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LOG112: Self-oscillation at point close to negative saturation.

Vyacheslav Krutov
Prodigy 50 points
Other Parts Discussed in Thread: OPA4277, OPA4251, LOG112

 

Notes on scheme: OPA4251 soldered instead of OPA4277. Q1 is not soldered( MOSFET and JFET selection).

 

 

Then photodiode is close to full dark log amplifier  starts self-oscillating. It could be observed on pin 5 (LOGOUT) and in ADC input pin.

I tried change voltage offset on photodiode cathode from equal  VCM (1.25V)  to 1.9V to ensure it reverse bias, but this not gave any positive result.

Increasing/decreasing  compensation capacitor  gives no positive effect too.

I checked M1 (reference current source MOSFET) bias voltage and found that it's changed in accordance with output oscillation. This means that collector current              Q2(inside LOG112) is changed and current source feedback loop compensates it.

     Reference current to LOG112 is 200uA. Then VLOGOUT (pin 5) goes to  -2.30 V (photodiode is close to full dark), oscillation appears. After entering full dark, oscillation is disappeared and VLOGOUT becomes - 3.89V.

    Diagram in LOG112 datasheet  (page 11) shows that negative log out saturation should be between -3 .. -3.5V for  Iref =200uA .

Does anyone has any idea how to eliminate this oscillations ?

 

  • 0
    TI__Expert 7185 points

    A log amplifier is essentially a transimpedance amplifier with a feedback equivalent to 1/gm = 1/Ic/Vt.  With small values of photodiode current the feedback approaches open loop which might be the cause of the oscillations.  If this is the case you can eliminate these oscillations by injecting a small amount of DC current which can flow through the LOG112 feedback diode even under dark conditions.