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high bandwidth op amp with same other feature as LM 741

manu francis
Prodigy 60 points
Other Parts Discussed in Thread: LM741, OPA690, TINA-TI

Hi,

   I am trying to create an "auto balancing bridge" for impedance measurement in the range of 1 MHz to 10 MHz. For that purpose I need an op-amp . Can anyone suggest?  I tried with LM 741 for low frequency,it is working properly. I tried with AD 8055 instead of LM 741 for high frequency impedance measurement, I am not getting any output.. Is any gain restriction for AD 8055 ? So I need an op amp with all same features except band width, need high band width upto some 300 MHz. .. 

 Regards ,

              Manu

  • 0
    TI__Genius 13335 points

    Hello Manu,

    The LM741 is a very old but very general purpose amp. Typically for auto balancing bridge applications, the frequency is much lower so you can employ a simple op amp where you get ok accuracy. However, as you start increasing the frequency you may have to add other components to detect nulls, phase, loop filter, and a vector modulator to give working/accurate results. So using a higher frequency generic amp isn't the best way to go.

    Do you have a schematic of the approach you've tried thus far?

    Thanks,
    Luke Lapointe
    High Speed Amplifiers

  • 0 in reply to Luke LaPointe
    Prodigy 60 points

    Hi Luke,

                       I just used simple auto balancing bridge. The input and output voltage acquired using NI 5122 card and impedance calculated using labview software. For MHz range operation which op amp is suitable?

     

  • 0 in reply to manu francis
    TI__Genius 13335 points

    Manu,

    For this configuration, the total dynamic range of Zx will be limited by the selection of R and the gain bandwidth of the amplifier. Not to mention if Zx has some sort of capacitance/inductance, then you could easily end up driving the gain extremely high as the frequency is increased and cause potential instability or oscillations. For example, assume R=1kΩ, Zx=1kΩ || 3pF. At low frequencies the gain will be 0dB so you can assume Zx has a resistive element of 1kΩ. Then as the frequency is increased and the capacitor starts kicking in, the total impedance of Zx starts to decrease which causes the amp gain to increase. Although you can you use this information to calculate the size of the capacitor, you are also driving the gain higher and higher until eventually (depending on the gain-bandwidth product of the amplifier) you cause the amp to go open-loop and your phase margin will likely be very low and likely unstable.

    You can see that the phase margin is about 40degrees here in the simulation. In reality it may be even lower and you could notice some oscillations on the output even without a signal present. This effect will only get worse with higher capacitive values. I've included the simulation file below which uses the OPA690 (which is a very good general purpose amp) so you can experiment with different values of Zx.

    2084.Auto Balancing Bridge.TSC

    You can open this in our free simulation program TINA-TI.

    In reality, you may want a different approach altogether. Agilent wrote a good app note on different impedance measuring techniques: http://cp.literature.agilent.com/litweb/pdf/5950-3000.pdf

    They've included some theory on auto balancing bridges on page 25. You're more likely going to need a more complicated circuit like that shown in figure 2-5.

    I hope this helps. Let us know if we can help for any other amplifier related questions.

    Regards,
    Luke Lapointe
    High Speed Amplifiers

  • 0 in reply to Luke LaPointe
    Prodigy 60 points

    Hi Luke,

                      Thank you so much for this good reply. I will try my circuit with OPA690.

                                                   Thanking you,

                                                                       Manu