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op AMP matched for the ADC

suy
Expert 1065 points

Other Parts Discussed in Thread: ADS54J20, LMH3401, LMH5401, LMH6401, ADS54J60

Hi

  I am using the ADC （ADS54J20）in my design，but the input signal driver strenth is weak，thus it need to add the differential op AMP，how do i select the op AMP and which parameters need my attention for match the ADC chip。

  • 0
    TI__Mastermind 45670 points

    Hi Suy,

    Please look at the following datasheets to help decide what is the best amplifier for your application.

    LMH3401

    LMH5401

    LMH6401

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Expert 1065 points

    Thanks rob，but i have two question for the following picture：

    1，On the right it shows RIN&CIN is 1.2K&2.8pF，but this pair of  values are not matching the figure 149&150 of ADS54J60 datasheet

    2，What does the resistor（ 54Ohm）do？When the Zin is 100Ohm，should impedance matching be considered for the LMH6401 output to ADC？If so，i calculate it by 100Ohm = R || （RIN || CIN），am right？

  • 0 in reply to suy
    TI__Mastermind 45670 points

    Hi Suy,

    I agree, I am not sure why the internal shunt C is not correct. Could be a datasheet error.

    Is this the application you are going to design?

    The shunt 54ohms bring the total impedance to 100ohms differential or (1.2k || 108). Then you add the 40ohms as well into the load that the amplifier if driving, 140ohms total. That way the amplifier sees roughly a 200ohm load, this was the amplifiers characterized load impedance. You want to close to that impedance to achieve best performance.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Expert 1065 points

    Hi Rob，

       Thanks for your answer！

       Yes,i am going to design according this application，not ADS54J60 but ADS54J20 

       First you mean the 54ohm is the parallel resistor in order to 100ohm differential impedance，don't i consider the the capacitive impedance of the ADC input impedance？And how did you calculate the total impedance  200ohm ？Doesn't  the total load impedance of LMH6401 is 10*2+20*2+100=160ohm?

       Another question the LMH6401 output add a 0.01uF capacitor，is it the AC coupled capacitor？When the Vcm of Op Amp is the same as the ADC we can use DC coupled directly，if not we need add the DC coupled capacitor in the above application，right？

  • 0 in reply to suy
    TI__Mastermind 45670 points

    Hi Suy,

    Yes, that is correct, the 54ohm in parallel times 2, is to make the analog impedance 100ohm differential. No need to consider the capacitive term. Your calculation is correct, 200ohms is ideal for the LMH6401 to drive but 160ohms should be fine as well. Just need to be in the range.

    If you do not need to DC, then yes, add a 0.1uF series cap on each output of the LMH6401 before the ADC analog inputs.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Expert 1065 points

    Thanks for  you,rob.