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I am using a high-speed op-amp (700 MHz unity gain bandwidth). I have got two questions.

1) It has voltage supply marked Vs+, and Vs- but no ground. So say I supply with say +5, and -5V. But there is no ground connection. The circuit which will feed this circuit, has a ground reference. But the op-amp does not have one. So how it can judge when the input voltage is negative.

2) To test, I have set the negative voltage supply to GND. and set it up as voltage follower. The diagram is the output. Yellow is the input, and green is the output. You can see lot of jitter in input and output, the moment I connect the sine wave input to op-amp. But before connecting input was smooth. Input frequency is 4KHz.

Can someone please tell me why?

OP-AMP Part # OPA2652U (Texas Instruments)

  • \$\begingroup\$ The opamp doesn't need a ground reference. So connect 0V, NOT -5V, to ground. And you may need to improve decoupling and input screening to solve the stability issues. \$\endgroup\$
    – user16324
    Commented Mar 9, 2017 at 11:58
  • \$\begingroup\$ Olin makes an excellent point that the part may not be unity gain stable. What is the part number of the device? \$\endgroup\$ Commented Mar 9, 2017 at 12:20
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    \$\begingroup\$ If you have not read it yet, Opamps for everyone is a very good starting point to learn about opamps, it's free and can be found here: web.mit.edu/6.101/www/reference/op_amps_everyone.pdf \$\endgroup\$ Commented Mar 9, 2017 at 12:33
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    \$\begingroup\$ Your opamp is oscillating, most likely due to bad layout. Please post a schematic and picture of your PCB. This is not an opamp for beginners. Why pick a 700MHz part? \$\endgroup\$
    – bobflux
    Commented Mar 9, 2017 at 13:05

1 Answer 1

has a ground reference. But the op-amp does not have one

This is a basic opamp question, and is not particular to your case or your opamp. Opamps are driven from the difference between the two input signals. Ideally, the average value of the two inputs doesn't matter. As long as the inputs are both within the input common mode range, the opamp will function correctly.

Again, this is basic opamp stuff. Go learn about opamps.

high-speed op-amp (700 MHz unity gain bandwidth)

This is not a good opamp to learn the basics with, which you need to do before getting into advanced topics. Consider anything over 10 MHz as advanced. Go get some LM324 and start experimenting. Put this opamp aside until you have learned how to make the LM324 do a few useful things.

High speed opamps are "twitchy" by nature, and you have to be extra careful in design and layout to make sure all the stability criteria are met. You're not ready yet to even understand what they are.

What is happening is that the opamp is going unstable during the high parts of the sine. Are you really sure it is supposed to be stable at unity gain? Look over the datasheet again carefully. Try using feedback that results in 5x gain, for example, and see what you get.

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    \$\begingroup\$ It seems like my op-amp does not have enough gain/phase margin. May be because I am setting it up on breadboard(cheap one). And yes the datasheet says it is unity gain stable and internally compensated. @OlinLathrop \$\endgroup\$
    – niki_t1
    Commented Mar 9, 2017 at 13:58
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    \$\begingroup\$ @niki: Breadboards are not suitable for testing 700 MHz signals. This kind of opamp needs careful layout and probably a ground plane. Usually the datasheets for such opamps go into layout issues. \$\endgroup\$ Commented Mar 9, 2017 at 14:22

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