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OPAMP selection - LOW consumption, 2 gains level

Francois Coulloudon
Prodigy 110 points

Other Parts Discussed in Thread: OPA333, TLV369, OPA369, OPA2333, TLV379

Hi all!

I'm currently working on an optimized low current measurement circuit.

The inspiration is coming from the ucurrent sensor found here: https://www.eevblog.com/projects/ucurrent/

However, our problem is very specific:

1) Supply voltage is 2.5V

2) I need one OPAMP to create a virtual ground @ 1.25V (2.5V / 2)

3) I need one OPAMP to amplify the voltage generate bu the current passing by the low value resistance. (mucurrent has two but if we could do with one it's better)

In our case, we would like to have an output of 0 to 2.5V output by having a current of -500ma to 500ma into a 0.1ohm resistance. This means an amplification of 25. For this, a simple non-inverting circuit is doing the job.

4) It's going to be AC current with max 60 hertz frequency where we will only sample few cycles to get maximum and lower value on a given cycle.

and we have one constraint:

5) Consumption must be as low as possible

To sum it up, I need two OPAMP for two different features: virtual ground @ 1.25V (gain of 1) and 25x (gain of 25) amplification of voltage across resistance due to current.

Is there one type of OPAMP who could do both with very low consumption and 2.5V supply? Or do I need two references?

I'm trying to read specs and I'm getting a bit lost on which one to choose. Could someone guide me just to make the right choice?

Thanks for you help!

  • 0
    TI__Mastermind 18900 points

    Hello Francois,

    The op amp you are looking for is the OPA2333—which is the OPA333 op amp in a dual package. This rail to rail op amp with low offset voltage, low supply voltage, and the BW you require will work perfectly.

    We have a TI Precision Design (TIPD175) that has different devices but presents a very similar application to what you are trying to accomplish. It contains a reference circuit—which provides your “virtual ground”—and a difference amplifier configuration—which will sense the current. The document also contains theory of operation, error analysis, simulation, PCB Design, and verification and measured performance. It should greatly help you in your design process.

    Hope this helps,

    -Cole

  • 0 in reply to Cole Macias
    Prodigy 110 points
    Exactly what I was looking for! Thank you. I had the same reference in mind. Altough some are even consuming less ( below 0.01 mA)... The zero drift feature for us is good but why would you choose this reference (OPA333) over very low power OPAMP?
  • 0 in reply to Francois Coulloudon
    TI__Mastermind 18900 points

    Great question, I see 3 or 4 important factors in your design:

    • Low Power (low bias current, low supply current per amplifier)
    • Accuracy (low offset voltage, low drift, low noise)
    • Price
    • Layout (in the form of board space)

    While I see you've mentioned power as your most important factor, I would like to say that the trade off with accuracy is very tight. You're right to say that the OPA2333 is not the lowest power option we have--I would recommend you take a look at TLV369, TLV379, or OPA369 for better performance in that area.

    Since these devices have traded for accuracy, they will give you far worse performance when acting as a difference amplifier. They do not satisfy your BW requirements, they produce more noise, drift more over temperature, and have more offset voltage. However, these devices may be better suited in the role in creating your reference voltage (virtual ground).

    When you compare the devices, the OPA2333 is competetive in price and better for your layout. 2 amplifiers takes up more space than a dual package and more space is lost to the extra decoupling capacitor that is needed for your supply voltages. Do a comparison and see if doing well in 3 categories (accuracy, price, and layout) beats the importance of your first category (low power).

    -Cole

  • 0 in reply to Cole Macias
    Prodigy 110 points

    Thanks a lot for your time! This is really useful. 
    This is the kind of support that makes us choose your products over the rest.

    You're right in design factors, however Low Power is order of magnitude more important than space.
    But I do need accuracy: low offset voltage, low drift, low noise is important. Especially that the device will be exposed to very high and very low temperatures.

    I'm thinking about going for the OPA369 for reference voltage and OPA333 for signal amplification. For OPA369, 0.4uV/C makes a 0,00004V difference over a 100 celcius range which is more than enough. Only think is that temperature rating is a bit lower (85C max). I will maybe need to compensate for Offset voltage (0.75mV).

    12 bit analog acquisition on 3.6v range means ~0.9mV resolution.

    I will have the best of both world. What do you think?

  • 0 in reply to Francois Coulloudon
    TI__Mastermind 18900 points

    You're welcome; I'm glad you found this helpful.

    Based on your analysis, I would say that the OPA369 is a great solution. Feel free to make another post here or in the Precision Amplifiers forum if you have any other problems.

    Best,

    -Cole