Using a photodiode in near darkness

[Artlav]

Another option for you, if you need higher gain (without requiring the huge feedback resistor whichi adds noise) and faster response is not necessary is to use a phototransistor instead of the photodiode. Not sure what your application is, but if you're dealing with low light, and speed is not an issue, they are better suited.

Phototransistors are kind of rare for non-visible and ir bands, and the idea of my project was to use various photodiodes to see in various bands, from ~5um to ultraviolet.

Basic description of the project is here:
http://orbides.1gb.ru/photobot.php?lng=eng

The bandwidth needed is only about 1KHz - the motors don't go faster.

The sensitivity at 2.2GOhms is already somewhat too much, actually - it gets off the scale in broad sunlight. At the same time, indoors it's almost too faint to see, so i tried some higher values with little success due to resistor's structure.

Another option yet is to use a T-network feedback scheme which will enable you to increase your gain significantly without having to resort to huge valued resistors...

I can't google up much on "T-network feedback", can you provide any details?

 

[yungman]

I have been busy as my wife just had hip replacement on Monday. Really don't have time to think about this for now.

Regarding on the sticky at 0, I have no idea as the circuit recovery time is so much shorter than what you are describing.

Regarding to too much gain in bright light and not enough in dark, use two different feedback resistors and use relay to kick it in an out. Say you use two feedback resistor, one is 500M, the other is 5G. One end of both resistor connect directly to pin 2 of the opamp. The other end connect to the relay. The relay is to switch the output of the opamp to either the 500M or 5G. You then switch the relay depend on the brightness of the surrounding. Easy. That's what we do all the time, you don't get enough dynamic range from one resistor.

 

[Artlav]

I have been busy as my wife just had hip replacement on Monday. Really don't have time to think about this for now.

No problems, I'm in no hurry. Good health to her.

When you get some time, or someone else look at this, here is he situation so far.
I've been trying to get negative supply for the amp. The best idea i had is MAX660 charge pump chip, turning +5V into -5V. 10uF cap for the pump, 100uF one to filter the output voltage, but I'm still getting a load of noise just from the thing being here. Also tried at different frequencies and with different caps - the noise does not go away.

So, is there either a way to set up the negative voltage properly, or a way to remove sticking without using that?

 

[Artlav]

The solution have found the problem.
Not surprisingly, it was all in the manual.

AD820 have two NULL pins for offset voltage compensation, by linking these to +5V with 4.7K and 10K resistors, i got the output 0 to be in the 9 to 30 range (1=5mV), depending on the PD used.
And presto, nothing sticks to anything, without any troublesome negative voltages.

 

[yungman]

Glad you find the problem. Are you going to put in the relay to extend the range for different situation? I think it should work as you can have one setting for indoor or night, and the other setting for bright outdoor.

The T-Network is a common scheme that use a voltage divider with low value resistor and the big resistor connects to the divider. eg. If you need a 1G resistor. You can use a divider using 9K and 1K to divide the output by 10, then you only need to use a 100M resistor to connect to the divider to pin 2 and get the same gain. The big down side is the noise is going to increase by 10 times and the offset will increase by 10 times. The good point is you get to use a smaller feedback resistor and therefore increase speed.

I did not suggest the T-network because you are dealing with noise already at the time. It is a standard practice to avoid using T-network unless you really run into speed problem. This is no different than using a 100M resistor and then put a X10 gain stage following it. All the problems are amplified by 10 times using this.

 

[Artlav]

Glad you find the problem. Are you going to put in the relay to extend the range for different situation? I think it should work as you can have one setting for indoor or night, and the other setting for bright outdoor.

Something like this, quite likely. A simple manual switch with three positions should suffice - sunlight, overcast, indoors. 500M, 2.2G, ~10G respectively. It's not moving around fast enough to warrant the complexity of a relay.