not be able to give an output at ground. Typically, the output will only get to within 18mV of the negative supply. If you're lower range is guaranteed to be 4mA, then you're probably ok.

While the ADS1120 that Joachim suggested may work for you. However, it would be more helpful to know more about what you are measuring. The ADS1120 has only four single-ended input channels and the max data rate of 2kSPS (split among the four channels). If you need more accuracy then the ADS1220 may work. Other solutions could include the ADS1115 and ADS1118 if a lower data rate can be used.

What data rate does your customer require? How many channels will they need? Do they have a preferred communication - either SPI or I2C?


Joseph Wu