In order to reduce noise, i use a JFET BF862 before the amplifier ltc6268-10,but i find a large offset current in oscilloscope. I want to know how solve it.

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    •  Analog Employees 
    on Sep 14, 2018 8:44 PM over 2 years ago

    Hi Bupt-xu,

    Can you tell me what is your applications and what are you trying to do?

    Are you expecting that the current between the two photodiodes should be balance? That node will have specific voltage magnitude and depending on your bias voltage, it may results to offset current which flow to your transimpedance amplifier.

    Also, how can you tell that there is a large offset current on your circuit and how do you measure it?

    We've seen this circuit that adds Jfet in front of the amplifier to reduce the leakage current. Can you help me understand how this configuration help to reduce the noise on your circuit?

    Best regards,


  • Hi Emman.A,
     I want to design a balanced homodyne detector, so the current between the two photodiodes should be balance.I use Agilent oscilloscope to measure the output of the signal,  and i find a lagre bias voltage about 1V.
    As for add jfet in front of the amplifier to reduce the noise on circuit, i learn it from a paper called "An ultra-sensitive balanced detector with low noise for continuous-variable quantum key distribution", now i also can't understand that why 
    jfet help to reduce the noise. In order to verify it,  i design a new PCB with JFET, we can find that the niose reduce about 10dB from the spectrum analyzer.
    Best regards.
  • I am not sure about the JFET and TIA, but you need to pay much attention about the consistency of the 2 photodiodes when designing balanced detector. the balanced detector is used a lot in interferometer receivers that the 2 PDs will cancel out the DC part(same for the 2 PDs) and only the AC part goes into the TIA. If the responsivity/dark current of the 2 PDs is not the same, there will be offset at the output of the TIA.