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LMH6881 missing specifications

Other Parts Discussed in Thread: LMH6881, LMH6521, LMH6882

Could someone fill in the missing details from the 6881 spec:

Differential input resistance is 100R typical, what is the tolerance?

Single ended input resistance is 50R typical, again what is the tolerance? 

If I set the output common mode range to 2.5V (1.25 on OCM) how much will the output common mode differ from 2.5V? (It's normally a few mV for most differential amps, It affects ADC linearity)

If I configure the device for DC coupled single ended signals the max DC input is 0.7V (page 15, 10dBm). 

The absolute max ratings specify -0.6V to 5.5V, operating 0V to 5V. Is there any way I can configure the device to accept larger single ended signals? eg use a 100R resistor on INPS instead of 50R in figure 39?

  • Hello Lee,

    The differential resistance is set by on chip resistors which have a process variation that can be as bad as +- 10%, however the distribution on a given wafer is much better. 

     I could not find data for the Vcm offset voltage.  The gain of the common mode offset circuit is set by on chip resistors which should match very well, so this should be well controlled, but I don't have the data on hand to say what the limits should be.  

    The LMH6881 has a fixed gain pre-amplifier on the input, large input signals will saturate the output of this pre amp, so aside from an external attenuator there is no way to increase the maximum input signal. 

    Regards,

    Loren

  • Hi Loren

    Thanks for the quick reply. It would have been nice to have some more safety margin to make the clamp circuits easier, we'll have to engineer around it.

    Regards

    Lee

  • Hello Lee,

    I have a further question close to the above: In the data sheet on page 12, figure 28 it can be seen that the typical differential input impedance at 200 MHz is about 100 Ohm - j 8 Ohm. The - j 8 Ohm part; how much variation should we expect on that?

    Best regards

    Klaus Moelholm

  • Hello Klaus,

    This is the bond wire inductance and input capacitance, it should consistent to within 10%.

    We used low loss/low k (Rogers) dielectric on the LMH6881 evaluation board to keep parasitic capacitance to a minimum.  You will need to simulate or measure this on your PCB to have a final value.  The chip portion of the input impedance may be a small fraction of the board value. 

    We found this to be the case with our LMH6521 which used FR4 material and had longer input lines.  The input reactance of the LMH6521 was so small we could not measure it on the eval board, the SOLT calibration errors were larger than the LMH6521 reactance values. 

     

    Regards,
    Loren

     

     

    Regards,

    Loren

     

  • Hello Loren

    Thank you for the quick and very usefull answer. I am looking in to this matter because we see some variations on the return loss on some inputs where this amplifier is used. I am aware that some of the other components in the circuit probably add more to the variation, but I am checking all to make sure.

    One of the other components is a ADT1-1WT-1 transformer from Mini-Circuits. We use this transformer to convert from the single ended instrument input to the differential input on the LMH6881. An improvement might be to remove this transformator and instead connect the instrument input directly to the INMD input on the LMH6881 and connect the INPD input to GND through a resistor and capacitor like on figure 37 in the data sheet. The instrument input measures on E3, E4 and STM1e telecom signals. If we do like this would we need to add some extra ESD protection on the INMD input or is the LMH6882 robust enough to survive this environment without extra protection?

    Best regards

    Klaus Moelholm