INA285: INA285 design for RJ45 connector...

Part Number: INA285

Dears,

Below is my customer design concern.

He applied INA285 in each RJ45 data pair.

As below picture, he use INA285 on pin 1, 3, 5, and 7. 

And for these pins, they use below circuit on it.

However, after applied INA285, all 4 pairs had cross-influence with each other.

As I know for INAxxx porduct, they are major for current measure for power trace, not for data trace, right?

If it also can use on data trace, could you help to give me some advise for customer issue and some design suggestion.

Thanks.

8 Replies

  • Hello Peter,

    Thanks for using Texas Instruments. I suspect that if your customer only had one RJ45 channel running, your customer would still see an incorrect output. I presume your customer is passing really high bandwidth signals through the RJ45 data pair. If these signals' frequencies are outside of the bandwidth, then the INA285 will not be able to keep up. Additionally if the PWM rising/falling edges are faster than the part's slew rate, the device will not be able to keep. There is also the issue of settling that can affect your measurements. what is the common mode voltage range and what signal speed is your customer trying to measure? If your customer is measuring data signals in the MHz range, our current shunt monitor devices will not work for them.

    Best Regards,

    Patrick Simmons, TI Sensing Products Applications Support

    Getting Started with Current Sensing Video Training Series

    TI makes no warranties and assumes no liability for applications assistance or customer product design. You are fully responsible for all design decisions and engineering with regard to your products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning your designs.

  • In reply to Patrick Simmons:

    Hi Sir,

    Yes! Below is my customer NEXT test (Near End crosstalk Test) result!

    It can be see that after add INA285, the signal low than 50MHz will be influenced.

    As customer RJ45 support PoE function, that means common mode voltage should be more than 48V, and they hope the G>=500!

    Therefore, is there any other solution suitable for my customer if INA285 cannot use for such design?

    BTW, current shunt product also can design for data signal measurement?

    Thanks.

  • In reply to Peter Tseng:

    Hello Peter,

    If your data is transmitted in the MHz range, our devices cannot handle that. Our devices are meant for low bandwidth signals. If your data signal is in the 100s of Hz range then, it possibly can be used to measure the data signal. If your data signal is in the KHz range or higher, then you should not use this device. I would recommend you use a high-speed amplifier configured for current sensing. As such I am forwarding your challenge to someone in the high speed amplifiers group.

    Best Regards,

    Patrick Simmons, TI Sensing Products Applications Support

    Getting Started with Current Sensing Video Training Series

    TI makes no warranties and assumes no liability for applications assistance or customer product design. You are fully responsible for all design decisions and engineering with regard to your products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning your designs.

  • In reply to Patrick Simmons:

    Dear Patrick,

    May I have your suggestion that if customer want to build the circuit with amplifier instead of INAxxx product, what solution will be better for them?
    Thanks.
  • Hi, 

    You may be able to use a fully differential amplifier such as the THS4541 to amplify the signal. What is the value of the shunt resistor, what is the current expected through the shunt resistor, and what voltage is needed at the output? 

    Regards, 

    Jacob Freet 
    High Speed Amplifiers

  • In reply to Jacob%20Freet:

    Hi Jacob,

    As customer information, the Rsense they will use 2mohm, and the current will be 1~2A on the power rail.

    For voltage, since the application is POE related, they will need up to 48V POE power for current shunt design.

  • In reply to Peter Tseng:

    Hi Peter, 

    Thank you for the information. CAn you clarify what you mean when you say they will "need up to 48V POE power"? Do you mean that the amplifier must be powered off of 48V or that the voltage at the shunt is 48V? 

    Thank you, 

    Jacob Freet 
    High Speed Amplifiers

  • In reply to Jacob%20Freet:

    Hi Jacob,

    As my attached pictures and description above, customer use current shunt in pin1,3,5, and pin7 of RJ45 connector.

    Due to their application is PoE related product and the PoE voltage must be carry via data pairs from PSE to PD.

    The current shunt will face the 48V high voltage from PSE.

    That's what I mean.

    If my understanding is wrong, just feel free to correct me.

    Thanks.