Operational Amplifiers - Differential Amplifiers


  • Hardware Contributor

    Would anyone around here have much experience with using op amps in a differential amplifier setup specifically or even just using op amps that would be able to throw some light on a issue i have?

    The first question, can we use op amps at their extremities with accuracy? Lets say my op amp is proved with the power supply of GND and 5V. Is it okay that my sensor will produce 0V at the output naturally, would the accuracy of the op amp be compromised by sitting it at 0?

    I have a sensor/current output of very very low, PA range, and i'm amplifying it with an op amp in a non inverting setup, however i need to make a bias/offset/baseline of 1V when the sensor is reading 0. I've been discussing this in various areas of electronical communities and i'm yet to find a solution that i can understand, there are several people giving me information but none of it makes any sense at all to me.

    If you would like some information regarding my current circuitry or any parts, please just ask.


  • Hero Member

    @Samuel235 rather hard to understand what you are planning to do. Looks like you need a "rail to rail" type. Once found read the data sheet and/or put it in simulation to see if it behaves like you want to.


  • Hardware Contributor

    @AWI I do indeed have a rail to rail op amp. I have a TLC272 (Datasheet). I have it working for the sensor currently however the 0ppm on the sensor is reading 0V on the op amp output. However, the datasheet of the sensor would like it to naturally sit >1V for my application. Now, i'm struggling on how to get the op amp to do that.

    My current circuit is;
    alt text


  • Hero Member

    @Samuel235 hard to find a reason why you would do that.. The easiest way is to detach the connection to gnd (little triangles in circuit, not the op amp power) and connect these to a voltage divider between vcc and gnd. The whole circuit is biased by a Vref from the divider.

    Be careful to bias the whole circuit cause the sensor hates voltages to be applied to its connections.


  • Hardware Contributor

    @AWI So if i remove the connection to gnd on the inverting side of the op amp and use a voltage divider instead. Leaving the non inverting side (side the sensor is connected to) connected to gnd?


  • Hero Member

    @Samuel235 Nope, connect them both to the divider...


  • Hardware Contributor

    @AWI, right okay. Is this safe for the sensor as like you said the sensor doesn't like voltage connections. Would it be wise to use any diode to protect that terminal of the sensor thats connected to the divider?


  • Hero Member

    @Samuel235 Kind of hard to say.. as I am not familiar with the rest of your circuit.By biasing the complete circuit as drafted in your previous post there will be no external current/voltage through/ over the sensor. The way I see it is a straightforward current amplifier with a 1000x gain. The comparator has a high impedance so the the sensor s only "load' is a 1kOhm resistor.


  • Hardware Contributor

    Well that seems to have done the trick, looks perfect!

    I will get my full circuitry up here for you to have a quick look at if you don't mind and then i'm one step closer to completing this board. Thank you for your help, time to learn the theory behind this now 😉


  • Hardware Contributor

    Okay, so i have this working perfect with the voltage divider running through another op amp to create a voltage follower. Now, i have removed all of the gnds from the circuit for this sensor and replaced them with the voltage follower, and it works flawlessly. So is there no where in this sensor circuit that needs to be gnd'ed apart from the gnd pin on the op amp, correct? I will post a schematic for this and the rest of my circuitry some time today.


  • Hardware Contributor

    Here is the schematic for the sensor:

    0_1463234709893_SensorSchematic.jpg


  • Hero Member

    @Samuel235 Nice job.. No need for additional grounding. The measuring circuit has the reference as a "virtual ground".


 

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