negative voltage problem

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amredries

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measure negative voltage microcontroller

i am design simple project using 16f877 it is used for measure 4 voltage the problem is this voltages are change from -2 volt to 5 volt but i found that my pic read any negative voltage aas zero so Any one can help me to measure negative voltage ?
 

pic adc negative voltage

there is no way to measure negative voltag directly in pic.
You will need to perform signal conditioning in H/W.
You will need to have +v dc shift then you will need to compress your voltage range to be within 0 to 5 volt.

This is simply done using Opamp circuits
 

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    amredries

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pic negative voltage

I think you can use negetive and positive referenc pin s.
 

measuring a negative voltage with pic

You CANNOT- MUST NOT use a negative Voltage Reference with the PIC16F877(A)
see data sheet (page 194):
https://ww1.microchip.com/downloads/en/DeviceDoc/39582b.pdf


VREF Reference Voltage (VREF+ – VREF-) 2.0MIN -- VDD + 0.3VMAX

VREF+ Reference Voltage High ........ AVDD – 2.5VMIN -- AVDD + 0.3VMAX

VREF- Reference Voltage Low ........ AVSS – 0.3VMIN -- VREF+ – 2.0VMAX

VaIN Analog Input Voltage ........VSS – 0.3VMIN -- VREF + 0.3VMAX

ZaIN Recommended Impedance of .......... 2.5kΩ MAX

regards ... Polymath
 

measuring negative voltages

You can not use negative voltage with PIC rather you hace to shift the voltage from -2 to +5 to 0 to +7 using level shifet using clampler circuit using opamp.
The otput of clamper or level shifter may be used with PIC MICROCONTRLLER.
 

mega measure minus voltage

That is a very common issue since ADCs are usually unipolar. Time ago I saw at Maxim website a simple but effective 3 Rs front-end solution which address your problem: ADC input connected to a voltage divider from Vcc (2 Rs) and another R from there to your input (negative sometimes) choosing proper R values you move lower negative input to 0V at ADC input. Of course ADC input span won't fit full-scale if signal is not amplified after R frontend but if you don't need full resolution could be unnecessary to add an op.amp. circuit.

Cheers ;-)
 

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