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Building a balanced detector

by BonoEtMalo
Tags: balanced, building
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Mar3-14, 10:46 AM
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So my PI suddenly decided she wants me to learn electronics. And so she started me off building a balanced detector. So here I am with this circuit I know nothing about, knowing nothing about electronics...

Anyways, quick question on reading Op-Amps. In my circuit diagram, there are two Op-Amps. They're listed as MAX406AESA and MAX4162ESA. I've looked up their datasheets, but in the reference detector I was given (one that was built a while ago) they used Op-Amps labeled 120-1636H on the visible face of it

How should I go about reading Op-Amps, and what should I be looking for when I see a name like MAX406AESA?

I've attached the circuit diagram if I wasn't clear in my description
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Screenshot from 2014-03-03 10:39:05.png  
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Mar3-14, 01:40 PM
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Welcome to PF.
Start with this data;
Then ask more questions.
Mar3-14, 02:03 PM
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The battery powered circuit you provide is not an easy one to analyse for a beginner.
There are many small complexities to distract the beginner.
The MAX406 is being used to generate a ground reference at half the battery voltage.

MAX4162 adjusts its output to make it's inputs to be at the same voltage.
The difference in current between Photo Detectors PD1 and PD2 flows through R5, R7 or R8.
Those resistors set the current to voltage conversion gain.
The low impedance op-amp output voltage is proportional to differential PD current.
The circuit output has a series 50 ohm resistor to better drive a coaxial cable.

Mar3-14, 02:25 PM
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Building a balanced detector

Quote Quote by BonoEtMalo View Post
they used Op-Amps labeled 120-1636H on the visible face of it

How should I go about reading Op-Amps, and what should I be looking for when I see a name like MAX406AESA?
Often, one chip manufacturing company designs a new part and gives it a number like LT1636, where the "LT" indicates that particular manufacturer. If it is a widely used part, other chip makers will produce equivalent parts, with the same number (1636) but a different manufacturer's prefix. Often, you don't care who made it (so long as it is a reputable company) - you buy the cheapest equivalent that is actually in stock when you want it!

The suffix letters often describe the packaging (physical size and shape, number of pins, packaging material e.g. plastic or ceramic, temperature rating, etc) for the part. Obviously specifying the package type you want is important when you buy a part, but not so relevant for identifying a chip that is already on a circuit board. The data sheet for the chip will show what the suffix letters mean.

Reading the labels on chips is something you learn by experience. Other numerical codes like "120" are often the manufacturer's batch number, or the manufacturing date in coded in some way.
jim hardy
Mar3-14, 03:03 PM
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That's one heck of an assignment for a beginner.

To your question:

How to learn opamps ?
Texas Instruments has online a wonderful book, "Op Amps for Everyone". A quick Google on that phrase, with SLOD added should turn it up. Print yourself a copy it'll be a lifelong resource.
Sure enuf,

That 1636 opamp might be this one, which looks similar to that Maxim.

When reading datasheets look at power supply range, gain-bandwidth product GBW, and input offsets.
Judging by the slow opamps used(200 khz) i'd guess you're decoding a fairly slow optical signal.

Baluncore gave a good circuit analysis.

IF i understand the circuit,,,,
At the modest frequency this thing can reach coaxial cable output seems extravagant, maybe the 51 ohm is to protect against short circuits. We'd have to ask the designer.

Unequal Currents through those two photodiodes cause the second opamp to make up the difference via that RC feedback network C9 and whichever resistor is selected. That turns photodiode current unbalance from light into voltage.

25 picofarads C9 isn't much.
What is this supposed to measure?
I don't think it'll go much above 40 khz. It runs out of opamp bandwidth.
jim hardy
Mar3-14, 11:42 PM
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You know what ?

That thing is DC coupled.

Were the two photocells on either side of a vane that's pointed at the sun, it would tell you which one was in the shadow.
That could actuate a motor to make something track the sun.
Is this a solar power experiment?
just curious....

by the way - when looking for opamp datasheets , you can google on the part number.
That produces a zillion hits from annoying middleman sites that are loaded up with advertising and poor copies of datasheets..

I always try to get to the manufacturer's site. You'll learn a lot by reading his application notes.

Maxim is a significant manufacturer of IC's with an informative and educational website. If the part number begins with MAX it's one of theirs.

That's the good and bad of it,,,,,,,

good luck

old jim

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