Precision transimpedence amplifier for photodiode..

Hi

Based on the datasheet for the Li Cor 190SL sensor, a transimpedance amplifier is included and the output is calibrated for a 10mV full scale output . 

Does your data acquisition have a programable gain amplifer front end?

Regards

Dear Sir,

Thank you for the info, I believe the Li Cor 190Sl is actually a Li Cor 190SA preceded by a precision resistor calibrated to give mV output, I doubt whether it has a transimpedence amplifier since ther is no power lines to the sensor. have you got the schematic of the Li Cor 190SL?

Assuming I have the Li cor 190SA (Raw PAR sensor) what would you recommend as the highest precision/lowest drift and stability OP amp? what about the OPA 2380?

Sorry I dont have any info of the Li cor sensor to identify what photodiode they use and its characteristics like capacitance etc.

Appreciate your help!

Alvin

Alvin,

The information on your sensor is confusing and ambiguous. I agree with Walter that it appears that it is intended to have a direct voltage output. It may be a photodiode loaded by a low value resistor. In any case, it does not appear that it is intended to be used with a transimpedance amplifier stage.

I suggest that you seek further information from the sensor manufacturer before attempting to construct an amplifier.

Regards, Bruce

Alvin:

I reviewed additional datasheets on the Li Cor 190xx sensors. They do offer two versions, one with a current output and one with a voltage output. Based on your comment that there are no power leads to the sensor, the voltage output version has a resistive shunt network across it.

Your device is the current output sensor. The OPA2380 could be used for your transimpedance amplifier. It has significantly more bandwith then is needed but does have good offset and drift specifications. The sensitivity is specified as  5µA per 1000 µmol s^-1 m^-2  typical. The gain resistor value will be Rf = (full scale volts) / (sensitivity x max value of measured variable). The sensor's capacitance is not specified so you will have to adjust the compensation capacitor, Cf,  for a stable operation.

Regards

Hi Walter,

Thank you for your valuable info. is there a more precise lower offset & drift OP amp than OPA2380? I am just finding the edge here, so i wouldn't have to worry about accuracy/drift/repeatability etc.

I also need bandwidth since though i intend to use it as a traditional PAR sensor, I will also use it in pulses dose analysis experiments in photo syhnthesis experiments. I am researching on LED light output on plant/algae growth etc. I used very short duration pulses of low freq/duty...

Further more I must agree also with Bruce since ther is a low value resistor in parallel (around 180 Ohm/precision) to the photodiode output, from your experience putting just a low value resistor in parallel lessens the dynamic response of the photodiode doesn't it :-)

Is there any in-depth datasheets Li cor has apart from the ones available on the net? I mean on photodiode characteristics etc.

Is there a simple way to analyze the photodiode capacitance?

thanks you for your time and knowledge sharing!

Alvin.

Alvin,

You have not been specific on your bandwidth or sensitivity requirements. This could influence the selection of the amplifier that is best for your application. The OPA380 may be a good choice but some more detail would confirm this. What is the current range you expect from the photodiode? What frequency response is required. What output voltage range do you want from the amplifier?

Walter's information on the sensors is helpful (thanks, Walter). My best guess is that the voltage output version is a photodiode in parallel with a low value resistor to produce a low voltage output in quasi-photoconductive mode. The current output version is probably just a photodiode, intended for use with a transimpedance amp. The physical appearance with long leads suggests that they do not expect it to be used for high frequency applications. It appears to be a large photodiode. With the additional capacitance from the lead wire, its capacitance is probably pretty high. Knowing the total capacitance of this whole sensor could be important to optimizing the performance of the measurement. It sure would be helpful to get this detail from Li-Cor.

Regards, Bruce.

Hi Alvin and Bruce:

I only have the datasheet and applications information for the Li Cor sensors from their website. 

1. There are two versions of the 190 sensor, The 190SA is the photodiode with a current output of 5 microamps per unit. The 190SL has an attachment to convert the current output to a voltage.
2.  Base on data from Li Cor website they use a 604 ohm 0.1% resistor in parallel with the diode to generate the millivolt output.
3. You are correct that using the resistor shunt will have a small effect the linearity of the sensor. The max specified output voltage is 10mV and Li Cor state this has minimal effect on the sensor's linearity.
4. Based on the specified accuracy of the sensor +-5%, not including time and temperature drift,  the OPA380/2380 will not add significantly to the measurement error.
5. The specified response time of the 190 sensor is 10usec. Li Cor does not specify if this is 3, 4, 5, or 6 time constants. Also, they do not specify if the response time measurement is made in the photoconductive mode or in the photovoltaic mode.
6. You did not state what duty cycle and frequencies you are planing to use. The minimum measurable pulse width of the 190 sensor will be about 20 usec

Hope this helps. You should contact Li Cor for additional technical data.

Regards

Walt

We use the 192SB quite a lot for oceanographic applications to measure photosynthetic ambient radiation (PAR).  You can add the 604R resistor and get a linear output with current but, if you wish to measure a wide variation in light, then you have to resort to a log amp.  For oceanographic applications, it is quite common to measure 8 decades of light range from full sun to a few nano Einsteins.  Way back we use to use the ICL8048 but now we are using the IVC102.

Hope that helps.

Dear all,

I am totally overwhelmed by all the comments thanks :-)

to proceed futther I will find out more technical details of the photodiode in the PAR sensor, no sooner i get it i'll keep all of you posted.

thanks!

Alvin.

Hi All,

Below are some info i received on the photodiode

Active area    5.1mm2
Capacitance    150pF
Shunt resistance    >100MΩ
Noise current    15 10-15 A/√Hz
NEP (900,10,1)    2.5 10-14 W/√Hz
NEP (200,10,1)    18 10-14 W/√Hz

hope this helps..

Alvin,

As expected, the capacitance of this photodiode is very high. I'm not sure whether this includes the capacitance of the cable. This could add additional capacitance of approximately 25pF/foot.

Assuming you use the OPA380 (or OPA2380 dual version), you can use the graph in the upper-right of page 7 as a guide. We don't know your bandwidth or gain requirements so this may take some experimentation on your side. If the bandwidths shown for the various gains are much greater than you require, I recommend increasing the value of the feedback capacitor by 2x. This will assure that you have no problems with oscillations.

Note that the circuits in OPA380 data sheet show a photodiode with a grounded anode. I believe that your photodiode probably has a grounded cathode. You can verify this by checking the unloaded output voltage. If the output is positive with respect to the shield conductor it is a grounded cathode. In this case, you must bias the non-inverting input of the amplifier to a positive voltage of approximately 3V. The output of the amplifier then is negative-going with respect to this 3V offset. Is this clear?

Regards, Bruce.