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OPA2674 abnormal operating

Other Parts Discussed in Thread: OPA2674

Hi supporter,

I'm using a high speed OPAmp OPA2674 within a gain=+4 configuration which mentioned in data sheet fig. 1. Please have a reference:

In my practice circuit, i replaced the Rg and Rf to 150 and 500 Ohms, respectively. (Vsupply = +-6V, Dual supply), my input conditions are Vin = 2 Vpp from 100 Hz to 40 MHz

Here I have some detailed technical question about the OPAmp

1. The data sheet mentions that the output swing of the OPA2674 can achieve 10 Vp-p, but under my configuration, the output voltage swing clamped at +- 2.5V and cannot rise up even the Vin is increase.

2. The gain of this circuit was stay at G=+2 (Vin = 1 Vpp--> Vout = 2Vpp,  Vin = 2Vpp --> Vout = 4 Vpp, freq. = 100~10M Hz), were there any wrong on my setup?

3. The G=+4 large signal frequency response (plot in page 7) mentions that the 3db frequency was higher than 100 MHz, but under my configuration, the response can't reach 40 MHz (in 30 MHz test, Vin = 1 Vpp --> Vout = 1.6 Vpp), were there any mistake or misunderstanding on my circuit setup?

Thanks for your help and I'm very look forward to someones reply!

Lin

 

  • Hello,
    That is certainly strange and unexpected. I have a few questions on your setup-
    1. Can you please provide a schematic of the circuit, including the load you are using.
    2. Are you using a 50Ohm source from the function generator and do you have a 50Ohm termination on the non-inverting pin of the amplifier?
    3. I assume that you are expecting a gain of 4.33V/V (1+500/150)
    4. Is the scope in 50Ohm mode or high-Z mode?
    5. How much resistance do you have at the output of the amplifier?
    6. Please make sure that the unused 2nd channel of the amplifier does not have floating inputs. Railing out one of the amplifier channels may cause the other channel to behave funny.
    7. What quiescent current are you measuring on the positive and negative supplies when you don't have a sine-wave input?
    If you can send me a photograph of your board that will also help me immensely in debugging.
    -Samir
  • Hi Samir,

    Thanks for your reply.

    1. My circuit schematics was same as the Figure 1 shown in datasheet, here I replaced the 6.8uF capacitance to 10uF, the Rf and Rg replaced by 500 and 150 ohm.

    2. The function generator was set as a 50 ohm source mode, and the 50 ohm termination resistance was also set at the input like the figure show.

    3. Yes, the gain I expected is around 4.33.

    4. the oscilloscope was set as 50 ohm mode.

    5. the output resistance of the amplifier was 50 ohm, like the figure show. (output pin connected a 50 ohm in series and connected to the oscilloscope)

    6. the unused channel is floating now, i'll connect them to ground and try again later.

    7. the quiescent current were 18mA on both positive and negative channel when there were not input.

    Is the output swing of the configuration related to the output resistor? because the data sheet show that the figure 1 circuit configuration has ability to output a 10 Vpp swing.

     

     page 7

     page 9

    Thanks for taking your time and i'm very enjoy having discussion with you

    Lin

  • Hello Lin,
    Your setup seems correct as you describe it. While the unused channel may not be creating a problem for you now, it is an uncertain state so you never know when it may start to cause issues. I would configure the amplifier similar to figure 1 without any signal driving it just to be on the safe side. Now back tour problem, I think the circuit is working properly, however the reason you are seeing half the gain is because you are losing 6dB due to the 50Ohm output resistor and the 50Ohm termination to the scope. Can you please put your scope in high-Z mode and recheck if you get the correct gain?

    -Samir
  • Hi Samir

    Thanks  for your reply!

    If we only can use high-Z mode as termination to the scope?

    If there is any method to have 50ohm as termination and 10Vpp output?

    (In our future work,we need 50ohm as termination.)

    Thanks for taking your time.

  • Hello Lin,
    With the 50Ohm double termination you will always have 6dB loss. If the OPA2674 is very close to the next stage that you are planning on driving you may be able to get away with no termination, however this is something you will have to experiment with and see if the signal integrity is acceptable.
    If the 50Ohm "termination" is fixed, then the other choice is also to reduce the value of the series output resistor from the current 50Ohm. If you can make that, say 10Ohm, then the attenuation will be 20log10(50/60) = 1.6dB (vs. 6dB). The distortion will degrade a little bit as you can see in the figure on Page 9 (top left).
    -Samir