I'm using opa657 see datasheet for my photodiode, but when I connected the photodiode as the 1st schematic in the data sheet, I got a sine wave of Vpp=2Vs at the output of the amplifier even without any input on the photodiode.
and when I put an incident light the sine wave will disapear and no signal appears to the output of the amplifier.
Where does this sine wave come from and how fix this problem?
Best Answer
You can't run the OPA657 at unity gain - it oscillates - even when configured as a TIA you can run into trouble with it. Here's the open loop gain and phase (red): -
The op-amp's phase margin is such that at a closed loop gain of about 6 to 8dB the negative feedback will be positive feedback i.e. open loop phase is about -180 degrees at about 850MHz and the gain hasn't gone below unity.
You could bypass the photodiode with a capacitor - see figure 3 in the data sheet - it shows a PD with quite a high self capacitance - maybe you should try adding capacitance across the PD?
This ensures that at high frequencies there is gain (because negative feedback has reduced) even though that gain may not be apparent either to the untrained eye or the photodiode. If you had a signal connected to the non-inverting input that gain would be very apparent and of course there is a signal at that pin - it's the equivalent noise of the op-amp's input - this is why, on transimpedance amplifiers it's called noise gain - you add capacitance across the photodiode to add stability i.e. a reduction in feedback and you find self-oscillation has been traded for a much noisier signal. TIAs are not trivial to design.
With a feedback resistor of 200k and a capacitor of (say) 47pF, the fed back signal will have reduced by 3dB at : -
F = \$\dfrac{1}{2\pi R C}\$ = 16.93kHz
By 100kHz the fed back signal is way down and no longer posing an oscillation threat.
Also, you need to ensure the chip has adequate decoupling capacitors on power lines to the device - they need to be close up to the chip.