TIDA-00554: How accurate is the data from photodetector in lamp?

Frank(EP) Liu

Expert 3765 points

Part Number: TIDA-00554

Dear Sir,

My customer read the photodetector value in the lamp. The value is not consistence with different board.

Test flow:

-input the fixed voltage(0.45V) on sense_1 as channel 2 in waveform below.

-read the photodiode value through NIRSCAN NANO GUI. And the data is different with board. it seems there is 20% deviation with correct value. And the data changes with board.

Can you explain how it reads the sensor data?

How accurate is the result and how to improve it?

BR,

frank

over 5 years ago

0 Philippe Dollo over 5 years ago

TI__Mastermind 28005 points

Frank,

Are you performing unit calibration on each NIRscan Nano EVM that you are testing? If improperly calibrated variation in measurement is common.

Best Regards,
Philippe Dollo

0 Vivek Thakur over 5 years ago

TI__Mastermind 22830 points

Hi Frank,
Is this variation for one lamp with multiple boards or different lamp with each board?

Lamp output varies with temperature and unit to unit. Other issue could the lamp drive i.e constant voltage vs constant current. TI NIRScan Nano EVM uses constant current driver for the lamp.

Regards,
Vivek

0 Frank(EP) Liu over 5 years ago in reply to Vivek Thakur

TI__Expert 3765 points

hi Vivek,

They input the fixed voltage to replace the photodiode output in lamp. So, they can make sure the signal quality is good and stable.

Even the fixed voltage, the data from TIVA\DLPC has ~20% deviation with different board. They don't know how to use this value.

BR,

frank

0 Frank(EP) Liu over 5 years ago in reply to Philippe Dollo

TI__Expert 3765 points

Philippe,

You mean the TIVA would change the photodiode data with calibrated variation?
if there is no calibration process, is the result is consisted?

BR,
frank

0 Frank(EP) Liu over 5 years ago in reply to Frank(EP) Liu

TI__Expert 3765 points

hi Philippe,

Can you show me where it change the photodiode data? I cannot find it in TIVA's code.

BR,

frank

0 Hirak Biswas over 5 years ago in reply to Frank(EP) Liu

TI__Genius 10925 points

Hi Frank,

The photodiode values are not changed on the TIVA firmware side. The values are adjusted according to reference calibration data before plotting on GUI side itself. Kindly see the subroutine Spectrum::Plot(PLOT_TYPE type, QGraphicsScene *scene) in the file spectrum.cpp inside the GUI Source folder. It calls the necessary functions/macros (GetAbsorbance(), COMPUTE_REFLECTANCE_VAL etc.) to adjust the absolute photodiode values before plotting.

Regards,
Hirak.

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TIDA-00554: How accurate is the data from photodetector in lamp?