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I am using AD826 op amp to convert differential current sinked DAC output operating at 50 mega samples per second. DAC is fed a 3MHz sine wave by a FPGA. Output of the DAC is fed to the opamp with following configuration:

enter image description here

The DAC is LT1668.

This configuration works well until around 1MHZ, but the output of the opamp reduces rapidly after that frequency. Also, the output wave seem to get distorted.

When simulated through LTSPice, I see that frequency response of this circuit (I have modeled the DAC using current sources) is flat even after 10MHz. For this simulation I have used Analog Device's spice model for AD826.

I tried changing resistor/capacitor values, but frequency response is same.

The circuit is currently built on a breadboard.

What could I be doing wrong?

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  • \$\begingroup\$ markup schematic to correct errors or show deviations \$\endgroup\$ Jul 2, 2022 at 18:50
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    \$\begingroup\$ If GBW is only 40 MHz nominal for Av = 20 then you have only 2 MHz nominal BW. wrong OPA \$\endgroup\$ Jul 2, 2022 at 19:08
  • \$\begingroup\$ slew rate might be also a limit at such a large wave, you need 10*2*pi*3M= 200V/us. Just confirm this when selecting another part with better GBW product. Also breadboard is terrible at those frequencies \$\endgroup\$
    – tobalt
    Jul 3, 2022 at 4:58
  • \$\begingroup\$ Where did you get the schematic from? \$\endgroup\$ Jul 3, 2022 at 5:36
  • \$\begingroup\$ The schematic is from the datasheet of the DAC LT1668. \$\endgroup\$
    – peter
    Jul 3, 2022 at 17:38

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