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THEPICUser
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| Confused - Analog to Digital |
 Posted: Fri Jun 08, 2007 2:51 pm |
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I wrote code for a PIC to do Analog to Digital and to display the result on my LCD. I have provided my code below.
When I connect a potential meter to my AN0 input, and vary the voltage from 0 - 5V, it works perfectly fine. I even confirmed with my volt meter.
However the problem I have is with reading another set of analog values. These values come from a motorolla mpx10D pressure sensor which produces values in the mV range. I amplify these values with a 741N opamp so that the voltage entering the pic is in the 0.7 - 1.2V range.
I check the values entering the PIC AN0 with my volt meter and it reads them to be about 0.70V - 1.2V. But the values my analog to digital conver captures and displays on the LCD keep fluctuating by a lot. So for example, the volt meter will output 0.70, 0.71, 0.72, 0.70, where as my PIC will output to the LCD 0.42, 0.81, 0.54, 1.10, 0.40.
What could be the problem? Could the opamps be doing something, but then why is my volt meter reading a stable value? I could really use some help with this one. Thanks.
| Code: |
#include <16F877A>
#device ADC=10
#use delay(clock=20000000)
#include <flex_lcd.c>
void main ()
{
unsigned long value;
float voltage;
set_tris_b(0);
set_tris_a(1);
set_tris_d(0);
setup_adc_ports(00000011);
setup_adc(ADC_CLOCK_INTERNAL);
set_adc_channel(0);
delay_us(10);
lcd_init();
while(1)
{
value=read_adc();
//output_B(value);
voltage = (float)value*(5.0/1023.0);
delay_ms(1);
printf(lcd_putc,"\fVolt is %f",voltage);
delay_ms(1000);
}
} |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Fri Jun 08, 2007 3:07 pm |
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| Quote: | | setup_adc_ports(00000011); |
Don't use "magic numbers" for the CCS function parameters.
Use the constants that are given in the 16F877A.H file.
Also, your number above is interpreted as Octal format.
In decimal, it's equivalent to 9. Hardly anyone in the C world
uses Octal. We use either Hex or Decimal (or maybe binary). |
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gammla
Joined: 08 Jun 2007
Posts: 3
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| Posted: Fri Jun 08, 2007 5:55 pm |
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Hello!
Try to lay a resistor around 10kOhm or higher between your output of the opamp and your adc input to ground.
This should solve your problem!
best regards,
gammla |
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ElectricalNoob
Joined: 17 May 2007
Posts: 15
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| THEPICUser |
| Posted: Fri Jun 08, 2007 10:36 pm |
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Thank you PCM Programmer and gammla. I'm going to try the resister idea.
When it comes to practical applications, I am really new at this. I'm trying to learn as much as I can in the shortest amount of time. I'm still puzzled with little things like. When my AN0 analog input is in a continuous reading loop, and nothing is connected to the pin. The results of the analog to digital conversion will fluctuate. I don't understand the reason for this. But it seems like it might be similar to the reason why my pressure sensor has fluctuating problem. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Fri Jun 08, 2007 11:55 pm |
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| Quote: |
When my AN0 analog input is in a continuous reading loop, and nothing is
connected to the pin, the results of the analog to digital conversion will
fluctuate. I don't understand the reason for this. |
Connect a 47K pull-down resistor to the input capacitor of your op-amp
circuit. That way, when no signal is present, the input will be at 0 volts. |
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