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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

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  • \$\begingroup\$ What is the output impedance of the source? \$\endgroup\$ Jun 10, 2015 at 17:51
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    \$\begingroup\$ Is your pin already setup as an Analog input pin??? Your plot gives me the impression that your ADC pin in is fact set as an Digital output and it is driving the pin high. \$\endgroup\$
    – Kvegaoro
    Jun 10, 2015 at 17:59
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    \$\begingroup\$ It looks to me that the Arduino pin is outputting a square wave - the frequencies of the two waveforms are different. (Why not set the 'scope to display the frequency of Channel 2?) \$\endgroup\$ Jun 10, 2015 at 18:24
  • \$\begingroup\$ @PeterBennett I think you are correct about two waveforms on the scope, the frequency is 137Hz. Are you suggesting that I've defined my analogue pin as a digital output pin? \$\endgroup\$ Jun 10, 2015 at 18:30
  • \$\begingroup\$ @TomPriddle: yes - Kvegaoro and I are both suggesting that the Arduino pin you are using is set as a digital output pin. \$\endgroup\$ Jun 10, 2015 at 18:33

1 Answer 1

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[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.

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  • \$\begingroup\$ Thanks for the suggestions. I've already tried adding a voltage buffer but it didn't solve the problem at all which was strange as I though the buffer would isolate. Your "dirty fix" worked although I was wondering if there would be a voltage drop due to the 1K resistor? I tried to find out using the oscilloscope although there doesn't appear to be a significant voltage drop \$\endgroup\$ Jun 10, 2015 at 18:53
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    \$\begingroup\$ @TomPriddle: The input impedance is much higher than the added resistance, so you would only get a very small voltage drop. \$\endgroup\$ Jun 11, 2015 at 14:24

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