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Operation Amplifier for video-ADA4891-1

avis
on Oct 6, 2010

Hi,

I'm using the ADA4891-1 in a non inverting application - With amplification of 2.

I'm using VS=3.3V , RG&RF = 604 Ohm, Input resistor-Rt=200 Ohm, I have 100n and 10u Capacitors betweren Vs and Gnd, Input Voltage=1V, output Voltage=Ringing!!!!!!. I change Rf to 0 Ohm and I get a "1" Amplifier without ringing!!!. What may be the reson for that?

Best Regards

Avi

  • 0
    •  Analog Employees 
    on Oct 6, 2010 11:03 PM

    What is the load for the amplifier?

    What is the complete definition of the input signal? Is it a sine wave? A video format signal? What are the min and max voltages?

    What does the layout look like? What is the grounding?

    What type of capacitor (dielectric) is the 100nF?

    Hank

  • 0 on Oct 7, 2010 11:02 AM

    Good morning,

    The load is an AFE WM8214 Wolfson, via a series resistor of 33 Ohm).

    The input signal will be a video signal(From sensor), but I test the circuit

    with DC.

    Regarding the layout, all the capacitors are within less than 5mm from the

    IC, and I have a Gnd plan layer. There weren't any other special

    consideration.

    The capacitor is ceramic capacitor from X7R family.

     

     

    Thanks

     

    Avi

  • 0
    •  Analog Employees 
    on Oct 13, 2010 12:46 AM

    OK, now I am a bit confused.

    You say the input is DC and you have ringing. Ringing is typically on a fast edge. With DC in it would be more precisely oscillation. While related, they are not the same.

    Does the oscillation change with DC level? What is the input you see oscillations on?

    Have you tried an AC (sine wave) signal? What does it look like?

    One thing I would try is a load resistor (say 150 ohms) from the op amp output to gnd.

    Any chance you could give me some scope grabs to look at?

    Hank

  • 0 on Oct 13, 2010 10:50 AM

    Good morning Hank.

     

    The Oscillation level changed with the DC level!!!!

    I did not try an AC yet.(I try AC with Gain=+1 = Changing Rf to "0" ohm).

    I try using Rf and Rg at 604 Ohm and the phenomena is the same.

    I replace the ADA4891-1  with AD8091ARTZ  and the circuit work great with

    Gain=2.

    Can you test the same circuit (It should be easy), with 3.3V Power supply.

    The circuit is as follows:

     

     

    I replace R55 and R56 to 604 Ohm, I also remove R121 (For tests), and even

    add a 100p Capacitor in parallel with R56.

     

    The problem is in pin 1.

    I bought 3000 PCS, Date Code :0951 Lot id: 1878291-1

    Pkg ID: adi+12825998, CUST PO: P1003214.

    The delivery sent from Singapore.

     

    Best Regards

    Avi

  • 0
    •  Analog Employees 
    on Oct 20, 2010 1:31 AM

    Sorry for the delay on this. I have been traveling on business.

    OK, you tried a different op amp and it worked. Did you try a different sample of the ADA4891-1?

    In looking at the data sheet for the amp, specifically page 18, note that for single supplies the suggested circuit includes a DC blocking capacitor for the gain setting resisor. Also, the  input is AC coupled and has a resistor network to bias the input to 1/2 the supply.

    Try this configuration and see if this helps. Remeber that DC inputs won't work.

    03c2b2fb082f83543482e7a3ba8f0a6b.html8561.03c2b2fb082f83543482e7a3ba8f0a6b.htmlAlso you might want to check out the ap note referenced on the same page, just above Fig. 61

    Hank