Discussion in 'Embedded Systems and Microcontrollers' started by ecaits, Jul 30, 2014.

1. ### ecaits Thread Starter Member

Jan 6, 2014
51
0
Dear Friends,

I have two-three silly question in my mind...

1. If I give more than 5V, say 10V to analog pin of ADC then what happen???

2. If I give negative voltage to analog pin of ADC then what will happen???

3. If I have 4-20mA output of sensor, can I use it as analog input??? OR First I need to convert 4-20mA to equivalent 0-5V??? If yes, How can I covert the same???

I am using PIC16F877.

Last edited: Jul 31, 2014

Apr 16, 2011
249
82
You forgot to give any information on exactly which ADC you are using.

Some ADCs will happily accept +10 volts and negative volts, others will not.

3. ### Papabravo Expert

Feb 24, 2006
10,021
1,757
A resistor of 249 Ohms 1% will convert currents as follows:

.004 A * 249 Ohms = .996 Volts
.010 A * 249 Ohms = 2.49 Volts
.020 A * 249 Ohms = 4.98 Volts

You can use an opamp configured a a voltage follower to avoid having the ADC input load the current loop.

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,294
482
There are voltage ADC, you will need to convert your current to voltage.

5. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,313
1,570
Generally you make any analog device very unhappy when you apply a voltage at any terminal greater then the positive supply, or less than ground (or the negative supply).

Most input pins these days have "ESD" diodes to conduct energy to the power rails in case of a sttic zap. Often you can use these to keep your device safe from occasional short term overvoltages, but you need to insert a resistor in series with the input line.

6. ### MrChips Moderator

Oct 2, 2009
12,228
3,280
Putting voltages greater than Vcc (Vdd) or lower than GND (Vss) isn't going to sit well with integrated circuits.

In a worst case scenario, a CMOS circuit will latch-up, i.e. go into a SCR parasitic short circuit state that literally fries the chip.

As Ernie points out, you need to put a resistor in series to limit the current plus voltage clamping diodes if the chip does not already have input protection diodes.

7. ### wmodavis Well-Known Member

Oct 23, 2010
737
149

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8. ### ecaits Thread Starter Member

Jan 6, 2014
51
0
I am using PIC16F877.

9. ### ecaits Thread Starter Member

Jan 6, 2014
51
0
I am using PIC16F877.

Apr 24, 2011
7,313
1,570