OPT3101: OPT3101 photo diode input circuit description and spice model

Mark,

 Your circuit will not work. The OPT3101 has an analog front end designed in. This circuit measures the current in nano-amps across the photodiode. There is a parallel capacitor that must match the capacitance of the photodiode with a 1Vdc bias on both. Typically around 6pf. Next the AFE measures the current of the photodiode for ambient light and uses the parallel capacitance as a reference. Then cancels out ambient light. Then the AFE will look at the electrical noise in the photodiode and the parallel capacitor and cancel that out. 

The whole circuit operates as an I/Q data demodulation circuit and determines the distance by calculating the phase angle of the quadrant that it is operating in. (Determined by the distance and amplitude of the returning light or Q data) If you add any circuit to the photodiode of the emitter, you will create a delayed effect that will void the ability to measure the distance. This part is not a DTOF (Direct Time of Flight) part it is an ITOF (Indirect Time of Flight). It is much much much more advanced than a DTOF and you do not have the ability to modify any part of the circuit and have it operate correctly. It is time sensitive to within a few nano-seconds. 

Look up I/Q demodulation for a better understanding of how this part measures distance.  

Also note, the OPT3101 operates using a 10Mhz modulated output. also not adjustable. (except for a 10Mhz to 12Mhz shift, then back used for de-aliasing the modulated signal for extended range.)

Hello Gordon,

I appreciate your quick response and the detailed explanation. We do understand that the analog input to the OPT3101 is expecting a low capacitance photodiode. I should have mentioned that our system requires the use of a green Laser source and its subsequent modifications. We are using the OPT3101 TXO output to drive a fast Current Mirror circuit which keeps the TX0 within about 1v to about 3 v, depending on system conditions (temperature, part to part variation, etc.) and how it is operating. This portion seems to behave well. But for detection, we are using a SiPM device in order to gain adequate response to the green wavelength. The SiPM is biased in the 24 to 32-volt range (again, depending on system conditions).

Because the photodiode is not sensitive to green wavelength, we are mimicking a photodiode using a transimpedance amplifier (TIA) with a small value capacitance to couple the output into the OPT3101. To mimic the photodiode ambient current condition, we are using a low capacitance current mirror (sink), understanding the OPT3101 expects to see a photodiode biased to 1 volt, that has any ambient, as well as 10MHz “signal” impinged onto the photodiode.

In our implementation as described above there are two paths connected to the OPT3101 analog input. The first is an “in range” 10-12MHz AC signal coming from the TIA coupled from the TIA into the OPT3101 with a small value capacitor. Second is a current mirror (filtered to allow ambient but no “signal” information to be sent to the OPT3101 to mimic the photodiode’s current. 

The inputs to the OPT3101 are carefully balanced.

Gordon said - The whole circuit operates as an I/Q data demodulation circuit and determines the distance by calculating the phase angle of the quadrant that it is operating in. (Determined by the distance and amplitude of the returning light or Q data) If you add any circuit to the photodiode of the emitter, you will create a delayed effect that will void the ability to measure the distance.

Mark B - We are adding a laser and laser drive circuitry in addition to the SiPM and Transimpedance amplifier.

Gordon said - This part is not a DTOF (Direct Time of Flight) part it is an ITOF (Indirect Time of Flight). It is much much much more advanced than a DTOF and you do not have the ability to modify any part of the circuit and have it operate correctly. It is time sensitive to within a few nano-seconds. 

Mark B - Does this imply that an additional fixed time delay beyond a few nanoseconds cannot be accommodated? Our understanding is that the offset registers allow the OPT to handle additional time delay that we are imposing.

Thanks - Mark B

Mark,

  First point.. TI does not support or recommend using a part outside of the normal use as specified in the datasheet. 

With that, a current mirror on the emitter should work as it should not add much additional time. The register offset may compensate for a small delay, but you will have to determine if the timing and accuracy is correct.

I am most concerned about the photodiode. The INM and IMP must have the same electrical noise or it can not cancel out the noise. The ambient light is also considered noise and will be referenced against the parallel capacitance to cancel out ambient. Now that noise and ambient are removed the AFE will measure the modulated 10Mhz 50% DC to determine that the returning signal is in fact its own. This is where is gets tricky. You can have more than one OPT3101 working in an area and it can determine that its own signal is returning. This is part of the IQ demodulation technique. The I the Q, and the ambient are all used to determine the distance. The ambient helps to determine which quadrant it will calculate the distance in up to 15 meter max. Then it's up to the de-aliasing to determine which multiple of the quadrant will be used. 

There are several photodiodes that can detect laser green. It will need a filter to allow only the green laser through. Most wavelength specific photodiodes have the filter on the photodiode. It will also need a lens to bring the reflected signal to a fine point directed at the photodiode. Using a laser diode, focused photodiode and de-aliasing, with the right setup you could get up to 150 meters. You will never get more than that for many reasons, mostly physics of the internal workings of the OPT3101. 

Thanks Gordon

We will evaluate your response and get back to you.

Hello Gordon, can I obtain spice models for the OPT3101 AFE (INP and INM) specifically the ambient cancellation and ToF circuits?

Hi Mark,

To my knowledge we do not have a spice model for the OPT3101. However, the OPT3101 estimator tool can help with selecting optical emitters and photodiodes and estimating performance, range, and accuracy for these components:

http://www.ti.com/tool/OPT3101ESTIMATOR-SW

Best regards,
Nicole