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LME49710 high impedance input design

Mike Borish
Prodigy 20 points
Other Parts Discussed in Thread: OPA1641, LME49721, LME49726

I'm in the process of designing a circuit employing the LME49710 that amplifies a source similar to an electret microphone. 

My circuit has a gain slightly larger than unity and employs resistors under 2K in the feedback and input sections of the inverting input to achieve higher than unity gain.  The circuit is AC coupled and has a set input impedance of 10M via a resistor from the + input to ground.

Unfortunately, I am experiencing more damping than I initially calculated from the input impedance loading the source.   I do not understand how this amplifier is internally biased and was wondering how high I can set the impedance of the + input before I start to experience unwanted side effects like distortion, resistor noise,  etc.  Also, I understand that I will experience large offset if I do not set up the amplifier in a classic differential configuration.  Should I set the amplifier in a classic differential configuration with very high resistors?  At what point will Johnson noise dominate current noise?

Is there a preferred way to null the offset at the output when configuring the amplifier in the way that I have presented?  My gut tells me that I should just tie a rail to the negative input but that almost seems too simple and I really don't understand how this amplifier looks internally. 

  • 0
    TI__Intellectual 1240 points

    Hello

    Unfortunately LME49710 is using Bipolar process, not CMOS process. So the input bias current is not so Low even this device is using bias cancelling technology. To reduce the offset voltage, You need to use very small input bias current op-amp. I'll suggest to use CMOS or Bi-CMOS process op-amp. LME49721 or LME49726 are CMOS op-amp with 5V supply. The input bias current is very small like fAmp order. Or JFET input op-amp like OPA1641 if you need wider output dynamic range.

    Best regards

    T NAOKAWA