Electronic – Arduino Uno – Analogue Signal Conversion Issues

adcamplifierarduinooperational-amplifieroscilloscope

The output of my Low Noise amplifier is a rough sinewave with a peak-to-peak of 1.7V and is shown in the oscilloscope trace below. Trace 1

I would like to use one of the analog pins on the Uno to perform the analogue-digital conversion. However when I connect the analog pin to the output of my LNA the output immediately changes from a sinewave to sqaure wave and the peak-to-peak voltage rises (see trace below). Trace 2

I even attempted to place a voltage buffer (follower) with no voltage gain in between the output of the LNA and analog Pin but this didn't solve anything. Again, as soon as I connected the analog pin the sinewave switched to a square wave.

int sensePin = 0;
int val = 0;
void setup() {
// put your setup code here, to run once:
Serial.begin(9600);
//pinMode(analogPin, INPUT);
}

void loop() {
val  = analogRead(sensePin);
Serial.println(val);
delay(200);
}

Any help/suggestions would be appreciated, I'm not sure what is wrong. Thanks

Best Answer

[For the most part, below is just my conjecture. Then again, troubleshooting often involves conjectures.]

When you connect the amplifier to the A/D pin, you may be increasing the load capacitance. which the amplifier output is "seeing". It could be due to the capacitance of the A/D input, or dues to the capacitance of the cable. This (suspected) capacitive load may be causing your amplifier to oscillate.

We don't know the model of the amplifier (?), so we don't know how much load capacitance it should be able to drive. We also don't know what you cabling looks like: is it 200mm or 20m?

A quick & dirty fix would be to add as series resistor (say, 1kΩ) between the output of the amplifier and the cable. I'm calling this a "dirty" fix, because you are increasing the impedance that the A/D converter will see.
A serious fix would be to drive the cable with a buffer.

Related Solutions

Electronic – arduino – How to have an Arduino wait until it receives data over serial

Here are some suggestions:

move declaration of ch above the while loop
you used = instead of ==, so while( ch = 0) will loop forever, because ch = 0 will always evaluate to true. It should be while( ch == 0)
refactor your menu display into a DisplayMenu() function, just because it's cleaner that way, then call DisplayMenu() from loop().

Electronic – Cannot get a digital square wave on oscilloscope from the GPIO of the Raspberry Pi

There are a couple of issues with your code that can be easily fixed. First, you are calling wiringPiSetup which means you will need to refer to the pins by their WiringPi designation which can be found on this page:

https://projects.drogon.net/raspberry-pi/wiringpi/pins/

You are then setting pin 7 (wiringPi designation) to be an output which is actually GPIO 4 (physical pin 7) and not the CE1 pin as you mentioned. CE1 would actually be wiringPi pin 11.

The next problem is in the while loop. You are doing a digitalWrite on wiringPi pin 4 (not 7 as you configured) and are always setting it low so it would never toggle high even if configured correctly. The following code should be what you are looking for but is not tested. This assumes you want to use CE1 which is normally used for SPI but should work with WiringPi as an general I/O pin:

  if (wiringPiSetup () == -1)
    exit (1) ;

  pinMode(11, OUTPUT);

  while(1) {
    digitalWrite(11, 0);
    digitalWrite(11, 1);
  }

Best Answer

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