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OPA847 used as a clamper

Paul Law49
Intellectual 260 points
Other Parts Discussed in Thread: OPA847, OPA698, TINA-TI, VCA821, VCA822

Hello, im planning to use the OPA847 in a clamper circuit.

I am using this circuit for high speed applications. My input signal will be around 10MHz.

The circuit involves the use of a capacitor and a diode.

1. Is there a specific type of capacitor and diode that i should use?

2. Are there necessary calculations that i should do?

  • 0
    TI__Genius 12631 points
    Hello Paul,
    Do you have a circuit schematic showing your proposed circuit. C0G and NPO capacitors typically show better linearity performance than traditional X7Rs. With regards to the diode, the capacitance of the diode needs to be taken into account as it will load the amplifiers output.

    Note that the OPA847 needs to be configured in a gain of at least 13V/V otherwise it will be unstable. Have you looked into the OPA698? This amplifier features an internal clamp/limiting circuit.
    -Samir
  • 0 in reply to Samir Cherian
    Intellectual 260 points
    Samir, can u recommend a clamping circuit using the VCA847?
    i will be using it at a frequency between 1MHz to 10MHz.
    I will be using the clamping circuit because my input signal is from -2mV to 2mV and i want to make it 0 to 4mV before i enter the to my ADC.
    Do i still need to observe the gain of 13V/V rule if im operating it at a such frequency?
    and i cant order OPA698 because im living outside USA and im tight on schedule
  • 0 in reply to Paul Law49
    TI__Genius 12631 points

    Hello Paul, You will still need to maintain a gain of 13V/V even if your signal is at the mV level.  The alternate is what is known as noise gain shaping. Please see the reference design below where this concept was used to stabilize a fully-differential amplifier. The concept for a single-ended opamp will be the same.

    Did you mean 0-4mV or 0-4V above? There is no effective clamp that I know of that will work in the mV range. Most diodes clamp at 700mV. Even a schottky diode will clamp at a few 100 mVs.

    -Samir

  • 0 in reply to Samir Cherian
    Intellectual 260 points
    Samir, sorry i gave u the wrong input.
    My input signal is from -1V to 1V and i want to clamp it to 0 to 2V using the OPA847 before the signal enters the ADC.
    I will be running at a 1MHz to 10MHz frequency.
    Can u recommend a clamping circuit for this?
    Can i do it without amplifying my input signal?
  • 0 in reply to Samir Cherian
    Intellectual 260 points
    Or rather is it possible to build a clamping circuit without using a high speed op amp? Knowing im operating at 1MHz to 10MHz
  • 0 in reply to Paul Law49
    TI__Genius 12631 points
    Paul, since you will be level shifting the signal, an opamp will be needed. However no amplification is needed so the OPA847 is not really the appropriate amplifier for this application. The OPA698 that I mentioned has the built in clamps that will make your solution straightforward.

    With regards to the need for a high speed amplifier, if you are concerned about the linearity and distortion at 10 MHz it is recommended that you use an amplifier with around 10x the bandwidth at least. If this is not a concern then you can certainly go with a precision amplifier. You should post your query on the precision amplifier forum as well to see what they recommend. I do not see too many queries for clamping needs so Im not really an expert on the subject.

    -Samir
  • 0 in reply to Samir Cherian
    Intellectual 260 points
    Samir, turns out that my input signal doesnt have to be -1V to 1V. I just need to output a 0V-2V waveform that will be entered to my ADC. I will be getting my input signal from a signal generator. I was thinking to still use the OPA847 as a clamping circuit, but this time with a gain of 20V/V. So if ill be inputting a -50mV to 50mV signal, and i use a OPA847 clamping ckt with a gain of 20V/V, will i now be able to produce my 0V to 2V output?
  • 0 in reply to Samir Cherian
    TI__Genius 12631 points

    Please see attached circuit. For clamps you can use a TVS diode (transient voltage suppressor) to do the job but be aware of the capacitive load from the diode.

    May I ask why you want to use the OPA847 and build your own clamps when a ready made solution exists in the OPA698?

    OPA847.TSC

  • 0 in reply to Samir Cherian
    Intellectual 260 points
    Samir, it's because i'll have to wait another 2-3 weeks if i order the OPA 698 now and i can't afford to wait that long. I am using this circuit for my project and i need to accomplish it as soon as possible.. Sorry Samir do you have other formats ? i can't seen to open the attached circuit u sent me because it is in .tsc
  • 0 in reply to Paul Law49
    TI__Genius 12631 points

    Paul,

     The file format uses TINA-TI, a spice simulator. It can be downloaded from here. I recommend that you download the tool since once you narrow down a TVS diode you need to be able to incorporate that into your design to see the effect of the diode on the amplifier.

    -Samir

  • 0 in reply to Samir Cherian
    Intellectual 260 points
    thank you Samir. I think this will work.
    are there other resistances that i can use besides 4.93k ohms and 47.5 ohms? can i use 1000 ohms and 10 ohms instead?
  • 0 in reply to Paul Law49
    TI__Genius 12631 points

    Paul,

    Sure 1kOhm and 10ohm will work. I modified the solution with a series output resistance and a zener diode. You will use an appropriately sized TVS diode instead. Couple of points:

    1. The series resistor is there to isolate the TVS diode capacitance from the amplifiers output. You may not need it but its good to have a placeholder in there and you can just make it 0 ohm if you don't need it.

    2. You may not be able to find a 2V TVS diode; in that case see if you can find a 7V TVS diode and bias the anode of the diode to the amplifiers -5V negative supply. This will ensure that you can clamp the output to 2V.

    Samir.

    2313.OPA847.TSC

  • 0 in reply to Samir Cherian
    Intellectual 260 points
    Thanks Samir. I'll try the using the circuit u sent me. Appreciate it.
  • 0 in reply to Paul Law49
    Intellectual 260 points
    Samir,you mentioned that the opa847 is unstable below gain of 12V/V.
    1. Where is that written in the datasheet?
    2. Can i operate vca821 or vca822 in unity gain? I plan to use it as a clamping ckt.
  • 0 in reply to Paul Law49
    TI__Genius 12631 points

    Please read the description on the front page. It is mentioned there. Also, please look at the characteristic curves on the datasheet where you will see peaking in the frequency response at low gains.

    I am not sure how you plan to use the VCA821 as a clamping ckt. It is not stable in a gain of 1

    You need to carefully read these datasheets. The answers to most of your questions are in there. Also please use TINA-TI to simulate your circuits when you are in doubt. If you are unsure on how opamps work please go through the TI Precision Labs training. It is a little long but quite extensive.

  • 0 in reply to Samir Cherian
    Intellectual 260 points

    How will i test if my opa847 is defective or not?

  • 0 in reply to Paul Law49
    TI__Genius 12631 points
    Paul, I do a couple of things to start with:

    1. Check the quiescent current and the output offset voltage.

    2. Put a slow sine-wave, 1kHz-1MHz, 100mV input and check the output on the scope.

    What issues are you seeing?