If I have 25 photodiode and one ADC,how to design transimpedance amp circuit

Hi Future,

a transimpedance amplifier need not to be expensive at all. In many cases a simple and cheap OPAmp is sufficient.

Can you tell more about your application? Bandwidth, photodiode current, type of photodiode, etc.?

Kai

Hi Kai :

The photodiode is OSRAM SFH_2704.

The issue is the sensor module size is very small.

The sensor array is about 12mm x 12mm.

The total sensor module is about 15mmx15mm.

There is no much space for the amp circuit.

Or I only can use a long flat cable to make sensor current to flow through a long 100mm cable.

And the amp circuit on the other board.

I do not know how long can the cable be ?

And there will be some other transimpedance circuit problem due to the long cable ?

Best regards

Hi Future,

your photodiode has about 14pF detector capacitance. A flat ribbon cable again about 10pF at 10cm, which adds to the detector capacitance. This should not be all too much "total detector capacitance" for a transimpedance amplifier design. Connect every second wire of the flat ribbon cable to signal ground at both ends. This will provide a useful shielding between the individual photodiode signals.

Can you tell something about your signals? What is the bandwidth of the signals? What is the photodiode current range?

Kai

Hi Kai :

If I connect 25 photodiode parallel, the total capacitance is 14*25 = 350pf.

What happened will these parallel photodiodes ?

Will the transimpedance amplifier unstable?

I have some space issue,there is not enough space for the PCB.

Or I need to connect a 30pin flat cable to the photodiode sensor circuit.

And I need a large space on the backend PCB.

best regards

Hi Future,

yes, the paralleling of 25 such photodiodes should not present a problem. You should be able to compensate the influence of total detector capacitance on the phase margin by a suited phase lead capacitance in the feedback loop.

Can you show a schematic?

Kai

Hi Future,

As Kai pointed out, what is your application or sensor's response time? Can you share the schematic with us? What is your sensor's low and max output current?

OSRAM SFH_2704 is green light photo sensor, sensor area is quite large. I assumed that the entire sensor area are sensed, not partial sensing area per your application. In other words, the incoming light source is collimated. 

If you do not have to measure 25 sensors simultaneously, you may use analog switch to mux through the sensor bank to transimpedance amplifier, but I need to know more about your application before I can recommend the best options. 

Best,

Raymond

Hi Future,

Since you have not provided detailed product design requirements, I will provide you some ideas. This is based on the information you provided so far. The following suggestion is one of the design options. 

1. Please take a look at OPA378/OPA2378. For transimpedance amplifier, youwill need to select zero-drift series (get rid of 1/f noise), low noise types. The OPA378 has GBW= 900kHz, $1.24/100 or $0.955/1k, low cost and small sizes. you may be able to get 25 of these for your application. The min and max sensor's current is determined by the incoming light intensity and sensor characteristics. The sensor has a NB green filter in front of Si diode. Photopic sensor is centered at 550nm, and your sensor's light wavelength is at 535nm. Also, you need to tell us if the incoming light source is chopped or continuous. In other words, there are electronic or mechanical ways to reset the transimpedance amplifier, depending on your application.  

2. You can feed 25 sensor's output signals through multiplexers, 2/4/8/16 mux channel types to a single A/D converter, if there is enough gain.  TI has many selections of low resistance analog mux switches. We can help you to select.

Alternatively, you may use low contact resistance analog switch to mux through the 25 different light sensors, but this will take time to complete and you are not looking at the incoming light signals at the same time. Since this is array configuration, it may not be feasible (but it can be done, if your event detection window is very long.) TI has have ACF2101,  IVC102 and other transimpedance amplifier ICs, but these are not low cost approaches.

We also have programmable gain amplifiers, which can do switching and improve gains from input signals, e.g. PGA116.  Anyway, there are many options to handle your design requirements. Each approach has pros and cons for a given application. That is why we are asking for application and design requirements to provide you with better design options. 

Best,

Raymond