Analog Volts

ZZX · Guest

Hi forum,
can the analog port read more than 5v, say 12v signal. If not, i can scale the voltage down using the divider circuit to read it. problem is that i have some resistive network further down the line and the current drops due to this divider circuitry.
Any suggestions? How can i read it without this problem?

asmallri · Joined: 12 Aug 2004 Posts: 1635 Location: Perth, Australia

rnielsen · Joined: 23 Sep 2003 Posts: 852 Location: Utah

The specs usually state that the input can reach .3V above VDD which the max is around 6.5v - 7.5v, depending on the PIC you are using. Anything above this and you will, most likely, fry the PIC and let out all of the magic smoke.

Ronald

asmallri · Joined: 12 Aug 2004 Posts: 1635 Location: Perth, Australia

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

ZZX · Guest

Thank u all for all the valuable suggestions,
Heres precisely something im doing :

14V --------/\/\/\/\/\---------- to load
14A | Rnet |
| |
| |
Div Network1 Div Network2

i.e im reading the voltages on both ends of Rnet(6.25milli ohms) and is meant to calculate the current. Now if my divider network 1 and two both have 3 resistors each of 4.7k, and not high impedance ones, since most current would flow straight to the load. Do u guyz think this solution would work provided i choose apropriate load which will be calculated as:

P= V x I
then R=P/(I)^2

ZZX · Guest

*The dividers in the schematic are at the ends of the Rnet resistors;

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

This article describes your exact situation. How to monitor voltage across a small shunt resistor in a +12V bus using an A/D that reads a small voltage relative to GND.
http://www.electronicdesign.com/Articles/ArticleID/12629/12629.html

Look for the "Associated figure" link near the bottom.
_________________
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ZZX · Guest

Hi folks,
Im back with the issue. I tried reading voltage across a shunt using dividers. Each divider has 3 x 4.7k Resistors, so 11.44 volts at both ends of the shunt gives about 3.8V across each resistor. Now everthings at hand except a small problem. In my divider circuit, one side measures 3.78v while the other 3.79 volts. This creates a problem when i upscale it i.e x3 to know the actual voltage. Hence at the terminals when the actual voltages are 11.44,11.44(across shunt), here i get 11.37 and 11.34v(upscaled calculated). ive used 1% tol resistors here.
Can anybody suggest how to reduce the gap? (0.03 difference )

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

Eugeneo · Joined: 30 Aug 2005 Posts: 155 Location: Calgary, AB

It looks like you have a power supply of 14 volts. Could you use an opamp that accepts inputs of 14 volts then follow and divide?

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

There are dedicated chips that will do this much better and cheaper than hand matching resistors. Look at Maxim MAX471 & MAX472. Maxim even gives free samples at their web site.

If you really want to roll your own try either of these two designs from the article I mentioned before.
http://www.electronicdesign.com/Files/29/12629/Figure_01.jpg
_________________
The search for better is endless. Instead simply find very good and get the job done.