16F870 ADC voltage limits

[sardineta]

1.- I am planning to use the 16F870 microcontroller to acquiere analoge signals, but I really do not know what are the voltage limits, I mean if I have a have an analog singal at a frequency of 60 Hz what are the maximun Vpp the uC can support.

According to some examples I have found on the web, the maximum voltage is 5 volts as upper limit and 0 volts as the lower limit and this signal varies by a potentiometer.

2.- Thanks a lot for your comments Nigel. I found useful the tutorial I will begin to program as soon as I have resolver my electronic designs and that's another history.

By the way, I want to sample an analog voltage at a frequency of 60Hz
the amplitud can vary from 100mV to 8V. I think I can use an opamp configuration to reduce the high voltage, does one pin can sample the incoming signal mounted on the DC level, isn't it?.

However this signal comes from a differential transformer, do you think I can put my analog signal so to say 100mV, into a DC level between 0 to 5 volts in order to sample it.

then when I want to sample high AC voltage level I could make drop its amplitud to put it in a dc level and then sampling it.
Am I right?
Does anyone can help me with these questions
Thanks a lot

 

[Nigel Goodwin]

I am planning to use the 16F870 microcontroller to acquiere analoge signals, but I really do not know what are the voltage limits, I mean if I have a have an analog singal at a frequency of 60 Hz what are the maximun Vpp the uC can support.

According to some examples I have found on the web, the maximum voltage is 5 volts as upper limit and 0 volts as the lower limit and this signal varies by a potentiometer.

Does anyone can help me with these questions
Thanks a lot

As you mentioned above, the maximum is the +ve supply rail (usually 5V), and the minimum the -ve supply rail (usually 0V). You can set different reference points though (within certain limits), but the negative one has to be at 0V or above, and the positive one at 5V or below, with the positive reference obviously higher than the negative one.

The voltage limits don't really make much difference though, if you want to read lower voltages simply amplify them with an opamp, you can make a mid point zero reference as well in this way (so zero would be 512, lower than that would be negative, higher would be positive). For higher voltages simply use a potential divider to drop the voltage.

You might find my analogue PIC tutorial helpful?.

 

[sardineta]

DC Level on RA0/AN0

Thanks a lot for your comments Nigel. I found useful the tutorial I will begin to program as soon as I have resolver my electronic designs and that's another history.

By the way, I want to sample an analog voltage at a frequency of 60Hz
the amplitud can vary from 100mV to 8V. I think I can use an opamp configuration to reduce the high voltage, does one pin can sample the incoming signal mounted on the DC level, isn't it?.

However this signal comes from a differential transformer, do you think I can put my analog signal so to say 100mV, into a DC level between 0 to 5 volts in order to sample it.

then when I want to sample high AC voltage level I could make drop its amplitud to put it in a dc level and then sampling it.
Am I right?

 

[williB]

welcome Sardineta from City of the Eternal Spring

does one pin can sample the incoming signal mounted on the DC level,

i have read this several times , and still dont know what you mean?

 

[Oznog]

An op-amp is used to make a signal larger. You'd need a simple voltage divider to reduce the signal level.

Note that the divider should have an output impedance of 2.5k or less to ensure full accuracy of the PIC's ADC. Also take into account that a voltage on the ADC pin which is higher than Vdd or less than Vss can damage the pin. Ensure that your input does not surge too high at any time.

 

[sardineta]

The signal that I want to read is an analog signal, like the 127 Volts ac, found in the wall outlets. but mi signal is a small signal with a frequency of 60 Hz but the amplitud is around 300mV

according on what you said me, thanks a lot, I should do the following
add mi ac signal to a DC signal so to say 2.5V in torder to the ac signal be between Vdd and Vss.

The figure represents the design and the output.

Thanks a lot guys.
All are wlcome to Cuernavaca

 

[sardineta]

figure

 

[robin santos]

clamp up

i think you have to ad a dc value to your ac input in such away your total signal swings symetrically between vss and vcc
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