INA199: INA199 going to positive saturation while connecting to -ve bus supply

Javier,

I understand that GND of INA199 and the low side sensing should be at same potential but this created a problem for me whenever I disconnect the load from the switch which is connected before In+ and IN- pins.

This led to complete 48V applied at IN+ and IN- through resistive load leading to damage of INA199.

Hence I isolated the GND of INA199 and the low side potential to be measured using an SMPS. So presently you can consider that inputs to IN+ and IN- pins are floating with respect to the GND of INA199.

I suspect that the internal 1M ohm connected between IN+ and Ref pin of INA199 can be creating some issues (as I have grounded the Ref pin and it is isolated from IN pins to be measured).

Can you please provide me a little more detailed internal structure of INA199 (other that that provided in datasheet, for ex how that protection diodes are connected and of what rating; any other details might be helpful) for analysis.

Regards,

Mayank Shukla

Mayank,

We do have other CSA(Current Sense Amplifiers) that are able to go above 48V common mode.  Are there specifications you must have from the INA199 that are required?  Have you looked at any of our other CSAs?  I can help you find an alternative CSA just let me know the must have parameters.  If not here are some more question below for the use of the INA199.

I am confused on floating vs Isolating with respect to GND.  Showing me a schematic may help me understand how you are doing this and what issues you could have.

Anytime the input pins are above the GND pin by more than 26V there will be a issue as that is above max rating.  Also anytime the devices input pins go below the device GND pin there will be issues.  The amount below GND pin is dependent on INA199 gain version you are using.

The ref pin is limited to the supply pins so it should have diodes to the GND pin and Vs+ pin.

The input pin has some protection above 26V for the high side of common mode and a diode to GND pin for the low side.

Javier,

Thanks for the response; the reason of using INA199 is that I had already used it in one of my other designs and using same board for Proof of Concept in another project.

In the previous design the board was powered up from the same 48V through which the current need to be measured (low side). Hence cutting 48VGND while keeping the load ON was not an issue as the board will also get switched OFF. In the new design the board must remain powered ON while cutting 48VGND; this created issue of having common mode voltage greater than 26V an INA199 sense pins. I can use other CSA's but for that I need to change my design (for different footprint / pinout) and as this is a Proof of Concept project I would like to get through with existing design and do gerber level changes once the design gets approved.

Below is the schematic:

Components mentioned NC are not mounted in actual board.

By isolating GND, I meant that the SH+ and SH- pins are not GND of INA199; it is separated by using an SMPS and I am using INA199 in differential mode wherein SH- is not same as GND of INA199.

Hope I am clear of my application.

If you can suggest CSA with minimum 60V common mode voltage range and same in SC70 package and same pinouts; i could order some samples and at least go through with design approval. Later I can change the design and use some other CSA.

Mayank