![](templates/subSilver/images/CCSLogo.jpg) |
![CCS C Software and Maintenance Offers](templates/subSilver/images/forumAd6.jpg) |
View previous topic :: View next topic |
Author |
Message |
Eugeneo
Joined: 30 Aug 2005 Posts: 155 Location: Calgary, AB
|
Photo sensor current too low |
Posted: Sat Apr 28, 2007 10:50 am |
|
|
This is a little off topic since it is not a compiler question but here goes.
I'm trying to read a sensor that has 40 nano amps max output. Here's my setup.
OPA129 opamp which has an untral-low bias current of 100fA max.
I'm using a 100 meg I feedback resistor, so in theory the output should be 4volts at 40 nano amps (40na * 100 megohm). Instead all my test equipment reads 1-20mv.
Board traces are goood and flux has all been removed.
Standard FR4 PCB board
When using a voltage follower with no gain the output voltage is over 1000mv, and I've tried different sensors.
Any thoughts would be greatly appreciated. |
|
![](templates/subSilver/images/spacer.gif) |
newguy
Joined: 24 Jun 2004 Posts: 1903
|
|
Posted: Sat Apr 28, 2007 11:27 am |
|
|
Can you post a schematic? It's a little hard to follow your description of the circuit. |
|
![](templates/subSilver/images/spacer.gif) |
Eugeneo
Joined: 30 Aug 2005 Posts: 155 Location: Calgary, AB
|
|
![](templates/subSilver/images/spacer.gif) |
Ttelmah Guest
|
|
Posted: Sat Apr 28, 2007 3:07 pm |
|
|
Do you really need to use the diode in photovoltaic mode?. This is much less sensitive than using photoconductive mode. Photovoltaic mode, is the preferred mode for really small levels (because of the lower noise), so this may be where you are coming from. The active area is small, so unless your source is very well focussed, a lot of the light may not be influencing the current. What is the source power?.
What is the wavelength of the source?. The 0.14A/W, is a peak figure, and if your light source is down nearer to 200nm, the responsivity falls to only 0.02A/W.
Remember that the cell itself, has a internal resistance (typicfied at 500MR), which is also loading the signal, as well as the current drawn by the inputs.
Best Wishes |
|
![](templates/subSilver/images/spacer.gif) |
SherpaDoug
Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA
|
|
Posted: Sat Apr 28, 2007 5:19 pm |
|
|
Try substituting a small battery and a large resistor for the sensor. Start with a combination that gives 1000X the current you are expecting from the sensor, a current you can actually measure and verify, and make sure your circuit behaves. Then work your simulated sensor current down to real levels, and finally use the real sensor. _________________ The search for better is endless. Instead simply find very good and get the job done. |
|
![](templates/subSilver/images/spacer.gif) |
Eugeneo
Joined: 30 Aug 2005 Posts: 155 Location: Calgary, AB
|
|
Posted: Sun Apr 29, 2007 12:41 am |
|
|
Thank you guys for the help
Ttelmah wrote: | Do you really need to use the diode in photovoltaic mode? |
I assumed that if I used this sensor in photoconductive mode, a source reference volage would be required (another source of noise) and the overal result would be simular since a current loop is still required. Also, when the sensor is setup in voltage follower mode, the measured output was close to 2 volts.
Ttelmah wrote: |
The active area is small, so unless your source is very well focussed, a lot of the light may not be influencing the current. What is the source power?.
What is the wavelength of the source?. The 0.14A/W, is a peak figure, and if your light source is down nearer to 200nm, the responsivity falls to only 0.02A/W.
|
I think you maybe right. I've done the test that SherpaDoug suggested, with some interesting results.
Doug's Test:
Using a 1.00volt supply connected to 50Mohm resistor supplied to the inputs resulted in a 1.998 volt output. When I reverse the input polarity, -1.987. This means even with a 5% 50Mohm resistor, bias current, and offset error, the output was pretty much that of an ideal opamp.
At this point I can only say the opamp circuit is functioning corretly at these current levels. I'll try some tests with 500M-1G ohm (almost seems silly to be using these values) to confirm this.
So... It must be 1 of 2 things. The sensor in which all three show the same results. Or the UV palm erase source which is supposed to be 200 to 240 NM and a butane torch (supposed to produce a measureable UV signature).
Thanks again guys, I'll continue forward. |
|
![](templates/subSilver/images/spacer.gif) |
|
|
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum
|
Powered by phpBB © 2001, 2005 phpBB Group
|