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Designing the right Amplifier

Emmanuel Mendez Alicea
Prodigy 80 points
Other Parts Discussed in Thread: INA121, OPA192

Hello,

I'm working on a project where I have to design an amplifier and a filter for a sensor. The sensor is a hydrophone and this generates a differential voltage proportional to movement in water. It has a piezo electric element inside. The output of this sensor usually is around the 10 mV so I decided to have an amplifier with a gain of 100 V/V (40dB) to obtain an output of approximately 1V. 

My question is, will a I need an amplifier with differential voltage inputs? Let's say for example like the instrumentation amplifier INA121. Or will the design of a single input non-inverting amplifier work? 

I ask because I'm a little confused with the connections. Also, the output of the hydrophone is a BNC cable so I would have to connect a BNC Female connector to the amplifier, right?

**The hydrophone output goes to the amplifier. The filter is already designed and is working**

Many questions so thanks in advance for taking your time :)

  • 0
    TI__Mastermind 25825 points
    Hello Emmanuel,

    Can you please share the data sheet for the hydrophone in question? We need to understand the input signal in greater detail (polarity, maximum voltage, frequency, impedance, etc.).

    Also, what voltage supplies are available for the amplifier? Finally, I assume that the output of the amplifier will be connected to an ADC, correct? If so, what is the input range of the ADC? I ask because it is likely to be greater than 1V, which means you could increase the gain and therefore obtain greater resolution of your input signal.

    In the meantime, here is a forum post that may be of interest:

    e2e.ti.com/.../1248748
  • 0 in reply to Pete%20Semig
    Prodigy 80 points
    Hello Pete,

    I was considering the Cetacean CR2 hydrophone due to the dimensions and price. Budget for us is a limitation for this year. They don't a have a certain datasheet, but they have information on the page. Here's the link: www.cetaceanresearch.com/.../index.html

    The voltage supplies available are from batteries. We have designed DC to DC converters with different outputs. One of them is a 12V output. We also have in mind using two 9V batteries for the Op-Amps voltage supplies but we have to run tests.

    We had in mind using the LPC1768. This board has ADC inputs so we won't need analog-to-digital converters, am I correct?

    **NOTE: I'm seeing that the CR2 hydrophone sensitivity response plot is not very linear, so the Sparton PHOD-1 Hydrophones are also to be considered, but this won't need an amplifier if I'm not mistaken. Maybe you can take a look and give me your opinion. I'm a college student and this is all new to me so any advice, recommendation, critique or any comment will be really grateful.

    Thanks for taking your time, I really appreciate it.
  • 0 in reply to Emmanuel Mendez Alicea
    TI__Mastermind 25825 points
    Hello Emmanuel,

    Thanks for the additional information. I have looked at the CR2 hydrophone that you're interested in and cannot find (or do not understand how to derive) the output voltage. I suppose this is because they have designed and sell amplifiers for use with their sensors, which you probably don't want to purchase given the financial constraints.

    It appears to me as though the Sparton hydrophone has a preamplifier whereas the CR2 does not. It also appears as though the Sparton sensor has a 'preamp calibration feature' which likely explains the linearity difference. My guess is that if you build your own amplifier for the CR2 you will have to calibrate it in order to get similar performance to the Sparton sensor. Please understand that I'm not an expert in the area of hydrophones, though!

    You are correct in stating that the microcontroller you're using has an integrated ADC. It looks like the reference voltage is 3.3V, so if we know the output range of the sensor we could design the signal conditioning circuitry accordingly. Note that the integrated ADCs are 12-bits...you may want to ensure that this gives you enough resolution for your signal.

    I did find a functional block diagram in the user's guide for the VP2000 preamplifier. The input to the signal conditioning circuitry appears to be single-ended, therefore you should be able to use op amps. It has a buffer stage followed by a bandpass filter and then finally the gain stage. If you have static (or pre-determined) cutoff frequencies of the bandpass filter and it appears as though you want a gain of 100V/V, you could use a tool such as FilterPro (e2e.ti.com/.../3076.filterpro-v3-1) to build the gain and filter stage.

    For the op amp, I suggest evaluating the OPA192. I selected it based on supply voltage, rail-to-rail input, and noise. My only concern is bandwidth (10MHz typical). If you do not think this amplifier will work in the design, check out our selection tool at www.ti.com/.../OPAMPS-SELGUIDE

    I hope this helps you with your project!