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# INA253-Q1: Extending the maximum common mode voltage

Part Number: INA253-Q1
Other Parts Discussed in Thread: INA253, AMC3302, AMC3301

Hello ,

I am designing a current measurement circuit and I want to use one of TI precision current sense amplifiers. My problem is that I need the device to tolerate high common mode voltages (Over 220V) I found some opAmps under "Difference amplifiers" that can tolerate up to 500 Volts but in their datasheets the shunt resistors used in the examples are very large in value (1-10 Ohms) and it's written that the accuracy degrades with decreasing the resistance (because the voltage drop also decreases and the effect of the offset voltage and many other parameters become more dominant). So my question is there a method to solve the accuracy problem when using low resistors such as 5 mOhms or 10 mOhms with High common mode voltage amplifiers? Or is there a method to extend the maximum common mode voltage of precision current sense amplifiers? I found an application note called "Extending Beyond the Max Common-Mode Range of Discrete Current-Sense Amplifiers". The only mentioned solution there was adding resistive voltage divider to reduce the voltage but won't this solution also affect the differential voltage ? Also based on the application note using resistors is risky concerning the accuracy.

Any ideas or solutions?

Thank you very much

Amr

• Hi Amr,

the add of an resistive voltage divider at the input of INA253 would total ruin its awesome common mode rejection of >120dB. With 1% toleranced resistors the common mode rejection would fall down from >120dB to about terrible 40dB, with 0.1% toleranced resistors down to 60dB. Even with ultra high precision resistors no  more than 70...80dB could be achieved because of the unavoidable uneven temperature and long term drifts of resistors.

Would an isolation amplifier help?

https://www.ti.com/isolation/isolated-amplifiers/products.html

Kai

• Actually ANC3302 might be what I have been looking for. Based on what I understood from the datasheet page 22 , the isolated power supply inside AMC3302 changes the reference ground (HGND) of the shunt resistor in order to reduce the common mode voltage on the shunt resistor that was significantly high due to the high voltage DC supply of the gate driver?

But my question is there a limitation on the value of the original DC supply or in other words on the original Common mode voltage? It's written in the recommended operating conditions that the common mode voltage should range between -0.032 to 1V but I guess that's during the operation of the Isolated power supply ?

• It's also written in the data sheet of AMC3301 that there is a restriction on the analog input to be within this range HGND – 6 VHLDOout+ 0.5 , my question is what's the suitable ranges for values of HGND or VHLDout how can they be calculated?

• Dear Amr,

Kai is correct here in that an iso amp may be a better fit here. As our parts are internal closed loop, placement of resistors on the front end fundamentally changes the gain of the device and adds error to the system, so while you can extend the common mode this way, you do so at the sacrifice of CMRR and gain error precision.

If you do not need galvanic isolation in the design, another option would be to use a current output device, and strap it 12V to force the device on top of the high voltage. We have an example of this circuit here. Again, there are some tradeoffs in the realm of error for an implementation such as this, but the document walks through error analysis of the circuit.

I'll move this thread over to the Isolation Amp team to get some answers to your question on AMC3301

• Thank you very much ! yes please i am looking forward to understanding AMC3301 better

• Hi Amr,

I support the AMC3302 and will be happy to address your questions. I think the AMC3302 would be a great device to select given the known criteria of your design. As you've seen, the AMC3302 has an internal DC/DC converter. This internal DC/DC converter will track with the high side voltage, hence why the high side ground (HGND) is connected to the shunt resistor.

In regards to common-mode input voltage, this restriction is why we recommend tying the negative input pin to HGND. This allows the positive input to swing +/-50mV with respect to the negative input and HGND, ensuring that the measurement stays within the common-mode input range.