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Entry  Tue Jun 9 19:39:03 2015, Koji, Electronics, General, Nick Lockerbie's precision current driver current_driver.png
    Reply  Mon Jun 29 21:28:46 2015, Koji, Electronics, General, 2ch Photo Sensor circuit PS_Calib.pngPS_Dark_noise.pngPS_DC_OUT.pngPS_AC_OUT.pngPS_Disp_Noise.png
Message ID: 1954     Entry time: Mon Jun 29 21:28:46 2015     In reply to: 1930
Author: Koji 
Type: Electronics 
Category: General 
Subject: 2ch Photo Sensor circuit 

Here the performance measurements for a 2CH reflective photo sensor circuit are presented.

Two sensor heads are driven by Nick's constant current driver. They are connected in series. Therefore the two heads need to be connected at the same time to make them work correctly.

The reflected light is detected by two photo diodes (per head) to cancel angular motion of the mirror. The photocurrent is amplified in the head and sent to the box.

It has a modulation input. One can connect a SYNC output of a signal generator to chop the LED current. The modulation freq of 3~5kHz gives the best result.
In order to use the modulation function, two internal jumpers should be removed to activate AC coupling cirucit.

The final outputs have 500Hz roll-off with 3rd order butterworth.

Attachment 1: Calibration of the photo sensor (CH1 only)

An Al mirror is placed in front of the photo sensor head1 on a sliding stage. When the sensor head is touch the mirror, the diplacement is marked zero. The sensor output depends on the distance of the sensor from the mirror. From the measured profile of the sensor, the near- and far-side calibrations were estimated to 2.6e3 V/m and 0.67e3 V/m, respectively. These numbers depends on the reflecting surface and if the modulation is used or not, as well as the modulation frequency. Therefore the calibration should be done everytime one sets the sensors up.

Attachment 2: Receiver performance

By disabling the current source, the dark noise level was tested. The PDs have the dark noise level of ~1pA/rtHz floor. This corresponds to the shot noise of 3uA DC current. This measurement has been done at the test points before the final roll off filters.

Attachment 3/4: DC RIN / AC RIN measurements

A head was positioned in front of the mirror with the distance where the response hit the maximum. This corresponds to the measurement of the RIN when there is no modulation. When the modulation is used, we can define a similar quantity to RIN, here we call this AC RIN. I don't see good suppression of the DC fluctuation. It might suggest that the DC amplitude of the LED current is changing this much and the ambient noise is not affecting the performance. So when an ambient noise is not an issue, DC meausrement gives us a better performance. When an ambient noise disturbs the displacement sensing, the modulation function should be used with the cost of noise floor level.

Attachment 5: Displacement noise without modulation

Finally, the displacement equvalent noise level was checked. This measurement has been done by placing Head1 in front of a fixed mirror with the distance about 1.5mm. This corresponds to the DC output of about 5V. The noise level is 0.2~0.5 nm/rtHz above 2Hz and 3/f^3 nm/rtHz below 2Hz. This noise curve actually includes the vibrational noise of the measurement setup.

Attachment 1: PS_Calib.png  280 kB  | Hide | Hide all
Attachment 2: PS_Dark_noise.png  716 kB  | Hide | Hide all
Attachment 3: PS_DC_OUT.png  530 kB  | Hide | Hide all
Attachment 4: PS_AC_OUT.png  544 kB  | Hide | Hide all
Attachment 5: PS_Disp_Noise.png  542 kB  Uploaded Tue Jun 30 04:46:24 2015  | Hide | Hide all
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