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OPA699 half wave rectifier

Other Parts Discussed in Thread: OPA699

Hello,

I'm Aktham.

I'm designing a high speed half wave rectifier using the OPA699 with a voltage gain of 6V/V.

My design is almost the same as in figure 8 of the data sheet (see attached Tina TI simulation file). The input signal is of 0.25 V or 0.5 V.

However, in my design, several surprising results do not seem to meet the specifications of the datasheet:

- There is a large distortion of the rectifier output at the rising slop. This distortion starts at almost 1 MHz and it is becomes worst  until several 10 MHz.

-This distortion is less important at higher frequencies (100 MHz), but the gain of 6 is not retained as in figure 9 of the datasheet.

Simulation results are almost identical to measurement results.

I'll be very thankfull if you could help.
have a nice day.
Aktham

OPA699_rectifier.TSC
  • Hello Aktham.

      With regards to the degradation at 100MHz, this is due to the slew rate limitation of the amplifier. A 3Vpk, 100MHz signal will require a slew rate of 2*pi*100e6*3 = 1885V/us. The typical slew rate is only about 1400V/us for a 2Vpk signal. This is why you are seeing the signal amplitude reduction. Thisbeing said I do think there may an error in the datasheet which has the Large Signal Bandwidth as 290MHz. This is quite an old part and I need to find an engineer who worked on this project to figure out the discrepancy in the Slew Rate-Large Signal BW numbers. My suspicion is there is some interaction with the clamp circuit as well which is limiting the output amplitude. Can you please have the Vlimit at -3V or so and see if you can see signal gain degradation?

    With regards to the distortion that you are seeing, it is because the clamp has a propagation delay when returning from a clamp situation. In the case of the OPA699, on page 4 of the datasheet this recovery time is typically 1ns. This is the reason you see the glitch/non-linearity in your signal.

    Hope this helps,

    Samir

  • Hello Akhtam,

     I tested the circuit with clamps at +/-3.5V and the output is still attenuated. It does look like there may be a datasheet error wherein the Slew Rate and Large Signal BW numbers dont correlate well.

    Samir