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OPA325: Precision Amplifying

MCAV
Prodigy 70 points
Part Number: OPA325
Other Parts Discussed in Thread: OPA328, LM7705

Tool/software:

Hello, I would like to increase the signal from a piezoelectric element by 600 times. The frequency range I am focusing on is between 0 and 20 kHz. I want the preamplifier circuit that I will connect to the piezo to have as little noise as possible. I have a few questions regarding the types of resistors I should use to achieve this. Which type of resistor will minimize noise in the system? In some applications, I’ve seen MELF resistors being used. While browsing forums, I also came across information that SUSUMU's RS series is often used in audio applications. I’ve read that wire-wound resistors produce less noise but their coil structure can create an inductive effect that might be detrimental. What path would you recommend in this case? Do you have a specific resistor type you would suggest? Considering the space limitations on the PCB, using SMD components would be ideal for me.

  • 0
    TI__Guru 67601 points

    Since the effective bandwidth (BW) is a function of the gain-bandwidth product (GBW) and close-loop Gain, BW = GBW/Gain, for OPA325 (GBW=10MHz) you may calculate:

    BW = 10MHz/600 = 16.7kHz;  This means that at upper frequency range of 20kHz, the AC gain would be attenuated to 51.79dB or Gain=388.6 (that's 35% error) - see below.

    For this reason I would recommend to use a faster part like OPA328 with GBW of 40MHz resulting in AC gain of 55.2dB or Gain=575 at 20kHz (4.2% error).

    Now, if you operate on a single supply, you MUST make sure that the output stays within OPA328 linear range of (V-)+100mV > Vout < (V+)-100mV (see AOL conditions).

    For  this reason, depending on the package you use (with different max offset), you must make sure you meet the minimum piezo input voltage in order to assure that the output is 100mV or more above the ground (see below).  Alternatively, you could drive the negative rail with LM7705 negative charge pump (-232mV).

     

    As far as your question regarding the type of resistors to be used, it really does not matter because the thermal noise of resistor is mainly a function of its size and temperature. Thus, in the case of above application using 100ohm input resistor with spectral noise density of 0.4nV/rt-Hz  will be negligible vis-a-vis OPA328 input noise of 6.1nV/rt-Hz.

    OPA328 piezo G=600.TSC