View previous topic :: View next topic |
Author |
Message |
naseerak
Joined: 08 Nov 2008 Posts: 138
|
Posted: Thu Jun 06, 2013 7:42 pm Post subject: ADC linearity |
|
|
Hello all
I want to make a milli voltmeter to measure the voltage across a shunt, I think I will use vref of 1 volt by divider resistors(1%) but have some concerns about the linearity of ADC module in Atmega8
(BASCOM-AVR version : 2.0.7.6 ) |
|
Back to top |
|
|
i.dobson
Joined: 05 Jan 2006 Posts: 1570 Location: Basel, Switzerland
|
Posted: Thu Jun 06, 2013 7:48 pm Post subject: |
|
|
Hi,
Read the pdf specification for the chip. and use a opamp to amplify the Signal.
Regards
Ian Dobson _________________ Walking on water and writing software to specification is easy if they're frozen. |
|
Back to top |
|
|
naseerak
Joined: 08 Nov 2008 Posts: 138
|
Posted: Thu Jun 06, 2013 8:41 pm Post subject: |
|
|
adding opamp will betray the accuracy actually this is an milliohm meter |
|
Back to top |
|
|
AdrianJ
Joined: 16 Jan 2006 Posts: 2483 Location: Queensland
|
Posted: Thu Jun 06, 2013 11:25 pm Post subject: |
|
|
Maybe we should take this discussion to forum Various, as it has nothing to do with Bascom.
The linearity of any reasonable op-amp is at least several orders of magnitude better than the ADC in an AVR chip. And those ADCS are already within +/- 2 counts or so of full scale ( 4096 counts ) in 12 bit mode. What exactly do you mean by linearity ?
<edit>
Sorry, my mistake, the AVR ADCs are 10 bit, but still well linear in that range.
And very good for measuring differential inputs with settable gains on the input pairs. Makes millivolt measurements quite easy.
My favourite ADC if you want more accuracy is the Texas TCL2543 or its variations. 12 bit, 11 single ended inputs.
Use with something like LTC1013 opamps for differential low level inputs. Been making current shunt measurements like that for years. _________________ Adrian Jansen
Computer language is a framework for creativity
Last edited by AdrianJ on Fri Jun 07, 2013 1:47 am; edited 1 time in total |
|
Back to top |
|
|
Paulvk
Joined: 28 Jul 2006 Posts: 1257 Location: SYDNEY
|
Posted: Fri Jun 07, 2013 12:20 am Post subject: |
|
|
Hello naseerak
I have tried to make a milli voltmeter to do this same job but the AVR ADCs are not good enough you need 12bits as a minimum 14bits to get good results. I tried all the methods oversampling ect but it did not work so I have been working with 12bit I2C ADCs you can find the code here for one of them http://www.mcselec.com/index2.php?option=com_forum&Itemid=59&page=viewtopic&t=11430
I have also 14bit and 16bit I2C ADCs that I am going to get working. With the meter I am building I will use the a ACS712ELC this is an IC that puts out a voltage in proportion to the amps flowing through it also it is isolated from the current it is measuring. In the tests so far with the device in the watering system blog I have had stable 5 digit readings from the 3volt back up battery for the real time clock. The other advantage of the device is its inbuilt high precision voltage reference.
Regards Paul |
|
Back to top |
|
|
naseerak
Joined: 08 Nov 2008 Posts: 138
|
Posted: Fri Jun 07, 2013 8:05 pm Post subject: |
|
|
Hi
Current measurement is not a problem here. I want to make some DIY shunts from iron wires which have relatively increased resistance compared to copper and aluminum wires, though 75mv shunts are available here but are expensive for the products which have to be mass produced. |
|
Back to top |
|
|
AdrianJ
Joined: 16 Jan 2006 Posts: 2483 Location: Queensland
|
Posted: Tue Jun 11, 2013 1:48 am Post subject: |
|
|
Pure metal shunts are a bit of a problem. The temperature coefficient of most pure metals is far too high for accurate work. Thick film resistors using somethig like nichrome or magnanin are avaialable, and relatively cheap.
See something like:
http://au.element14.com/meggitt/mr-tl3ar047f/resistor-nw/dp/9973354
But of course it depends on your application. _________________ Adrian Jansen
Computer language is a framework for creativity |
|
Back to top |
|
|
|