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INA170 Linearity Question and related error on datasheet

Other Parts Discussed in Thread: INA170, INA169, INA225

Hello , 

I'm looking at the INA170 data sheet (high side current monitor, with offset current for bidirectional measurements).

I'm looking at page 4 of the data sheet (SBOS193D), at the middle graph on the right (It would help if someone bothered to number the figures!)

This graph (and possibly many others) looks to have been copied from the INA169 datasheet  (SBOS181D) 

The problem is that the INA170 has an adjustable offset current, so Vin can range from a negative number to a positive number , so the graph might , perhaps, be close to what would be achieved with Ros set to infinity. . 

What I was hoping to see was a graph, say , ranging from -125mV to +125mV , annotated as "Ios=125uA" 

This would hopefully move the nasty nonlinearity off to -125mV. 

Also on page 3 , the Electrical Characteristics table, The nonlinearity needs to be better specified , what is in the table is only relevant with  pin 5 open (Ros=infinity) , and clearly the input can go positive and  negative unlike the 10mv to 150mV in the table.  

Also the conditions in this table may need to be reworded , as being over the range 10uA to 300uA of OUTPUT rather than 10-150mV of INPUT ? 

The other problem with the datasheet is that equation 4 is completely wrong, what gives with the "+/-" sign in the middle?, and you have wrongly added the offset?.  

Equation 4 should read 

Vout = (IS.Rs.RL/1k) - (Vref.RL/Ros

It would be nice to mention in the datsheet that if Ros is numerically equal to RL  then  the offset just becomes Vref. , and if using a differential input ADC it can be connected across pins 6 and 5.  

Likewise using a ( diff  ) ADC , and making Ros exactly twice RL , one can elegantly connect pin3 (Vref) to the reference of the ADC, and remove a lot of drift and second order effects.

  • Bob,

    Thank you for your feedback!  As you can see from the data sheet, this device is 13 years old and I hope you see our data sheets have improved considerably since then.  That said, you are correct that data sheet plots are sometimes shared when there is a common core technology in the ICs.

    I'm passing your notes along to our marketing team for review.  I hope you haven't been too inconvenienced by this presentation of information, and I'm happy to help if you have further questions.

    Again, thank you for evaluating and using TI products!

    Jason Bridgmon

  • Bob,

    A colleague of mine suggested that the INA225 has very similar specs and is a more recently released device.  I thought I would pass that along in the case that the INA170 has too many outstanding issues to your liking.

    Again, thanks for considering TI current shunt monitoring products!