Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

  • State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 49 subscribers
  • Views 950 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

OPA211: Regarding Megahertz frequency operation.

Gaurav Ramesh Datta
Prodigy 20 points
Part Number: OPA211

Greetings everyone,

I am planning to use OPA 211 op amp for ultrasound receive applications. Before using it to amplify actual ultrasound signals, i am testing the OPA 211 on a breadboard. I have the 8 pin SOIC  package shown below - 

I  am testing it with a non inverting gain configuration of 11 v/v (as shown in the pic below). Also I am testing on the Xilinx evaluation board.

Thus, the pin 2 is grounded through a 10 ohm resistor. A 100 ohm feedback resistor is connected between pin 2 (negative input) and pin 6 (output pin). pin 7 and 4 have +- 2.5 V respectively. Also, I have put 0.1 uF capacitors between supply pins and ground.

For operations below 1 MHz, it works fairly well. With an input signal of 200 mV (p-p) @ 500 KHz, I am getting an output signal having peak- peak magnitude of ~2 V. Attaching the output pic below - 

However, for inputs > 1MHz, the op amp starts showing anomalous behaviour. Attaching an output pic, when input is 1 MHz - 

I know that Breadboards are not the best for testing at higher frequencies. However, I do not think so it should be this bad at 1 - 4 MHz. Also, the bandwidth of OPA211 is 80 MHz for G = 100.

Am I missing something in my test setup? Would be glad is someone can help me out.

Gaurav RD

PA, USA

  • 0
    Guru 140200 points
    Hi Gaurav,

    breadboards usually have huge stray capacitances between the pin rows and should be avoided at all when experimenting with a HF OPAmp like the OPA211. It's very likely that these stray capacitances erode the phase margin and make the OPAmp to oscillate.

    Also, your feedback resistances are much too low ohmic! Increase them by a factor of 10.

    And: Don't connect the scope probe directly to the output of OPA211. Insert a small resistance of 47...100R. This will decouple the output from the capacitve load of the scope probe.

    Kai
  • 0
    TI__Guru* 93105 points
    Hello Gaurav,

    All of Kai's points are valid and should be considered and checked. Additionally, the waveform images appear to be amplitude modulated by a lower frequency source; more so on the 500 kHz waveform, but it is evident on the 1 MHz as well. I suggest that you place your OPA211 test board inside a metal enclosure to see if modulating signal is being picked up by the breadboard. We often use metal paint buckets as a test circuit enclosure and power the op amps using 9 V batteries placed inside the bucket.

    When an op amp oscillates due to eroded phase margin, it is usually at frequency near the unity-gain crossover frequency. The gain-bandwidth (GBW) of the OPA211 is about 45 MHz (G = +1 V/V) so I would expect such oscillation to have a frequency in the tens of megahertz. There are exceptions to this. If this is an oscillation it must be at a much lower frequency than tens of megahertz.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    TI__Genius 11135 points
    Gaurav

    We haven't heard back from you so we assume this answered your questions. If you need more help just post another reply below.

    Thanks
    Dennis
  • 0 in reply to Dennis Goeke
    Prodigy 20 points
    Yes Dennis, my issue has been resolved.

    Thank you Thomas and Kai for the valuable feedback.