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circuit design using OPA835

Ruipeng Li
Prodigy 10 points

Other Parts Discussed in Thread: OPA835

Hello,

 

I am doing research on piseoelectric sensor. This sensor can generate a signal within +-20mV at around 100kHz when knocked nearby. What I need to do is to magnify the signal for 20 times and offset the signal to the voltage range within 0 to 2V. I am using the circuit design of figure 1 of the file "OPA835 Ultra Low-Power Op Amp Applications".

Figure 1 requires Rf to be 2kohm, so I choose Rg to be 100ohm. V_ref is set to be 1.5V, which is provided by a battery. V_in of the expect value of V_signal adding V_ref is achieved by connecting the negative end of the piseoelectric sensor to the positive end of the battery and the negative end of the battery to GND.

However, I only observed that the signal was magnified by 20 times. The signal voltage was not lifted by 1.5V. Can anyone help me solve this problem?

Thanks,

 

  • 0
    TI__Mastermind 22825 points

    Hello Ruipeng,

    The circuit from the app note is shown below:

    http://www.ti.com/litv/pdf/sloa162

    The pizzoelectric device looks like a generator in series with a capacitor (no DC path):

    For the output to be sitting at 1.5V (with no input signal), the non-inverting input should be at Vref.

    So, if I understand how you've biased this circuit from your description, the 1.5V Vref voltage does not appear at the non-inverting input (unlike Figure 1 above where the signal is riding on Vref). Are you sure the non-inverting input is sitting at 1.5V average value? I'd expected your output to be railed to the V- rail (based on your description).

    It'd be helpful if you supply an actual schematic.

    Regards,

    Hooman