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    ADA4927 with common mode




      I want to use you product to drive a ADC with a common mode.

      My design is


      The CMIRef is the output common mode of the ADC. The left SMA is the input and the right sma are the output.

      I supply the product with V+=+5V and V-=0V. R15 and R7 in red is not fitted

      I think i respect your recommandation to design it but the output will decrease an lost 3dBm at 400MHz.

      In your datasheet, the product have a bandwith more than 1GHz.

      The output swing is about 500mVpp.

      Do you know why I have only a bandwith of 400MHz instead of more than 1GHz?


      Thank you


      Best Regards


      Man Long

        • 1. Re: ADA4927 with common mode

          The ADA4927 is a current feedback amplifier. Its Bandwidth is dependent on the value of Rf. The 1 GHz bandwidth in the datasheet is for Rf=301 ohms. As you increase the value of Rf, the bandwidth decreases. You schematics show Rf=715 ohms. Reduce Rf to get larger bandwidth. You would also have to reduce Rg to keep the gain the same.





          • 2. Re: ADA4927 with common mode

            I change Rf resistor to 300 ohms and Rg to 195 ohms but the same thing appears.

            I forget to said that Vocm is about 3.12V.




            Man Long

            • 3. Re: ADA4927 with common mode

              With the feedback resistor decreased, you should have seen some changes if the bandwidth is limited by the 4927. Did you see any peaking in the frequency response?


              How do you measure the bandwidth?

                              Using network analyzer? Scope probes and sig gen?

              Did you verify that the source you are using actually has 1 GHz BW at the input to the 4927?


              Is your PCB god for 1 GHz?


              Can you send the schematics with a white background as a file? I can get you a frequency response plot with your configuration but have a hard time reading off all the numbers from the picture you posted.



              • 4. Re: ADA4927 with common mode

                I did the measure and effectively the bandwith increase up to 600MHz. I decrease Rf to 191R and the bandwith is about 620MHz. I see a peak at 400MHz in the both case.

                I measure the bandwith with scope probes and I did it with spectrum analyser. I input a sinus waveform.

                With spectrum analyser, I check the input and I measure the output. All this, for all frequency.

                I think the pcb support 1GHz because I use another product of analog device, the ADA4960-1 for AC coupled in the same board.

                With this amplifier I have a bandwith wich can go up to 1000MHz.

                In which format do you want the schématic?

                So i change the background in white. I hope you can read it easier.


                • 5. Re: ADA4927 with common mode

                  I forget to say that when i decrease the Rf resistor, a 2nd harmonic appear from 150MHz. for example at 150MHz, the ratio signal 2nd harmonic is only 17dB and at 700MHz is only 2dB with Rf resistor at 191 ohms and the third harmonic appear from 200MHz. With Rf=300 ohms at 700MHz I have only 6 dB for the second harmonic. With this resistor I don't see the third harmonic. Of course at the input they don't have the 2nd and the 3rd harmonic.

                  do you think this appear because the input is not totally adapted? With calculation I have 33 Ohms for input equivalent resistor.

                  • 6. Re: ADA4927 with common mode

                    Please send me the schematic as a large, high-rez picture so I can open it wih Office.

                    • 7. Re: ADA4927 with common mode

                      Ok I send you in pdf file.

                      • 8. Re: ADA4927 with common mode

                        I bought the eval board of this product (ADA4927-1YCP-EBZ). I solded it with the ADA4927-1, with RF=300Ohm and Rg = 300Ohm to have a gain=1.

                        I put Rt=57.6 Ohms and Rts=26.7 Ohms.

                        With these I have a bandwith of 700MHz only with a peak at 400MHz. I don't understand why I can't have the 1GHz of bandwith as indicated in the datasheet. How can we flatten the peak to have a minimum of ondulation?




                        Man Long

                        • 9. Re: ADA4927 with common mode

                          My load is 100 Ohms. I never see in the data sheet RL=100Ohms, always 1kOhms or 200 Ohms.

                          Do you thinks that this amplifier can drive a load of 100 Ohms?




                          Man Long

                          • 10. Re: ADA4927 with common mode

                            Can anyone answer me?

                            • 11. Re: ADA4927 with common mode


                                You have a problem with the AD8031 circuit.  The input capacitance is about 1.6pF, so with a 10k

                              feedback resistor, you have a pole at about 10 MHz.  In addition, you have large caps on the output

                              pin.  Both of these will affect your stability.  I'm surprised you have not seen oscillations.


                              • 12. Re: ADA4927 with common mode



                                I'm sorry but I don't use the AD8031 circuit but ADA4927.


                                Man Long

                                • 13. Re: ADA4927 with common mode

                                  Your schematic "schema" shows the ADA4927 and the AD8031??

                                  • 14. Re: ADA4927 with common mode

                                    Ah ok excuse me.

                                    I'm thinking about the differential amplifier. So you said my feedback resistor is not appropriate to this amplifier.

                                    This amplifier is to buffer the input voltage to the output. The high value of the output capacitor are to stabilize the voltage. So if my schematic is unstable which value do you recommend?


                                    Thank you

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