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LMH6629 Evaluation Board - strange output signal

Guglielmo Sirna
Prodigy 310 points
Other Parts Discussed in Thread: LMH6629

Hi, I having trouble with  two LMH6629 Eval boards.

Both units  show either no output signal (for small input signal ~100mVpp, 10MHz) or a clipped waveform (for  input signal ~1Vpp, 10MHz) . 

I have set R10=0ohm and use single +5V supply.

I am using the standard non-inverting input. The current consumption is ~11mA;

The input and output impedance is ~50ohm; 

The SW1A (PD) behaves as expected, powering down the device. 

Here is the DC level I have measured with teh device powered ON and Idd~111mA

pin 1, PDZ: 5V

pin2, FB: 0.3V

pin 3, IN- : 0.02V

pin 4 , IN+ : 0V

pin 5, V- : 0V

pin 6, COMP: 5V

pin 7, OUT: 0.3V

pin 8, V+: 5V 

Any idea what is going on?

BR

guglielmo

  • 0
    TI__Mastermind 22825 points

    Hello,

    As Paul Grohe has pointed out (Thanks Paul), you need to be mindful of input and output voltage ranges, if you decide to test the board with a single supply.

    Here is the email communication copy for reference:

    From: Sirna, Guglielmo
    Sent: Tuesday, May 28, 2013 1:34 PM
    To: Grohe, Paul; Hashemi, Hooman
    Subject: RE: LMH6629 Evaluation Board - strange output signal

     

    Many thanks.

    This should explain and solve the issue.

    I will go back to dual supply mode,  since I prefer to not have to apply a DC bias to the non-inverting input,

    Many thanks

     

    From: Grohe, Paul
    Sent: Tuesday, May 28, 2013 6:07 PM
    To: Sirna, Guglielmo; Hashemi, Hooman
    Subject: RE: LMH6629 Evaluation Board - strange output signal

     

    Hello Guglielmo,

     

    Actually, Hooman Hashemi is apps for the high speed (LMHxxxx) amplifers.

     

    The demo board is designed for split supplies.

     

    Setting R10 to 0 ohms sets it to a single supply configuration.

     

    From the voltages you listed, you do not have a mid-supply bias on the input.

     

    In other words, with no input signal – what do you expect the output to do? It cannot swing negative since you have taken the negative rail away…so it just sits at “0V” or can only swing positive.

    You need to apply a DC bias to the non-inverting input to bring the output up off the negative rail.

     

    Remember that the gain is 10, so you would need to add 250mV to the input to get the output to 2.5V.

    Paul

  • 0 in reply to Hooman Hashemi
    TI__Prodigy 310 points

    Thanks. Please considered this answered