Adc

[vinke]

Hey guys,i was wondering what would happen if i put a negative voltage on an analog input pin of an adc module of a pic,given tat +Vref=5v and -Vref=0V.

 

[eng1]

Hey guys,i was wondering what would happen if i put a negative voltage on an analog input pin of an adc module of a pic,given tat +Vref=5v and -Vref=0V.

Applying a negative voltage would violate the electrical specifications of the device. See section 'Electrical characteristics' in its datasheet.

 

[vinke]

Would the pic burn if ever it happens?

 

[eng1]

Would the pic burn if ever it happens?

The PIC has internal diodes that clamp voltage spikes, but you shouldn't apply a negative voltage continuously.

 

[Hero999]

Nothing would happen if you limited the current to a level the diodes can stand with a series resistor.

 

[vinke]

Ok but i want to know what would be the digital result of this negative voltage if +vref=Vdd and -Vref=Vss=0V

 

[ericgibbs]

Ok but i want to know what would be the digital result of this negative voltage if +vref=Vdd and -Vref=Vss=0V

The device manufacturers produce datasheets for their products so we have guide lines on their use.

Even if you applied a -V to an ADC input, you would not be able to convert it to a usable value within your program,

so why waste time and money trying!.

If you can afford it, just do it and lets us know what happens

 

[vinke]

The device manufacturers produce datasheets for their products so we have guide lines on their use.

Even if you applied a -V to an ADC input, you would not be able to convert it to a usable value within your program,

so why waste time and money trying!.

If you can afford it, just do it and lets us know what happens

In the datasheet it doesnt specify what happens in such circumstances,else i wouldnt ask the question.
Im not trying to put a -ve voltage deliberately ,but im using a deflection bridge
which at some sensor extreme range may produce a negative differential voltage(less than -0.2V).I can design the bridge for this not to occur.Infact i was thinking to use that -ve voltage as signal that my sensor is under extreme conditions,thats why i was wondering what would happen if a -ve voltage appeared on an analog input pin.I didnt try it till now because I cant afford to burn my PIC

 

[ericgibbs]

In the datasheet it doesnt specify what happens in such circumstances,else i wouldnt ask the question.
Im not trying to put a -ve voltage deliberately ,but im using a deflection bridge
which at some sensor extreme range may produce a negative differential voltage(less than -0.2V).I can design the bridge for this not to occur.Infact i was thinking to use that -ve voltage as signal that my sensor is under extreme conditions,thats why i was wondering what would happen if a -ve voltage appeared on an analog input pin.I didnt try it till now because I cant afford to burn my PIC

hi,
Understood, I thought you were asking what if!.

I would expect the PIC to tolerate a -0.2Vdc signal without any problems.
I'll double check the PIC datasheet to make sure, will get back to you.

If the ADC input could go over say -1Vdc I would suggest a 220R in series with ADC input and a schottky diode from the ADC input to 0V, in order to clamp the pin at -0.4V

 

[vinke]

I would expect the PIC to tolerate a -0.2Vdc signal without any problems.
I'll double check the PIC datasheet to make sure, will get back to you.

If the ADC input could go over say -1Vdc I would suggest a 220R in series with ADC input and a schottky diode from the ADC input to 0V, in order to clamp the pin at -0.4V

Hi eric,
was thinking that it could be tolerated too.but my point is would the adc module read that -ve voltage as zero or something else.Ive been trying to simulate it using oshon soft simulator but the analog input cannot be decreased further below zero.

 

[ericgibbs]

Hi eric,
was thinking that it could be tolerated too.but my point is would the adc module read that -ve voltage as zero or something else.Ive been trying to simulate it using oshon soft simulator but the analog input cannot be decreased further below zero.

hi,
I would expect the PIC/ADC to see the -0.2V as 0.0V

I also use Oshonsoft, I believe the way Vladimir has written the software it wouldnt recognise it anyway.

Did you see the analog.zip for Oshonsoft I downloaded today to this forum?

The thread is named 'Oshonsoft Simulator'

https://www.electro-tech-online.com/threads/oshonsoft-pic-simulator.32483/

 

[vinke]

hi,
I would expect the PIC/ADC to see the -0.2V as 0.0V

I also use Oshonsoft, I believe the way Vladimir has written the software it wouldnt recognise it anyway.

Did you see the analog.zip for Oshonsoft I downloaded today to this forum?

The thread is named 'Oshonsoft Simulator'

https://www.electro-tech-online.com/threads/oshonsoft-pic-simulator.32483/

ok thanks dude.
No havent seen it,but ill download it and have a look.
Neways ill try and put a negative voltage while limiting the current and see what happen.Let u know afterwards.

 

[Hero999]

If you really want to measure negitive voltages then hook up a potential divier from the input to +5V. If you use say two 10k resistors, then it'll give 2.5V at 0V and 0V at -5V.

 

[eng1]

I would expect the PIC to tolerate a -0.2Vdc signal without any problems.

Well, he said "less than -0.2 V"... The minimum voltage is -0.3 V with respect to Vss (BTW which datasheet are you reading? which PIC?), but that's an 'absolute minimum'. I wouldn't think about applying a voltage below Vss (or Vref-) and above Vdd (or Vref+) though.

im using a deflection bridge which at some sensor extreme range may produce a negative differential voltage(less than -0.2V).

How are you measuring a differential voltage with a PIC? can you give more details? a schematic?
A negative differential voltage does not imply that the PIC has a negative voltage with respect to Vss on its analog inputs.

 

[ericgibbs]

Well, he said "less than -0.2 V"... The minimum voltage is -0.3 V with respect to Vss (BTW which datasheet are you reading? which PIC?), but that's an 'absolute minimum'. I wouldn't think about applying a voltage below Vss (or Vref-) and above Vdd (or Vref+) though.

How are you measuring a differential voltage with a PIC? can you give more details? a schematic?
A negative differential voltage does not imply that the PIC has a negative voltage with respect to Vss on its analog inputs.

hi eng1,
Are you asking me these questions or the OP?

 

[eng1]

Hi Eric, I was asking the OP.

My point is if the bridge is powered by a single supply source, the differential output can be negative but the two output voltages must be positive with respect to ground, do you agree?

 

[ericgibbs]

Hi Eric, I was asking the OP.

My point is if the bridge is powered by a single supply source, the differential output can be negative but the two output voltages must be positive with respect to ground, do you agree?

hi,
If its a bridge with a single positive supply I agree.

I'm not sure what opa configuration he is using.?

 

[vinke]

Hi Eric, I was asking the OP.

My point is if the bridge is powered by a single supply source, the differential output can be negative but the two output voltages must be positive with respect to ground, do you agree?

Im not connecting the differential voltage directly to the pic.im passing it through an intrumentation amplifier first then the output of the instrumentation amplifier to the analog input of the pic. if the differential output becomes negative will the output of the instrumentation amplifier become negative too?
Im balancing my bridge at the max resistance of the sensor which is a soil moisture sensor(simply two electrodes separated by 1mm apart).Im doing this because i want my bridge to output 5V when soil is wet (100% humid) and 0V when soil is dry(0% humid) ,as soil resistance decreases with increasing humidity. But the max resistance of soil seen by the electrodes is not constant ,it may increase,and if it increases above the value ive set in the bridge,the differential output will become -ve.Ive been thinking to solve that problem by placing a resistor,Rohms, in parallel with my electrodes so that when my electrodes reads a resistance greater than R,the resultant resistant seen by the bridge will never exceed Rohms.hence the differential output will never be negative,at the expense of a lower bridge output range.

 

[ericgibbs]

hi vinke,
Is the instrumentation amp powered from a dual supply or single.?

I assume you are powering the bridge from a single +V referenced to 0V?

The bridge output at balance should be +V/2.

 

[vinke]

hi vinke,
Is the instrumentation amp powered from a dual supply or single.?

Single supply.normally if the differential input becomes negatives so should be the output.

I assume you are powering the bridge from a single +V referenced to 0V?

Yep,a single supply

The bridge output at balance should be +V/2.

the output with reference to the ground!
that is a differential output of 0V (V/2-V/2)