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LMH6629: noise optimization

David D
Intellectual 580 points
Part Number: LMH6629

Hello,

On page 26 the datasheet advises: "If bias current cancellation is not a requirement, then Rf || Rg does not need to equal Rseq. In this case, according to Equation 3, Rf || Rg should be as low as possible to minimize noise."

I'd like to ask how low can one practically go with reducing Rf,Rg and what are the tradeoffs (except the mentioned offset cancellation)?

Rseq=50R.

If low noise is top priority, what else can be done to improve performance?

Thanks,

David.

  • 0
    Guru 48395 points

    Hard to answer without 

    1. Input impedance matching needed?

    2. AC coupled a possibility?

    3. Desired frequency span, 

  • 0
    Guru 140200 points

    Hi David,

    why not running a simulation with the

    LMH6629 TINA-TI Reference Design

    ?

    The model includes RF effects and noise:

    * LMH6629 SPICE MODEL PERFORMANCE
    *
    * MODEL FEATURES INCLUDE OPEN LOOP GAIN AND PHASE,
    * OUTPUT SWING, OUTPUT CURRENT THRU THE SUPPLY RAILS,
    * OUTPUT CURRENT LIMIT, CLOAD EFFECTS, RF EFFECTS,
    * SLEW RATE, COMMON MODE REJECTION WITH FREQ EFFECTS,
    * POWER SUPPLY REJECTION WITH FREQ EFFECTS, INPUT
    * VOLTAGE NOISE WITH 1/F, INPUT CURRENT NOISE WITH
    * 1/F, INPUT BIAS CURRENT, INPUT COMMON MODE RANGE,
    * INPUT OFFSET VOLTAGE WITH TEMPERATURE EFFECTS,
    * INPUT CAPACITANCE, COMP PIN FUNCTION INCLUDING
    * THRESHOLD AND INPUT CURRENT, AND QUIESCENT CURRENT
    * VS VOLTAGE AND TEMPERATURE. SHUTDOWN PIN
    * AND ITS FUNCTIONALITY IS NOT INCLUDED IN THE MODEL.

    Kai

  • 0 in reply to kai klaas69
    Intellectual 580 points

    Hi Kai, Michael,

    I'm using the Spice model to simulate in ADS as well.

    Input impedance : it's 50R to GND, driven by high-Z current source.

    No need to zero offset.

    AC coupling:  only at output, input must be DC coupled.

    Freq.: DC to ~30MHz.

    Gain: < ~6V/V (due to DC component at input).

    I was lowering the Rf,Rg in simulation to a very low values (tens of Ohms).

    Which kept improving the noise.

    I'd expect too low resistors to load the output stage...thus I was asking here for confirmation and other aspects I might have overlooked.

    Thanks,

    David.

  • 0 in reply to David D
    Guru 48395 points

    Well yes, just lowering the R's will incrementally improve the noise - you need to check the output current loading that produces and decide if you are ok with that. Eventually, if the Rf+Rg load gets to low, it will reflect into the internal gain node and start to bandlimit. I would probably not go below 100ohm Rf. But check the DC and AC current loading that implies against the spec output current available. 

  • 0 in reply to David D
    Guru 140200 points

    Hi David,

    can you show a schematic?

    Kai