Because of the Thanksgiving holiday in the U.S., TI E2E™ design support forum responses may be delayed from November 25 through December 2. Thank you for your patience.

  • State
  • Locked Locked
  • Replies 7 replies
  • Subscribers 47 subscribers
  • Views 928 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.

High CMRR at Mhz level Amplifier Selection

Pengzhi Yang
Prodigy 40 points
Other Parts Discussed in Thread: INA128, INA826

I'm trying to find a In-Amp with a high CMRR at high frequency as 2MHz and differential gain is not required. The input signal is regulated at 2-5 V. However, I can't find CMRR data in datasheet and also the CMRR in some in-amp is not supposed to be high such as INA128/129 or INA826. So I wonder if there's any other solution or amplifier selection? Thanks guys!

  • 0
    TI__Expert 3206 points
    hey penghzi ,
    can you please provide us with a little more details of the application and the schematic of the circuit.
    Also can you share the details of signal which you are trying to give to the In-Amp .
  • 0 in reply to Shreenidhi Patil
    Prodigy 40 points

    Hi Shreenidhi Patil,

    Thanks for your reply

     

    The simplified schematic is shown above, input signal is connected to a common mode source and a clean output is expected

    The green curve and purple curve are the step shaped input signal(raising edge frequency is 2MHz approximately )   , and the output of INA128 is shown as below. Is there anyway to suppress the CM mode noise further? Thank you.

  • 0 in reply to Pengzhi Yang
    TI__Expert 3206 points
    Hey pengzi ,
    What are the supply voltage of the IA INA128 ?
    Can you please share the gain of the INA128?

    Also can you capture the output ,the +ve input and the –ve input of the INA128 in the same screenshot of the oscilloscope and share it with us

    Is the red color curve in the scope screenshot you shared , the difference between positive and the negative pin of the INA128

    Also please let us know the voltage you have given at the Vref pin

    Thank you
  • 0 in reply to Shreenidhi Patil
    Prodigy 40 points

    Hi Shreenidhi,

    The supply voltage is +-15 V; Rg is flowing, so the gain is 1; I just have the screenshot as earlier post and the red curve is the difference between the input signal. We grounded the Vref as datasheet suggested.

    also, can you give some recommendation on Ti amp chip have high CMRR at high frequency as 2MHz? most of chips have CMRR data only to 1Mhz as below which is difficult for me to select .

    Thanks

    Pengzhi

  • 0 in reply to Pengzhi Yang
    TI__Expert 3206 points
    hey penghzi,

    we do not have data on the CMRR vs frequency at 2 MHz for the part that you are currently using INA128

    However i can see that INA128 is not a part with bandwidth of 2MHz
    Also because the input and the output are not on the same screen capture , i cannot say with surety that the low CMRR of INA128 at 2MHz is the reason for the noise in the output.

    Can you please use a signal generator and give in a step function with rise time equal to that of your current signal. If the output comes clean then for sure the device is not the issue. i can see a lot of noise on your input signal , which can also be a reason for the erratic noise in the output. Hence please confirm once using signal generator

    Thank you
  • 0 in reply to Shreenidhi Patil
    Prodigy 40 points

    Hi Shreenidhi ,

    Thank you for your reply,

    Based on the screen shot above, we can find the CMRR=-20log(Gcm/Cdm)=-20log(0.07/2.2/1)=22db, we also did some verification test like you recommended and got similar result. I just wonder if there's other amp chip have larger band width? and is there any other way can improve the CMRR at that high frequency?

    Pengzhi

  • 0 in reply to Pengzhi Yang
    TI__Expert 3206 points

    Hey Penghzi ,

    Sorry for the delayed reply

    TI precision opamps doesnt have any part with high CMRR at 2MHz.  However i would recommend you to go through the TI high speed portfolio http://www.ti.com/lsds/ti/amplifiers-linear/high-speed-amplifier-greater-than-equal-50MHz-overview.page

    Also can you please have a look at the circuit below. I have used a fully differential amplifier (example) with high CMRR at 2MHZ and tied it to our INA128 . I am not the high speed guy here but i think the concept will work provided the fully diff amplifier has sufficient bandwidth

    2MHZ.TSC

    Thank you