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Opamp selection

Hi,

I am interested in an opamp with the following specs and I was wondering if anyone can help me with finding it here.

  • Dual supply operation (+/-3V)
  • Ultra low supply current (less than 100uA)
  • Low offset voltage
  • 4 circuits
  • Minimum slew rate: 2V/us
  • Comes in 16-LFCSP packaging

I am interested in using this opamp to build a current source which will supply a current up to +/-100uA across a maximum load resistance of 50kohm.

Thank you very much.

  • Hi sahard

    Checking on Analog Devices’ portfolio you could look at ADA4096-4. ADA4096-4 meets most of your requirement, it is capable of +/-3V dual supply operation with a supply current of 65uA per amplifier. Its offset voltage is also relatively small with a typical value of 35uV. Your requirement of 4 circuits, which I think you refer to as 4 amplifiers in package, is also met by ADA4096-4 along with the package requirement of 16-LFCSP. It does not reach your requirement, though, of 2V/us Slew Rate, it can only provide up to 0.25 V/us Slew Rate.

    Often there is a tradeoff between slew rate and current. For faster devices, you would expect the part to draw more current. Why do you require a high slew rate? What will the current source be driving? If you really need a high slew rate in your application, you could also check AD8643 which reaches your Slew Rate requirement of 2V/us, its offset voltage is 50uV but the supply current is much higher at 195uA per amplifier. Could you provide more details about your application? With more information, I could provide a better part for you.


    Thanks


    Chard

  • It is a variable current source. For example, if the load is 50kohm, the output current I produce will not exceed 60ua.

  • Hi chard,

    Thank you for finding these opamps for me.

    The reason I would like a fast opamp is because I am generating current pulses used for neuro-stimulation. I would like the pulses to have a risetime of <10us. If I draw a maximum of 100uA across a 30kohm resistor, then the opamp should have at least a slew rate of 0.3V/us to meet specs. Looking at the options you provided in your first paragraph, 0.25V/us is a good option if that is the best we can do.

    I also, would like a low current consumption opamp because the device is battery operated, so it must have a low supply current of <100uA.

    Best,

    Sara

  • Sara,

      100 uA times 50k is 5V.  How can you do this on +/-3V??

    Harry

  • Hello,

    I purchased the ADA4096-4  and tested it. Since the slew rate listed on the datasheet is 0.25v/us, I was expecting that at a generated output pulse of 2.5V, Ithe rise time would be 10us. This is not the case. Rather, I got a rise time of a little over 75us. Can anyone explain this to me?

    Do you suggest any other opamps that truly provide 10us rise time for an output pulse that goes from 0 to 2.5V?

    Thank you,

    Sara

  • I am using the opamp as a current source. The output load is a 25kohm resistor and I generate 100uA of current pulses. I measure across the 25kohm load and expect to read pulses of 2.5V. The rise time of the pulses going from 0V to 2.5V is >50us.

  • Output signal.  I am measuring across the output load.

  • How are you measuring?

    Gain?

    rise time of signal source?

    Slew rate is a LARGE signal parameter.  You have to overdrive so that the input pair is

    completely cocked to one side.  See:

    http://www.ti.com/lit/an/snoa737/snoa737.pdf

    Harry

  • Is the 0 to 2.5V an input signal or output signal?

    Harry

  • Picture a block with a gain of +1 and the block has the capability of 100V/us slew rate.

    If I put a very slow waveform in, say 1V/second, I will never see 100V/us at the output.

    You need to put a step function INTO your circuit.

    Have you simulated the circuit?

    Harry