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OPA 659 noise reduction

Other Parts Discussed in Thread: OPA847, OPA656

Dear Sir / madam,

I want to design a high speed low noise amplifier for measuring very low current (nano-amp range). I want to amplify it 1000 times (to get output in micro-amp range). I came to know about opa 659 IC with very low input bias current (10pA) and quite high slew rate 2550 V/ns, but i want to know about the noise performance of this IC. Is it possible to design a 3-stage amplifier having gain of 10 at each stage with very low noise?

Earlier, I  used opa847 with input noise of 0.8 nV / rt Hz and i got significant noise while using it on pcb board. Now since opa 659 has got 8 nV / rt Hz input noise, will it be fine to replace opa 847 with it? Can you please suggest me which IC will be better for this application?

  • Hello Nishant,

      If the input current is in the nano-amp range you will need quite a bit of gain in each stage. This indicates that your dominant noise contributor will be the current noise of the amplifier * the feedback resistance. The JFET input amplifiers like the OPA656/657/659 will all be good fits for this end-application.

    Please see the attached application note on how to stabilize and analyze the noise in Transimpedance Amplifier applications. Your amplifier bandwidth requirements are essential in determining the appropriate amplifier for the application. The higher the bandwidth requirements, the less gain you can put in each gain stage. Also, please use TINA Spice (available for download on the TI website) to simulate your circuit and ensure you have sufficient stability, badnwidth and noise margin.

    http://www.ti.com/lit/an/sboa122/sboa122.pdf

    Samir

  • Thanks for your suggestions.

    I want to use a switch to disconnect certain part of my circuit. Can you suggest me any IC for this purpose, so that i can control my circuit electronically. That IC should not add large delay or noise in the output.

    Regards,

    Nishant

  • Hello Nishant,

      There are a awide variety of relays and switches available that need to be chosen based on the particular application:

    Panasonic & Omron make both Photomos Relays and more traditional Signal Relays, that will have some effect on loading.

    Also, there are Reed Relays and RF relays available that are more for high speed applications.

    TI does not manufacture these types of switches/relays so I cant really direct you to someone with this particular expertise.

    Samir