INA240: Output Oscillation

Hi Robert,

Thank you for the detailed report, well done.

The problems you are referring to in some past posts is this - the first release of the commercial INA240 can be induced into low amplitude oscillation when supply ramps fast, this is due to a race condition of the offset cancellation circuit. Even though the behavior is not desirable, the (range of) offset is within spec. Moreover, we’ve since release a revision to eliminate this issue. What you received could be residual revA material in the distribution channel.

However I recommend a further experiment to confirm it is indeed the same issue:

(1)   Short R247 (shunt) and connect to ground or the high side fixed voltage.

(2)   Power cycle and monitor U47 pin8 (OUT).

At the same time you could order some free INA240-q1 samples from TI Store. The automotive version was released on new mask set from day one. Therefore Q1 should work for you if it is indeed what we think it is. If you need help with ordering, please let us know.

An unrelated comment - I don’t see a need for certain resistors such as R253, R267 and R259, they can be deleted.

Regards, Guang

Hi Guang

Thank you for the prompt reply.

Since i posted the report i have been unable to repeat the problem.

This is not surprising since the interval between occurrences can be several days or even weeks during which times the supply is cycled only occasionally.

I shall try the tests you recommend next week and update you with the results.

Thanks for the comments regarding the resistors. They are included in the design to match the load conditions in which the outputs are specified for both the INA240 and the OP2313.

Refer to page 6 and page 6 of the respective data sheets if you wish.

In days gone by, and with certain topology's of amplifier outputs, it was, and still is, necessary to have a load resistor to achieve a near zero output.

I shall order some samples as you recommend and get them fitted at the earliest opportunity.

Once again, many thanks and have a nice weekend.

Robert

PS: Are you expert on the LTC3114-1? I shall be needing some support on problems i have found with a design using this device in the not too distant future.

Hi Robert,

The load resistors can be omitted, but you can keep them also. They don’t affect the IC one way or the other.

From your description, it does sound like the same power up issue. If A3V3 ramps faster than 25V/mS, there is no concern, otherwise there is a risk.

LTC3114-1 is an ADI/LT device. We’ll be happy to provide an opinion but ultimately you may want to get their support.

Regards, Guang