OPA855: AC coupled series photodiodes Transimpedance Amplifier

Hello Parveen,

   Here are some collateral we have on designing Transimpedance Amplifier (TIAs): Simple TIA Video, Choosing a TIA Video, Blog Series, Blog Series, Advanced TIA Topic.

    To simulate a circuit with the specifications you have listed you can either use Tina-TI or Pspice for TI. We provide models of our amplifiers in both forms which is located in the product folder of the part number under the design and development tab. For example, here you can find the OPA855 Tina-TI and Pspice Model.

    What is the current output of both photodiodes? We can help create a Tina-TI simulation example for transient/AC/stability of your design specs. 

Thank you,

Sima

Hi Parveen,

you could do it this way:

parveen_opa855_4.TSC

parveen_opa855_3.TSC

Kai

Morning Parveen, 

here is a Butterworth solution with a 750Mhz flat BW - notes

1. That 0.17pF feedback is about what a surface mount R is parasitic wise, might not need any extrinsic feedback comp C

2. That 100Mohm on the input is for sim only, 

3. The AC coupling in series with the inverting input is all you need, output AC coupling likely not - be careful of self resonance in those C, might want an X2Ycap for the 100pF one, 

And file, 

OPA855 low gain low Cs Zt.TSC

Hi Kai

Thank you for detailed explanation. However few things which are not clear:

1. R2=900 and C2=10u connection is not clear and how are these values are calculated? I think R2 is the equivalent resistance of series connected resistors R1 and R2 of photodiodes.
2. Why are you not using the coupling capacitor at the input of OPA 855? Is there no effect of coupling capacitor on stability?

Hi Michael

Thank you for your prompt reply. Few things that I wanted to know?

I read your article Transimpedance design flow using high speed opamps.pptx. In this article you have suggested to experimentally verify the frequency response with a Network Analyzer shown in figure below. Can you please explain? In the above mentioned circuit (OPA855 low gain low Cs Zt.TSC), why you have not considered the capacitance of second photodiode?

Thank you

Hi Sima

Thank you very much for providing the material.

Well the 3rd part in that original article series goes into that technique in some detail, here is a pdf of that scanned from the original, 

8662.Here's an easy way to test Zt article.pdf

I was not clear on your total source C using only a single 0.65pF element - doubling that to 1.3pF will require a 0.2pF feedback C and give about 620MHz BW in theory. 

Hi Parveen,

the 900R and 10µF is from a similar thread. The exact values don't have any influence on the simulation results. The same is valid for the omitted coupling capacitor. But feel free to use my simulation file and do your own simualtions

Kai

Hi Kai

Thank you for your help. I just wanted to know what will be the effect of the coupling capacitor on my circuit? If I remove or place a coupling capacitor will the stability and  the  bandwidth of the circuit remain the same?

Parveen

As I said, no influence. Fully stable

Kai

Hi Team,

I require an output of 10Khz lower cut off and 600MHz at upper cutoff in the above-mentioned circuit. I placed a RC high pass filter at the output of transimpedance amplifier to obtain a lower cutoff frequency of 10Khz. But when I am looking at the datasheet everywhere it uses a 200 ohms load. Can you please suggest whether is it ok to connect a RC stage after the TIA stage? The other alternative is to connect an active filter but I am unable to find the OPAMP which is used in active filters to obtain such a high bandwidth. After that I require an inverting stage of gain 7. Can you please suggest  if this alternative is any good?

OPA855 .TSC

Hello Parveen,

   This should not have an affect on the TIA stage. There is no need for another amplifier stage between the TIA and the Gain of 7 stage. I used Kai's stability Tina-TI file to show the circuit is still stable that I have attached below. 

5047.parveen_opa855_3.TSC

Thank you,

Sima

Hi Sima

Thank you for your reply. I was wondering whether is it ok to connect a current feedback amplifier (CFA) after the TIA stage. I am not sure how to connect the CFA after the TIA stage.  

Hi Parveen,

where do you see issues?

Why not showing a schematic?

Kai

Hi Kai 
Please find the attached schematic file. I want to replace OPA818 with THS 3491 to obtain high swing  and 500Mhz bandwidth, that is why I want to connect the output of TIA with CFA THS3491. But I am not sure will it be ok to connect THS3491 after TIA stage. 

OPA855_OPA818.TSC

Hi Parveen,

hhm, I ask myself why so many people are using inverting amplifiers instead of non-inverting ones. R10 is way too small and will short-circuit the output of OPA855. In a non-inverting amplifier, on the other hand, R10=75R5 as gain determining resistor going to signal ground wouldn't play any role.

Kai

Hi Kai

You mean that if I connect OPA818 in noninverting configuration then will I get a high swing of ±4V? I require a swing  of  ±4V with a large signal bandwidth of 400MHZ.

What I mean is that this inverting circuit will not work.

Kai

Hi Kai

Thank you so much for this invaluable suggestion. The other issue that I am facing is that I require a high swing of ±4V and large signal bandwidth of 400MHz. Can you please help me in this regard also? I shall be very thankful. Should I connect the THS3491 after high pass filter instead of OPA 818?

Hi Parveen,

a 4Vp sine of 400MHz means a slew rate of

2 x pi x 400MHz x 4V = 10053 V/µs

Kai

Hi Kai
Sorry, I made a mistake. I  actually need sine wave of 2v peak to peak. It means that the slew rate is 5026V/uS. Can we achieve this requirement by connecting THS3491 (CFA)  after TIA stage? Because I have checked in data sheet that the slew rate of 8000V/us. 

2V peak or 2V peak to peak?

Hi Kai
2V peak

parveen_thd3491.TSC

Kai

Hi Kai

Thank you for your reply. Can you please check the way I connected the THS3491 after TIA. If it is incorrect, please guide me as to the correct process.

OPA855_THS3491.TSC

Have you noticed that I have chosen a gain of 2 and not a gain of 5?

Hai Kai

I have noticed it. Is there specific reason to choose a gain of 2.

Hai Kai

You are choosing gain of 2 because output of OPA855 gives maximum output of ±2V. And if this voltage is multiplied by gain of 2 , we will get a swing of ±4V.Am I correct?

Hi Parveen,

what is the maximum output signal of U1?

Kai

Hi Kai

I need a total gain of 10KV/A. Actually I don't know the maximum output signal of U1. 

Hello Parveen,

   What is the output current range of the photodiode?

Thank you,
Sima

Hi Team

When we apply a sinusoidal signal at THD3491DDA with 2V peak to peak at a frequency of 400MHz, then the output peak to peak amplitude is ±4V.  How can I verify the same condition when simulating a square wave?

Hi Kai,

The maximum voltage that I have obtained at the output of U1 is 1V peak to peak.