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OPA928EVM:Rf switching

?? ??
Prodigy 50 points
Part Number: OPA928EVM
Other Parts Discussed in Thread: OPA928, OPA3S328

Tool/software:

I want to use an OPA928EVM and I want to change the Rf resistor depending on the situation. There are only two jumpers and no switching switch, and one of them has a capacitor built into the circuit, which I believe interferes with DC measurements. I am hoping to hear about measures regarding feedback switching etc. If the solution is to modify the circuit of the OPA928EVM itself, then that is possible with my technology.

  • 0
    TI__Guru 55026 points

    HI,

    The OPA928 is a femtoampere input bias current amplifier working from a  4.5 to 36V supplies.

    The OPA928EVM does not include switches, since very low current measurements would be sensitive to any leakage current of the switches. The OPA928EVM uses 1GΩ feedback resistor. Any solid-state analog switches or multiplexers used to switch the RF will typically exhibit leakage currents in the 10s to 100s of picoampere (or more) depending on the switch type, and this approach will not work if you are attempting to measure currents at the femtoampere or low picoampere range.

    We have other standard CMOS programmable gain transimpedance solutions; but of course, the input bias current of these is in the 10s of picoamperes. For example, the OPA3S328 is a precision, low-voltage, CMOS operational amplifier (op amp) with integrated switches that are optimized for flexible transimpedance applications.  The device supports a 2.2V to 5.5V supply and the typical input bias current is ±10pA max at the 0 to 85C temperature range.

    https://www.ti.com/product/OPA3S328

    If you require ultra-low current measurements while switching the RF feedback resistors, there may be ultra-low leakage reed relays with insulation resistances in the 10^14 Ω.  These are used in electrometer solutions and are produced by reed relay manufacturers. 

    https://standexelectronics.com/wp-content/uploads/Application_Alley_Electrometers1.pdf

    Also, below is a in depth application note describing the challenges and all the design considerations to perform femtoampere current measurements. The application note shows examples with the OPA928. 

     https://www.ti.com/lit/an/sboa597/sboa597.pdf

    Thank you and Regards,

    Luis

  • 0 in reply to Luis Chioye
    Prodigy 50 points

    We have found that solid state analog switches or multiplexers are difficult to instant y-tei when measuring small currents. I have two questions.
    What if I want to change the value of Rf?
    What type of switch should I use if I want to make a low current measurement?
    Thank you in advance.

  • +1 in reply to ?? ??
    TI__Guru 55026 points

    What range of current measurement is required?

    As we have discussed, there are challenges measuring ultra-low currents, and switching the RF resistors without adding errors into the measurements, and ultra-low leakage current switches may be required...  I am only aware of the reed relays mentioned on the post above that offer a very high insulation resistance.

    Regards,

    Luis