Transimpedance amplifier for photodiode for triggering the frequency 50-120 Mhz

Hi Andrew,

For the TIA section of your discussion:

Assuming your photodiode produces 6uA output (full scale, with VR = 5 V, λ = 870 nm, Ee = 1 mW/cm2), and assuming that your ADC needs 2Vpp full scale, this sets the TIA gain to be about 333 kV/Amp. If you use a single TIA gain stage, then that is the value of the feedback resistor you'll need.

From equation (9) of the LMH6629:

If you need a f(-3dB) of at least 120MHz (With RF= 333kohm, Cin = 15pF), then the OPA you use should have a GBWP of 452GHz (which is not a practical / realizable value).

Therefore, I think you need to follow your 1st transimpedance gain stage with more gain stages if you in fact need the 120MHz of bandwidth. This will allow you to drop the value of your RF resistor to boost the bandwidth, with the following gain stages increasing your amplitude to be suitable for interface to the ADC.

I've made a lot of assumptions above. May be you can look over my answer and update / correct my assumptions, and we can continue the conversation.

Regards,

Hooman

Hello Hooman,

thank you very much for your feedback, and sorry for letting the post abandoned for two weeks, but had some family reasons to do so.

yeah, I calculated as well the GBW with respect to my requirements and I received also this data(a bit higher) but thought that I made somewhere mistake, that is why I come to the OPA I wrote before. Thus if I want to achieve the disarable bandwidghth I have to combine several OPA in one amplification/filtering circuit, but here I come more to not open topic for me jet and some material for understanding how it works and how to calculate appropriate OPA for this purpose.

Before I created another board, where I took the circuit board from the 3D glasses and add comparator there and result was the following: here is the board

and here the comparison with the original signal which I took from laser and from the comparator. Blue line is from laser, the yellow from the comparator

from one point of view it couldn't happen due to weak OPA, but the delay I received almost comply with comparator I used there.

Now I decided before I continue with it, I need to test the workability of the system when I take the signal directly from the HFM of original source and connect it to the laser driver for modulation of the signal. Thus I will be able to test the system which I want to create with real environment(glass) and in case it works, I continue with development of the circuit board which I did in first post. For this I need to amplify the signal which I have from the source, because it only 1.6V and Vpp is the same as I received before 530mV. I need for modulation the following 

which OPA would fit to this requirement?

should you have any question, please let me know!

have a good day,

regards,

Andrew

Hello Andrew,

   Hooman is out of office this week and I can look into helping you. I am very unclear as to what it is you are asking and what you need to achieve. Can you please restate your problem. What is HFM? I have no idea what this means:

"For this I need to amplify the signal which I have from the source, because it only 1.6V and Vpp is the same as I received before 530mV."

What is the function of the OPA2348 in the circuit? What 2 signals are you trying to compare?

Thanks,
Samir

Hello Samir,

thank you for intent to help me while your colleague is unavailable!

I think we should start not from begining of the post but from the related to the post intermediate test which I want to perform in order to fully prove that the system has a chance to work. I general I have the following:

problem: TOF camera with IR laser illumination doesn't work through the glass due to rereflection of the IR light to sensor, thus the sensor is blind and sees nothing.

resolution concept: Move the IR light away from sensor at least by 20cm. It will affect the accuracy, but for the purpose I use the TOF camera it is acceptable.

restriction: not allowed to make changes in the TOF camera, means I'm not allowed to open new TOF camera and attach some cables to get the original signal which is synchronized with the sensor.

goal 1: testing the concept by taking the signal directly from the TOF camera.

goal 2: in case goal 1 gives positive result, take the signal from the photodiode attached in front of the IR laser emitter.

Thus let's come to goal 1, and here we have:
 the original laser driver(name closed under NDA), has ability to be managed via SPI and modulate the laser frequency by High Frequency Modulator. Now I'm able to connect to the HFM and to any other pins before and after the laser driver in order to get the necessary signal and apply to the laser driver I'm going to use. The laser driver I'm going to use is IC-HG, The driver requires only TTL signal for modulation of the frequency, and now we coming to the main question in this part, which OPA I could use to amplify the signal from the HFM in order to be able to run the IC-HG.

The last measurement which I did today, was from pins after HFM, the frequency vary from 48 MHz to 120 MHz and the amplitude of the signal 427mV.

should you have any questions, please let me know. I'm ready to perform any other measurements and tests in order to collect more information. as I told already in first post, I don't have enough knowledge in this area that is why some questions could be strange but I would appreciate to read any materials in this field in order to resolve my problem.

regards,

Andrew

Hello Andrew,

 I think Im a little clearer now. To summarize your setup and problem statement for the TIA:

1. Photodiode has 11pF capacitance.

2. Signal BW needed from TIA is  around a maximum of 120MHz..

3. Can you please let me know what the range of currents expected out of the Photodiode is and what is the full-scale maximum you expect from the measurement system? Once I know this we can work on look to see how much amplification we can have in the 1st TIA stage so that we can achieve our BW requirements and also how much gain we would need from the next stages. It would also be useful to know what you have after the gain stages...is it an ADC or do you have something simple like a comparator which would only give a binary output signal?

Please let me know if I have misunderstood anything. In the meantime here is a good article on understanding Transimpedance amplifiers

http://www.ti.com/lit/an/sboa122/sboa122.pdf

-Samir

Hello Samir,

thank you for you feedback, yes, you understood all correct.

The range of current from photodiode I expect 5-6 milliampere because the laser wavelength is close to the maximum sensitivity of this photodiode. After the amplification we should have the full-scale minimum-maximum 0.8-2 V. The ADC is not defined as well, due to not resolved first part of the system. Output Signal has to be TTL, or LVDS.

thank you for the link you have provided, I have already read it on TI and more simplified version on the TI blog.

regards,

Andrew

Hello,

is there someone who could help me with the several stages amp in order to maintain the 120Mhz frequency?

regards,

Andrew

Andrew,  

If you have 5-6mA of outnput current from the photodiode and you want about 2V output after the transimpedance stage, then 400Ohms of transimpedance gain should suffice. If so, the OPA846, OPA847 and OPA657 will be good amplifier choices.

You also mention 0.8V-2V min to max and then want an output TTL signal (0V-5V). Which one do you really need?

Hi Samir,

thank you very much for your feedback!

with respect to datasheet of the laser driver, I need the TTL signal.

regards,

Andrew

Andrew,

 Who needs to produce this TTL signal. From your block diagram in the 1st post, it looks like its the MCU. If so then you will have to post to the MCU forums to find the correct device.

Hello Samir,

in first post I took the same laser driver which uses the camera, but the supplier is not active in communication and finaly I even cannot buy this driver. I searched and have found another driver, which is more easy to supply and it needs the TTL signal for modulation of the frequency of the laser pulsation. Thus I still here :). Should you have any questions, please let me know.

regards,

Andrew