• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 48 subscribers
  • Views 732 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

My INA826 positive input common mode range is not meeting (V+ - 1V) spec.

Doug Billington
Prodigy 50 points
Other Parts Discussed in Thread: INA826, INA195, INA286

INA826_CIRCUIT_3-14-16.PDFI am using your INA826 as a bi-directional current sense amp with a gain of 100.

My differential signal is small,  but the common mode input swings from -5V to +5V. Power supplies are split with -7.5V and +7.5V

My input signal is +/-50 mV max for what should be +/-5V output max, but I get -5V to +4.8V at the output.

On the positive input swing side. when the common mode voltage reaches about 4.8V. The output distorts.

The output goes up to 4.8V, then inverts and starts going DOWN.

IF I RAISE THE + SUPPLY VOLTAGE FROM +7.5V TO +8V. the problem goes away.

The INA826 spec says the common mode input goes from -V to (+V - 1V) so I expected it to be good from -7.5V to +6.5V with my +/-7.5V supplies.

BUT there are graphs also showing common mode range that appear to say that the upper range can be much greater than (+V - 1V).!!

I have to guess extrapolate your graphs to my +/-7.5V supplies, but it appears that an upper Vcm value of (+V - 2.8V) <-- (my board) is a reality.

QUESTION: DO YOU HAVE ANOTHER INST AMP (pin for pin) THAT I CAN REPLACE THE INA826 WITH THAT WILL WORK UP TO 5V = Vcm WITH +/-7.5V SUPPLIES?

Or If I have to re-layout the board, can you recommend a current sense amp that works with common mode voltages of +/-5V?

Thanks

Note, in answer to your questions above: End Application is a tester.  INA826 is used in power output amp current measurement sub-system

  • 0
    TI__Mastermind 25815 points

    Hello Doug,

    I recommend using the Vcm vs. Vout tool located here.

    Given a +/-7.5V supply, G=100V/V, and 0V reference voltage, you will have the following operating regions given your common-mode range of +/-5V:

    Region 1: -5<Vcm<-4.4. -6.4<Vout_n5V<6.4
    Region 2: -4.4<Vcm<2.8. -7.4<Vout<7.35
    Region 3: 2.8V<Vcm<5. -3.1<Vout_p5V<3.1

    Note that Vout_n5V represents the output swing a Vcm=-5V while Vout_p5V represents the output swing at Vcm=+5V. The output swing in Regions 1 and 3 change with common-mode as you will see with the tool. Also note that the INA826 has level shifting, hence the uneven output swings in Regions 1 and 2. Here are links to an article series that explains all of this in more detail:

    www.edn.com/.../Instrumentation-amplifier-VCM-vs-VOUT-plots--Part-1
    www.edn.com/.../Instrumentation-amplifier-VCM-vs--VOUT-plots--part-2
    www.edn.com/.../Instrumentation-amplifier-VCM-vs--VOUT-plots--Part-3

    Reverse engineering this for an output swing of +/-5V, the common-mode range is approximately to -5.66V to 4V. I recommend increasing the positive supply to 8.5V in order for the output to reach 5V given a common-mode range of -5V to +5V.

    Unfortunately I can't find an instrumentation amplifier that meets your requirements. However, perhaps you'd be interested in a current shunt monitor. If so perhaps the INA286 or INA195 might work. If this seems like an option for you, I can move this post to the current sensing forum for further support.

    I hope this helps!