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OPA928: amplifier for constant current source supply

Ross Parlour
Intellectual 1540 points
Part Number: OPA928
Other Parts Discussed in Thread: LM118-N, OPA191, OPA2191

Tool/software:

Hi,

I am supporting my customer with the design of a constant current source which outputs 500uA to a 20k ohm load. It receives a 16V power supply to the op amp, a 5V reference voltage to the non-inverting input, and a maximum of 12V at the output.

Are you able to recommend a device here? 

I have looked at the LM118-N, however, the high slew rate or frequency is not required, with a greater focus on the integrity of the output current. I have also looked at the OPA928.  This meets the specifications, however it doesn’t mention current sources or supplies in the use application. Also, if possible, over voltage protection for the power supply would be a nice feature.

BR, Ross 

  • +1
    TI__Guru 54286 points

    Hi Ross,

    You have mention that you require a current source that outputs 500µA into a 20kΩ load, using a 16V supply and 5V reference.

    Can you kindly let me know more about the current source requirements:

    - Do you have any strict accuracy requirements? Any strict noise requirements?

    - Is this application only to source 500µA or do you require bidirectional current (Source and sink)

    - You have mentioned, high slew rate or frequency is not required. Is this only for DC current source?

    The OPA928 (ultra-low femtoampere input bias current op-amp) can certainly work, but this amplifier is mostly used in applications that require ultra-high impedance or on transimpedance amplifiers require to measure very small currents in the very low picoampere or even femtoampere level.

    If you need a simple 500µA current source into a 20kΩ load, using a 16V supply and 5V reference, a precision JFET or CMOS op-amp could work depending on the noise requirements.  Below is a quick example with a precision 5V reference, a precision 10k resistor, and the OPA191 (CMOS precision amplifier).

    Depending on the required accuracy or settling requirements, we can choose different amplifiers. 

    Also, if you require an AC current source, there are other topologies such as the Howland Current Source.  Below are some application notes that may be of interest.

    Thank you and Best Regards,

    Luis

    Make a Current Source Or Current Sink

    Analysis of Improved Howland Current Pump Configurations

    TINA file Current Source with Reference Example:

    OPA191_Current_source_example.TSC

    TINA File Improved Howland Current Source Example:

    OPA2191_Howland_Current_source.TSC

  • 0 in reply to Luis Chioye
    TI__Intellectual 1540 points

    Hi Luis,

    Thank you for the detailed response - incredibly helpful.

    The constant current source will be going to a Wheatstone bridge with a maximum total resistance of 10K ohms (excuse my previous typo). I will use a Howland current pump for my constant current source following your guidance, using the app note Analysis of Improved Howland Current Pump Configurations (Rev. A) .

    It will be a DC current, to one varying load, and the circuitry is noise sensitive.

    BR, Ross 

  • +1 in reply to Ross Parlour
    TI__Guru 54286 points

    HI Ross,

    The Improved Howland Current Source with the OPA2191 should work well.

    - You can use a low noise, low drift 5V voltage reference of your choice.  The REF7050 has very low 1/f (low frequency) noise and very low drift (2ppm/C).

    - On the Howland source, you could consider using the RES11A00 1kΩ matched resistor network for resistors R1-R4, and a precision/low drift  Rs 10kΩ resistor.  This will provide excellent accuracy and DC stability.

    https://www.ti.com/product/RES11A

    - At the bridge output, if you are connecting an instrumentation amplifier or fully-differential precision ADC, you may consider adding a differential R-C-R filter to limit broadband noise.

    See suggested circuit below, you can change per your requirements:

    Simulation File:

    OPA2191_Howland_Current_source_b.TSC

    Thank you and Best Regards,

    Luis