Little question about PIC16F

Thread Starter

FBorges22

Joined Sep 11, 2008
109
You don't need another 7805 to generate the VREF voltage. What you'll need is a 5V reference diode/module that is small and precise. You have to ask someone here for a particular part number. I only know of TL431 but there are more modern parts I don't know of that would do just the same function.
You mean that I should use a 5V zener diode to regulate the voltage?

The 741 Opamp will not function with just 5V total supply voltage. You'll need a rail-to-rail Opamp for this one. Or you can power the Opamp from the +12V supply. In this case, you are correct to add a 5V voltage clamp to the Opamp output.

It is difficult to recommend a part if you do not have your physical location filled in so we know which part of the world you're in. Also the transformer connection is missing a return path and your signal conditioner is "unusual" in that you have use the offset control pin to bias the Opamp output. People usually don't use that method but condition the Opamp inputs instead. Again you will need to ask for a suitable Opamp circuit here.
I am located in Sao Paulo - Brazil...

Your goal is to produce a +/- voltage swing at the Opamp output upon a fixed the 2.5V reference level for input to the ADC.

What is your design opamp output voltage pk-pk at the output of the Opamp? 4V pk-pk for 120V AC input?
Yes it is 4V pk-pk for the 120V AC input.... What software did you use to draw this simulation that you made?
 

eblc1388

Joined Nov 28, 2008
1,542
You mean that I should use a 5V zener diode to regulate the voltage?
No. Zener is worse than a 78L05. You can use Google and search for "Precision 5V reference" and that would give you a lot of choices. Some of them are hard to get and some are expensive. Perhaps you can use the REF02 instead. I don't know what part is on sale in your location. Perhaps other members can advise on a suitable part.

Yes it is 4V pk-pk for the 120V AC input.... What software did you use to draw this simulation that you made?
The software I use to do the simulation is the free & powerful LTSpice IV, which anyone can download from the Linear Technology Website here:

LTspice IV is a high performance Spice III simulator

You will also need the MC34071 model in order to simulate the circuit which I have included in the attachment. This file needs to be placed in the LTC\LTSpiceIV\lib\sub folder. Remove the .txt extension from the filenames as forum don't allow file ends in .sub extension.
 

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eblc1388

Joined Nov 28, 2008
1,542
What do you think of this regulator? MAX675
Yes, it will work. You can also trim its output voltage to 5.12V instead of the standard 5.000V so each bit of your ADC would mean exactly 5mV. This is acceptable even when you have a 5.0V PIC VDD supply because data sheet allows max. 0.3V over VDD for ADC Vref voltage.

I was thinking in using him with a active low pass filter...
The output of this regulator has low noise and is good enough to serve as the reference voltage for the ADC. Why would you want to add an active low pass filter?
 

Thread Starter

FBorges22

Joined Sep 11, 2008
109
The output of this regulator has low noise and is good enough to serve as the reference voltage for the ADC. Why would you want to add an active low pass filter?
Well... I am just being cautions...But since this regulator have low distortion I will not install the low pass filter... Tomorrow morning I will go to the shop to search this regulator...
 
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