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Topic: Amplifying weak signal to 2000x (Read 145 times) previous topic - next topic

YunuZ

Sep 16, 2019, 09:14 pm Last Edit: Sep 17, 2019, 10:47 am by YunuZ
Hi, I try to build a high precision optical rotary encoder.

Im using 5 small smd LEDs

The 3 ones to detect strips like this:
Code: [Select]

Led1 led2 led3
^^^^ vvvv ^^^^

_____________............._____________............. = paper strips


Led2 = illuminates the paper
Led1 & led3 sense the white or black strips

The 2 other are similar to 3 ones but only one sender & one receiver to see offset zero position mask.

The receiver LEDs produces approx 2-5 mVolt when see white strip and less than 0,5 mVolt on black strip


I need to gain the signal about 2000x,
so 2-5mV x 2000 = 2-5 volt.
The nice range to be measured by Arduino analog pin.

I need 3 op-amp, so I choosed LM324
The feedback resistor of non inverting op-amp are 2M ohm & 1K ohm. So the gain is 2001x (or about 2000x).

But the output gain was not as expected, like crazy graph in Arduino IDE.

Anybody know the problem?
Do you have any solution?

Note:
I cannot use photodiode or phototransistor,
the dimension is too big and limit the resolution of the reading.
I use the 0603 smd LED, placed very close each other.
Good enough to see strips as small as 2-3 mm.


wvmarle

You will need a multi-stage amplifier, this is too much for a single stage. Three stages sound more sensible (20x, 10x, 10x).
But with such low voltages a specialised part such as the HX711 may be a better choice.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

jremington

#2
Sep 17, 2019, 04:52 am Last Edit: Sep 17, 2019, 04:53 am by jremington
A single stage gain should not exceed about 100x, or it will tend to oscillate.

The LM324 is a very poor choice of op amp. Look for precision, low offset, low bias current op amps.

YunuZ

#3
Sep 17, 2019, 10:36 am Last Edit: Sep 17, 2019, 10:44 am by YunuZ
This is the problem of Lm324:

The input + and - emit voltage approx 11mV when checked using multimeter.
The 11 mv noise blinds the 2-5mv signal.

Even if I set the low gain of 10-100x , it likely not detecting the weak signal.

Do you have a suggestion of similar LM324 or another method?


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