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Selecting the best op-amp package

Jason Orchard
Prodigy 30 points

Other Parts Discussed in Thread: OPA378

Hi all,

I am currently working on a university project that involves designing a system that requires the use of amplification.

A series of measurements comes from a sensor which range between zero Volts and 12mV. These will then be sampled

by a ADC with maximum voltage of 3V. I need to use an op-amp (or a series of them) to map the voltages. I cannot seem to find

anything that is sensitive enough and provides enough Voltage gain (250 times). Any help or suggestions would be great!

Thanks,

Jason

  • 0
    Guru 21975 points

    Jason,

    Can you provide more information on the nature of the sensor and signal?  What type of sensor are you using? What is its impedance? What frequency response is required? What is the sampling rate of the ADC?

    Most any op amp is capable of providing a gain of 250. Can you describe what do you mean by "not sensitive enough?"

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 30 points

    Hi Bruce,

    Sorry about being too light on the details.

    The sensor is a 350 Ohm full-bridge strain transducer and has output ranging from -12mV (min) up to 12mV (max). I would like to use an op-amp circuit to increase the voltage to a maximum of +3V (for input = 12mV) and a minimum of -3V (for input =-12mV) (and voltages between mapped linearly). This range can then be sampled by a micro controller. Ideally the measurement could be recorded at a user set frequency anywhere between 0 Hz and 1000Hz max. I assume this is very difficult to do, but really the Higher frequency measurements are of higher importance.

    Shall we say a lack of knowledge (or general intelligence) made me comment on the lack of sensitivity, i take it back! I have since got some simulations working in TINA using the LM301A op-amp.

    So my issue at this stage is how to attain both positive and negative gain with only a single (positive) supply. Could you give me some suggestions on what type of configuration to use to achieve this?

    Thank you very much for taking the time to read this.

    Jason.

  • 0 in reply to Jason Orchard
    Guru 21975 points

    Jason,

    The simplest possible amplifier is a single op amp. See figure 31 in the OPA378 data sheet. Make R1 (both) equal to (RB/2)*G,  G is the desired gain and RB is the bridge resistance, 350 ohms. Vref is a voltage source equal to the desired output voltage with zero bridge output.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 30 points

    Thanks very much for your reply Bruce the solution is working well for me.

    Just out of interest; for what reasons did you suggest the OA378?

    Cheers,

    Jason

  • 0 in reply to Jason Orchard
    Guru 21975 points

    Jason,

    OPA378 has low offset voltage and low offset voltage temperature drift (often important in bridge applications) and has sufficient bandwidth for your needs.

    Regards, Bruce.