Quantum Sensor with Arduino

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Arundo
 
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Joined: Thu Jul 24, 2014 9:29 am

Quantum Sensor with Arduino

Post by Arundo »

I have a licor li-190 Quantum sensor that I want to opperate with an Arduino. The output of the sensor is uA (0-10uA) and I have it connected to a uCurrent board to transform uA to mV. When I measure the output voltage of uCurrent with a polymeter everything is ok but when I try to read the uCurrent output with an Arduino analog pin I get strange numbers.
With a polymeter I get a value of 136mV and with arduino, using value=analogRead(A0) I get random values regadless of analogReference(INTERNAL) or analogReference(EXTERNAL). With analogReference(INTERNAL) and 136mV input the value should be 1100*136/1023=146. What am I doing wrong? I connect the negative wire of ucurrent to arduino GND and the possitive to analog 0 pin. Here is the code:

Code: Select all

void setup() {
  Serial.begin(9600);
  analogReference(INTERNAL);
}

void loop(){
  int value; double result; double Vcc;
  value=analogRead(A0);
  result = float(value) * (1.1/1023)*1000;
  Serial.println(value,DEC);
  delay(1000);
}
here is the output

Code: Select all

0
0
0
0
1023
1023
0
951
1023
0
761
0
0
0
0
0
1023
0
1023
0
0
1023
595
Also in some cases the output voltage is very low such as 7.5mV. Can someone propose to me an op-amp x100 circuit?

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adafruit_support_mike
 
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Joined: Thu Feb 11, 2010 2:51 pm

Re: Quantum Sensor with Arduino

Post by adafruit_support_mike »

Let's try benchmarking the analog pin: Make a resistor voltage divider with a center voltage somewhere around 135mV (33k/1k from the 5v rail should give you about 147mV) and see if the pin measures that correctly.

Arundo
 
Posts: 4
Joined: Thu Jul 24, 2014 9:29 am

Re: Quantum Sensor with Arduino

Post by Arundo »

I made a voltage divider with a 39K and 1.2K resistors and with multimeter I measured 0.147 voltage. Arduino measured from 0.143 to 0.155 which I think it's ok but when I connected the sensor with ucurrent and ucurrent with arduino I got values around 0.130 and after 15 seconds the value start decreasing becoming 0 at the end without any change. Any ideas why is this happening? No matter how long I have the ucurrent connected to multimeter the output is stable.

I connect the minus output of ucurrent to arduino ground and the plus output to arduino A0.

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adafruit_support_mike
 
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Re: Quantum Sensor with Arduino

Post by adafruit_support_mike »

Hmm.. that sounds like some kind of capacitor discharge.

The Arduino uses a successive approximation ADC with a sample-and-hold circuit at the front end. That works by repeatedly zeroing then charging a small capacitor (about 14pF) at a frequency of around 15kHz. The uCurrent uses a chopper-stabilized op amp that runs at an internal frequency of 10-15kHz, and the gain circuit uses a 10pF capacitor to prevent high-frequency noise.

It's possible that those are interacting in some unexpected way. Try adding a 1uF capacitor between the ADC pin and GND. That should swamp out any other capacitive effects.

Arundo
 
Posts: 4
Joined: Thu Jul 24, 2014 9:29 am

Re: Quantum Sensor with Arduino

Post by Arundo »

I have changed the setup. Quantum sensor is connected to a TS1001 opamp with 100K resistor as it is proposed from Kerry Wong in http://www.kerrywong.com/2013/01/17/cur ... ng-ts1001/ and the output varies between 0 and 740mV.
The output read with a multimeter is stable while with arduino I get an output like this
http://postimg.org/image/r779jameb/
How can I stabilize it? Each line in graph is a measurement under different conditions. Each measurement last 20ms.
Adding 1uF in the previous setup,between ground and V+ made the system fail.

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adafruit_support_mike
 
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Re: Quantum Sensor with Arduino

Post by adafruit_support_mike »

When you say the measurement period is 20ms, does that mean each reading lasts 20ms or the entire 1000-reading sequence takes 20ms?

Arundo
 
Posts: 4
Joined: Thu Jul 24, 2014 9:29 am

Re: Quantum Sensor with Arduino

Post by Arundo »

the whole 1000 readings take 20ms. In more detail 24ms

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adafruit_support_mike
 
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Re: Quantum Sensor with Arduino

Post by adafruit_support_mike »

Thanks. I wasn't sure if the ripples were fast or slow.

Since they're fast, I can think of a couple possible sources of noise:

1) To get 1000 samples in 24ms, your sampling rate would have to be around 41ksps. That's faster than the Arduino's usual limit of about 10ksps. Try adding a few microseconds of delay between samples to take the rate down to about 8ksps. If the shape and frequency of the ripple changes, we'll know that the noise is in the ADC. If you get the same basic pattern at lower resolution, it means the problem is outside the ADC.

2) The spikes look kind of like switching regulator noise. What are you using as a power supply for the sensor and op amp?

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