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Author Topic: [Help] IR Sensor schematic reading  (Read 1149 times)
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Sepiroth
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« on: July 14, 2010, 09:20:12 09:20 »

Hi all,
First of all i'm sorry if i misplaced the topic. While I was looking for thermopile sensors and circuits around net, I've came across this:"http://paparazzi.enac.fr/wiki/Image:IR_double_small_schematic.png " As you all can see, it uses two thermopile couples for stabilization system of an UAV. Sensors are MLX90247, with pin out of
(1)OUT_IR+ IR positive out
(2)Rsense
(3)OUT_IR- IR negative out
(4)Vss-thermistor ground
What I didn't understood from the schematic is, what's the use of connecting Vcc to ground pins and one of positive pin, and connecting negative outputs of a pair together? I usually use differential input opamps for sensor interfacing with usual setup, just wondered whats the principle of this circuit scheme. Thanks for the help,

King Regards,
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MoarK
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« Reply #1 on: July 14, 2010, 09:52:51 09:52 »

http://media.digikey.com/pdf/Data%20Sheets/Melexis%20PDFs/MLX90247.pdf

It looks like the thermopile sensors are set up to provide a difference signal -
the op amps are set up as integrators, presumably to reduce jitter.
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Sepiroth
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« Reply #2 on: July 14, 2010, 10:13:52 10:13 »

yup seems so. the only reasonable thing i've thought is they connected vcc/2 to ground to offsett the output, bc they are using single supply opamp or so..but still didn't get the idea why connecting sensors outputs together :\
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MoarK
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« Reply #3 on: July 14, 2010, 10:41:25 10:41 »

It may be that one is meant to sense ambient and cancel for the other sensing an object?

Posted on: July 14, 2010, 10:21:52 10:21 - Automerged

I just had a look at a different MLX  chip- the plug n play thermometer-
that's really sweet- complete.   

Posted on: July 14, 2010, 10:27:19 10:27 - Automerged

As I understand it, they are photovoltaic sensors, so IR1 is referenced to +3 and provides the negative ref for IR2.
The more active IR1 is, then, the lower the reference for IR2, which lowers its output by the amount registered by IR1.
So it looks to me like IR1 is cancelling IR2.

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