Hi All, I am designing an Amp-Hour meter to log the current going into my solar batteries, which is charged by a PWM regulator. The regulator is pulsed at 100- 150hz. In my design I am using a shunt resistor to measure the current and a micro calculate and display the Amp-hour value. I need the current reading to be =
You are using this on DC I asssume.
20 * 0.005 = 0.1VYou need a low offset voltage op-amp to amplify this up to a better voltage for the ADC's input. Since this is DC, you don't need much bandwidth. If you want the most accurate results you can get, look at chopper stablized op-amps.
Next you need to limit the band width of what you put into the ADC to be much less than 1/2 the rate the ADC is sampling at. Again since this is DC, you don't need to provide much bandwidth. The cut off frequency of the filter will be set by how large of capacitors and resistors are practical. You want to use either plastic or NPO/COG capacitors.
Once you have figured out the filter's bandwidth, multiply that by, lets say, 10 and you have the minimum sample rate.
Chances are you will want to use a 12 bit ADC. The accuracy you need comes from its offset voltage and its linearity not simply from the number of bits.
Assuming you have a micro reading the ADC, you can further improve the readings by averaging the ADC output over 0.1 second periods. This filters out most of the AC hum you may have picked up.
something like this maybe?
martin
Marky hath wroth:
If you want to buy one instead of designing one from scratch, see:
You might want to compare specifications.
Well 1% is 100 "samples" over the range. Good form is to have +/- 1 LSB accuracy, so that's 200 "samples". 256 bits is 8 bits of resolution.
The 100-150Hz will result in some calculation problems, so it's probably best to just integrate the detected current across your shunt with a big cazapitor, and deal with slow changing DC instead of rapidly pulsing DC.
Full scale voltage = 20A * 0.005 ohms = 0.1V A gain of 20 will give you 0 to 2 volts to feed to the A/D. No clue what you're using for an A/D converter but whatever you select will need to work with a 0 to 2V input range.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
No reason, I thought it was a good starting point.
I will give this a try.
Interesting, I will order a freebie sample from TI to play with.
I have plenty of 10k and 200K resistors in my tool box.
Thanks guys, your help is appreciated!!
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