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LMH5401: How to reduce the DC sink current from LMH5401(AFE)

yao xiao
Prodigy 20 points
Part Number: LMH5401
Other Parts Discussed in Thread: LMH6401, TIDA-00654

Hi, 

I design an AFE schematic , which use  LMH5401 + LMH6401 to test   +/- 50mv signal (1.5GHz 50 ohm  DC-couple)  , the schematic is all the same of   TIDA-00654 (http://www.ti.com/lit/df/tidrgy0/tidrgy0.pdf?ts=1590200707928) ,

the Vcom of  ADC is +1.8V , the Vdiff  is 400mVpp ,

the power of LMH5401 and LMH6401 ，VS+: +3.8V ,VS-: -1.2V.

It's very hard to satisfy the less VICM of LM5401 and the larger Vocm of  LMH6401,I use Tina to simulate:

If I set the Vocm of LMH5401 to 0V, the singal  out from LMH5401 can't  get the VICLR of  LMH6401(0.3V) .

So  I set the Vocm of LMH5401 to 0.5V , the singal  out from LMH5401  get the LMH6401 ,but the sink current from LMH5401 (Vicm) to  signal Generation, there is 2mA, It's too lager for a +/-50mv sigal, I need sink current not more than 0.1mA.  could you give me same advises to slove this problem. 

Thank you!

  • 0
    TI__Genius 15555 points

    Hello yao,

    Have you tried increasing your gain setting resistors RF & RG to meet your requirements? While this may have a negative effect on your noise and bandwidth, it should lower the amount of current going back into the source.

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    Prodigy 20 points

    Dear  Babiker，

    Thanks for your reply, I have try to connect the resistor of RT  to  the negtive power, VICM is lower. 

    There is anther question ,how to increase Input Impedance of the FDA , I get the Equation from the datasheet of LMH5401 ,if I increase the value of RG, my adc only can get a small signal,Could give me  same advises to slove this problem,too.

    Thank you!

    Best wishes.

    Xiaoyao

  • 0 in reply to yao xiao
    TI__Genius 15555 points

    Hello Xiaoyao,

    I'm not sure I follow here, could you provide some plots to better describe what is happening when you try and increase Rg? Are you also increasing Rf to maintain the same gain?

    Best,

    Hasan Babiker