All About Circuits Forum  

Go Back   All About Circuits Forum > Electronics Forums > The Projects Forum

Notices

The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

Reply   Post New Thread
 
Thread Tools Display Modes
  #1  
Old 09-24-2008, 07:08 AM
vortexmc vortexmc is offline
New Member
 
Join Date: Sep 2008
Posts: 3
Default 4 digit Auto-ranging Voltmeter

Hi everyone, this is a new project that i have undertaken, and i was hoping too get some advice on how to accomplish this.The project requires the construction of an auto-ranging volt meter that can measure positive DC voltages in the ranges 0v-1v, 0v-100mv and 0v-10mv. The display should 4 digits up to a maximum value of 1.000 volts. There should be an indication of some form when the input voltage is out of range. The one restriction is to use a PIC processor of 20 pins or less.
Reply With Quote
  #2  
Old 09-24-2008, 01:33 PM
beenthere's Avatar
beenthere beenthere is offline
Senior Member
 
Join Date: Apr 2004
Location: Missouri, USA (GMT -6)
Posts: 15,815
Blog Entries: 10
Default

You will need at least 10 bits for the resolution (don't expect the last two to be stable). Convert the voltage reading after conditioning, output the number to the display.

You will need some external hardware for the voltage ranging.
__________________
First comes the hardware, then the software.
Reply With Quote
  #3  
Old 09-24-2008, 03:02 PM
scubasteve_911 scubasteve_911 is offline
Senior Member
 
Join Date: Dec 2007
Posts: 1,202
Default

What about running an ADC and a DAC together?

The scheme would be, the DAC would be responsible for changing the reference voltage on the ADC. It would start out high, then climb downwards to autorange the signal. You need a really nice reference and a high resolution DAC for this, since you're relying on its precision.

The ADC should also be accurate. You need to run a slight-negative rail on the input stage. So, it would go, voltage input to clamping diodes on the input to an opamp buffer. The buffer should be powered below ground. Or, perhaps it is best to add a bit of known bias on to the input. This is because there is usually a minimum input voltage to the ADC. Also, you need to find low-input/output offset amplifiers/ADCs. Even a pretty good opamp has about +/- 1mV input offset voltage. You can get them down to about a microvolt though using a chopper-stabilized amp.

Steve
Reply With Quote
  #4  
Old 09-24-2008, 04:52 PM
liitlefan liitlefan is offline
New Member
 
Join Date: Sep 2008
Posts: 7
Default

Quote:
Originally Posted by scubasteve_911 View Post
What about running an ADC and a DAC together?

The scheme would be, the DAC would be responsible for changing the reference voltage on the ADC. It would start out high, then climb downwards to autorange the signal. You need a really nice reference and a high resolution DAC for this, since you're relying on its precision.

The ADC should also be accurate. You need to run a slight-negative rail on the input stage. So, it would go, voltage input to clamping diodes on the input to an opamp buffer. The buffer should be powered below ground. Or, perhaps it is best to add a bit of known bias on to the input. This is because there is usually a minimum input voltage to the ADC. Also, you need to find low-input/output offset amplifiers/ADCs. Even a pretty good opamp has about +/- 1mV input offset voltage. You can get them down to about a microvolt though using a chopper-stabilized amp.

Steve
i agree.great idea ,i like it .but it cost a lot ~~
Reply With Quote
  #5  
Old 09-24-2008, 05:32 PM
scubasteve_911 scubasteve_911 is offline
Senior Member
 
Join Date: Dec 2007
Posts: 1,202
Default

Quote:
i agree.great idea ,i like it .but it cost a lot ~~
This is how I roll when I design

I can design a lot of things really poorly, but the challenge is doing it well. Although, there are advantages to making something inexpensive.

OP,

You PM'd me about clarifications, but I would like to keep the discussion public so that others may benefit.

If you want to design a good autoranging meter, you should be using a microcontroller. The microcontroller is loaded with code to execute the task at hand. There are many available to use, any relatively modern one will work for this project. I recommend Cypress PSoC microcontrollers, since they're easy to use. You can get a programmer and a mini-development board from Digikey for about 55$

http://search.digikey.com/scripts/Dk...me=428-1585-ND

It may be a bit much to spend right away, but think of all the fun things you will be able to do. I learned how to program with the microcontroller in both assembly and C when I was 17. Now, I use more powerful microcontrollers, but you need to spend money to get the debugging tools and software.

I would use a quality analog-to-digital converter. In your case, you do not need to take samples very quickly. Resolution is the most important factor, since you want a bit of precision. There are many errors associated with ADCs and many books written on the subject, so try to find the lowest DNL, INL possible. There are other things, but these are probably the most important.

http://en.wikipedia.org/wiki/Analog_...tization_error

So, basically, an ADC will give your microcontroller a code that is a function of the reference voltage you supply and the input voltage. I am suggesting that you change the reference voltage dynamically with your input voltage. For example, an ADC has a 5V reference applied and a 2.5V signal is input. This gives you a half-scale digital code. If an 8-bit converter is used, you should get 10000000 in a perfect world. Realistically, you can get steps of (5V / 256 (8bits) roughly 19.53mV. So, your input voltage is now ambiguous. Depending on what sort of precision you require, you should increase the resolution to achieve this.

An autoranging ADC can do a bit better. For instance, if you had a 1V reference instead of a 5V reference, you will get 5 times better resolution. You can provide a varying reference voltage by means of a DAC. Clearly, your fully relying on the precision of the DAC, so it needs to be as accurate as possible. So, high resolution with low drift. Preferably one that you can attach a really nice voltage reference to.

The process would be, you write the maximum voltage you would expect to the DAC. Measure the ADC, then find out where you should set your DAC to. Maybe keep within sensible ranges, for easier calculation.

The main problem with measuring signals that are extremely low is that opamps and ADCs don't like anything near their supply rail. So, if you're powering by a single supply, then you might only be able to get with 10mV of each rail at best. This is why I propose using an opamp meant for dual supply operation and bringing the negative supply input below ground. You should be buffering the input before the ADC to guarantee enough output impedance to drive the ADC, plus an anti-aliasing filter (either in the opamp circuit, or after before the ADC). Look up this term and research the need, it's beyond the scope.

Another good way to do this is to have many opamps with different gain. You can fix the reference to a set voltage. Microcontroller usually come with comparators with different voltages to select from, or at least a crappy ADC. You can read in directly what voltage it is, then select an appropriate gain stage with analog switches. You can then figure out what the voltage is knowing the gain setting, fixed reference voltage, and ADC value. Of course, this can be a bad idea for high precision because of the variance of offset voltages encountered, gain errors, tempcos of gains, and overall matching of the circuit.

Or, the absolute simplest way would be to bring in a voltage directly into the microcontroller's ADC with a 1.25V reference. You need to protect the inputs with some zener diodes to supply rails and have a resistance in series beforehand. This wouldn't be good for any high impedance voltage source to be read though, so consider adding a buffer first. You can also go to town with multi-point averaging to rectify any random noise you might encounter.

Good luck,

Steve
Reply With Quote
  #6  
Old 09-24-2008, 06:07 PM
vortexmc vortexmc is offline
New Member
 
Join Date: Sep 2008
Posts: 3
Default

Thank you steve for your clarifications, it makes alot more sense with what tou have said, i have chosen to use a PIC 16f690 microprocessor. Would u mind giving me a couple of ideas regarding the code, steps i should follow, in conjuntion to the auto-ranging, and with the general approach to the project.Again thank you
Reply With Quote
  #7  
Old 09-24-2008, 09:38 PM
scubasteve_911 scubasteve_911 is offline
Senior Member
 
Join Date: Dec 2007
Posts: 1,202
Default

You're welcome. I would try to do a block diagram of all the functional parts of the system. Then, fill in the blocks with actual components. Produce a schematic of the work, then post it to ensure that you didn't miss anything.

When I work with micros, I treat them as black magic boxes that will do what I want in the end. As long as the IOs are able and my processing/storage/timing demands are reasonable, then it is a safe bet. Then, I build the hardware and start trying to build the software 'framework'.

This approach has been good to me, so I suggest you do something similar.

Steve
Reply With Quote
  #8  
Old 09-24-2008, 11:27 PM
Pich Pich is offline
Senior Member
 
Join Date: Mar 2008
Location: Vancouver Canada
Posts: 119
Default

Try this link for your ADC converter, http://www.selectronic.fr/includes_s...il/ICL7109.pdf
it will handle 200mv and 2 volts at 4 digit resolution, in order to diplay 20mv a chopper stabilized opamp will be needed as mentioned by scubasteve 911 with a gain of 10 using the 200mv setting on the ADC
Reply With Quote
  #9  
Old 09-25-2008, 12:28 AM
scubasteve_911 scubasteve_911 is offline
Senior Member
 
Join Date: Dec 2007
Posts: 1,202
Default

This ADC is perfect for you! Oh, and I forgot to suggest, I would extend your measuring range a bit, 0-1V is a bit sad. You can do at least 0-5V and it will be much more impressive.

http://www.linear.com/pc/downloadDoc...52,P1636,D1887

Linear Tech LTC2400

Steve
Reply With Quote
Reply   Post New Thread

Tags
, ,


Related Site Pages
Section Title
Worksheet Digital logic signals
Textbook Basic voltmeter use : Worksheet
Worksheet DC metrology
Worksheet Design Project: Voltmeter
Worksheet Voltmeter design
Textbook Rate-of-change indicator : Dc Circuits
Textbook Potentiometric voltmeter : Dc Circuits
Textbook Voltmeter usage : Basic Concepts And Test Equipment
Textbook Multimeters : Dc Metering Circuits
Textbook Voltmeter impact on measured circuit : Dc Metering Circuits


Similar Threads
Thread Thread Starter Forum Replies Last Post
Digital voltmeter with pic dhc7 The Projects Forum 18 06-26-2008 08:27 AM
Assembly Program (digit conversion) frogacult Programmer's Corner 4 05-11-2008 05:15 PM
Relative addressing mode problem with HC05 JohnnyD Embedded Systems and Microcontrollers 2 07-06-2007 11:23 PM
Help with High Impedance Voltmeter snarf The Projects Forum 32 06-13-2007 08:01 PM
Digital voltmeter Mr.Smoke The Projects Forum 5 06-02-2006 11:36 AM

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT. The time now is 06:24 PM.


User-posted content, unless source quoted, is licensed under a Creative Commons Public Domain License.
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.