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DRAGSTER
CONFIDENTIAL
DRAGSTER
Calibration
Revision : 0.0.04
Date : 25/06/09
Contact :
AWAIBA GmbH
Am Wegfeld 30
90427 Nurnberg
Germany
www.awaiba.com
1 Introduction
DATE: 25/06/09
DRAGSTER Evaluation
Version 0.0.04
PAGE: 1/6
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DRAGSTER
CONFIDENTIAL
This document gives a brief explanation about how to configure the Dragster sensors in order to
reduce the effects of the mismatch between pixel segments and to make the light response
linear for all sensor.
2 Setup
For this calibration the system will include an Eval board configured as medium camera link, a
Dragster sensor V3-8K-3.5, the communication software DragsterComm and the frame grabber Me4
from SiliconSoftware.
Although this calibration is made with a V3, the process is similar to the other versions.
3 Calibration
The final goal of the calibration is to tune the ADC offset and gain error between segments so
than we get a uniform image out of the sensor. For that 2 steps are necessary. One calibration step
without light and another with the light, preferentially green (~ 520nm) at about 80% of saturation.
The image histogram is the guide over all the calibration process.
Before starting it's recommended to reset the sensor and sending the default values with the
DragsterComm. Check that from pixels 4 to 7 there are the black and white reference pixels and after
that only black pixels are present until the 63
rd
.
3.1 Black level calibration
The goal is to get both segment histograms right over 0 and one over the other.
DATE: 25/06/09
DRAGSTER Evaluation
Version 0.0.04
PAGE: 2/6
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Figure 1: Image in light with default settings and no calibration.
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DRAGSTER
CONFIDENTIAL
Prevent any light of getting to the sensor by switching off the light and placing a black cap over
it.
Make a “Send All” of the default settings with DragsterComm. First we'll make a rough tuning
so that all segments show a histogram over 0. Once this is achieved a fine tuning of the individual
black levels has to be made so all histograms overlap and look like a single one.
Activate the image histogram. If it is clamped to zero, decrease the black level in “All
segments” and send this register. If both segments are far away from zero, increase the black level until
one of the histograms start getting close to 0.
After this the histogram should look like this:
Now the fine tuning has to adjust the higher histogram to go over the other. For this we need to
identify which of them is the higher one. By setting a black level ± 10 of what was set in the Black
level All Segments it should be easy to identify it.
Note: To set the black level of individual segments use only the 4K_1 SPI and 8K_1 SPI.
After the fine calibration, the black level should look like this:
DATE: 25/06/09
DRAGSTER Evaluation
Version 0.0.04
PAGE: 3/6
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Figure 2: Initial black level
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DRAGSTER
CONFIDENTIAL
After this we should make the offset correction in order to use again all the ADC range,
otherwise the first DN are wasted for the black signal and we'll never get pixels with values under what
was set. This should be made on chip using the Control 2 and Control 1 register. The next sequence is
the most adequate and although others might also work these steps are the safest.
Using the “Send” buttons of the individual registers, follow this sequence:
1. Start by making sure that in Control 1 "Subtract offset" bit is 0 and "Update request" to 1. Click
“Send”
2. Set the sensor to dark condition and adjust the black level to the smallest output value over 0,
like described in this document. Avoid any pixel value of 4DN,i.e., the lowest ADC output
value.
3. Set in Control 2 "Write Offset" bit to 1. Click “Send”.
4. Using Control 1 send the "Update request" on 1. Click “Send”. The image will ALWAYS
freeze, i.e, is not light sensitive, and the new acquired values are written to the internal offset
register. Looking at the histogram, if it did not freeze, the sequence has to be restarted.
5. Set in Control 2 "Write Offset" bit to 0. Then “Send”.
6. Set in Control 1 "Subtract offset" bit to 1 and "Update request" to 0. Click “Send”.
7. Set again in Control 1 the "Update request" bit to 1 and click “Send”.
Now the dark image will be subtracted on chip from the illuminated image.
When turning "Subtract offset" bit in Control 1 to 0 the stored offset image is lost and needs to
be reacquired with the same step sequence.
DATE: 25/06/09
DRAGSTER Evaluation
Version 0.0.04
PAGE: 4/6
Illustration 1: Black level calibrated
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DRAGSTER
CONFIDENTIAL
3.2 ADC Gain calibration
To calibrate the gain the best is to use a very stable light source with the possibility to regulate
the light energy. LEDs are recommended. Use the maximum light energy available or tune it so that the
histogram is between 3000DN and 3500DN
Set the sensor to the operating point, especially the gain programmed on the sensor to the
desired setting in your application. Tune the analogue gain on/off ( bit 5 Control 2) and the inverse
ADC gain register.
The histogram should look like this:
Use the 4K_1 SPI and 8K_1 SPI to fine tune the Inverse ADC gain of the individual segments.
Set again a value ±10 of what was set in the Inverse ADC gain of All segments so that we can identify
which segment to tune.
Once the histograms are one over the other the calibration is done and the sensor shows a good
response over all range, just with the remark that the saturation level is not 4096 any more. Because the
on chip offset subtraction is activated, the ADC range is smaller.
As last step, the “End of range” register should be increased so that the ADC covers again a 12
bit range. In the Illustration 1 it's shown that the black level was set around 100DN so this is the value
that is recommended to be increased in the “End of range” register.
DATE: 25/06/09
DRAGSTER Evaluation
Version 0.0.04
PAGE: 5/6
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Figure 3: Initial ADC gain calibration
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DRAGSTER
CONFIDENTIAL
DATE: 25/06/09
DRAGSTER Evaluation
Version 0.0.04
PAGE: 6/6
Figure 4: ADC gain calibrated
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