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LMH ADC Driver CM input impedance

1886332
Expert 1325 points

Other Parts Discussed in Thread: LMH6554, LMH6552

Hi,

our ADC output a voltage to feed ADC Driver VCM.

We would like to understand what is the Impedance on the common mode pin in a few scenarious.

The questions should regard to all the LMH series

For the LMH series of TI ADC drivers, I would like ask the following :

 

  1. What is the VCM to GND impedance while the LMH IC is not powered.

As we intend to work with negative voltage, that means V+ and V- pins are floating as well as VEN

 2. What is the VCM to GND impedance while the LMH is powered on but disabled using VEN

This means : V+=+5V, V-=-5V, VEN<0V so that the IC is in disable mode.

Thanks.

  • 0
    TI__Genius 13935 points

    The LMH series amplifiers are not all designed the same.  Some amplifiers have a resistive divider that sets the Vcm pin to be at mid supply (V+ - V-)/2.  Other amplifiers have no resistive divider, so the Vcm pin is effectively the base of a very small transistor (very high impedance).

    Many amplifiers have no internal connection to ground, so if both supply pins are floating then the amplifier has no connection to ground.  In that case it would be the external components that would determine how the ground connection will impact the Vcm pin when the supplies are floating. 

     

    Can you give the part numbers for some of the parts you want to use?  Can you give a sample schematic?

  • 0 in reply to Loren Siebert 1
    Expert 1325 points

    Hi Loren,

    We are talking on the LMH6554 family.

    Please consider this as the main device upon the questions are about.

    Thank.

  • 0 in reply to 1886332
    TI__Genius 13935 points

    The LMH6554 has a high impedance Vcm input, as it is directly connected to the base of a small transistor.  There are no on chip bias resistors.   

     The Vcm pin is specified as 180k Ohms and 6uA of bias current in the datasheet.  This is when the device is actively powered on.  It should be higher impedance when the supplies are not connected. 

    The Vcm pin will have no internal impedance to ground, if the supplies are truely floating then the Vcm pin will be floating as well for small signals.  For large signals the base to emitter junction of the input transistor and the ESD protection circuits cause a much lower impedance, but still with respect to the supplies. 

    Because the LMH6554 requires external components it is possible that the ESD protection circuits will divert current to the pins with external resistors to ground. 

    When the power is on, but the amplifier is not enabled the Vcm pin impedance will also be very high for small signals.  Again, for large signals the ESD circuits will become an issue. 

  • 0 in reply to Loren Siebert 1
    Expert 1325 points

    Hi Loren,

    Thank you for the answer.

    What you wrote on the LMH6554 is correct also for the LMH6552??

     

    thanks.

  • 0 in reply to 1886332
    TI__Genius 13935 points

    Yes, the LMH6552 and LMH6554 are the same architecture.  The LMH6552 has a higher supply voltage range, the LMH6554 is faster and lower distortion.