ADC Selection

200A across a 1mOhm shunt generates 200mV and dissipates 40W -- can you stand that much power in your shunt? 200A / 5mA = 40,000. That implies that a 16-bit ADC will have sufficient resolution, assuming that you're not misusing the word. You can expect both precision and accuracy (yes, they're different words) to be worse than the resolution.

You want to take your 200mV +/- 5uV signal from the shunt resistor and amplify it by an order of magnitude to match the reference voltage of your ADC. Assuming it's a 2.5V reference, that's a gain of 12.5. You'll need an amplifier whose offset and temperature drift are commensurate with that 5uV resolution, even when it's sitting next to something with

200A flowing through it. Even if your resistor has one end at "ground" you should contemplate how meaningful "ground" is when there's 200A running around. Then you should make a Kelvin connection to the shunt, and feed that into a differential amplifier that can take whatever sort of common-mode voltage you've got on the shunt.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

nt

The 200A will probably never happen, and if it does it'll only be around fo r a few msec.

I need to make two of these measurements as close to simultaneously as I ca n get. At first I was thinking about using a sigma-delta, something like t he LTC2442, but I can't tell how quickly the MUX switches, or how close (in time) the voltage readings are to each other. Do you know what parameter o n the datasheet tells me that information?

or maybe a SAR with a mux input is a better option... maybe an 18bit SAR wi th a MUX input...

thanks!

is this AC only? if so, use a cheap soundcard input, you get at least two channels, simultaneous [within the delay of your input filtering differences] know it's usually better than one tenth of 1/48000 ?? what

2uS, plus you can calibrate down closer. very useful if you're trying to preserve phase to measure power through this thing. uh, be careful of AC mains isolation etc.

No it's a DC application, power monitoring

Unless otherwise stated, it's the conversion time. Sigma-delta converters do a lot of tricks to increase accuracy at the expense of latency and oddball filtering -- it's a good idea to read the data sheet VERY CLOSELY, and make damned sure you know what you're looking at.

I'll try not to give you the full "use engineering units" scold, but "as close to" isn't very informative -- one person's "so close it doesn't matter" may be a year, while someone else's may be a picosecond. If you really want to get as close to simultaneous as humanly possible, then build a pair of sample & hold circuits (I assume you need to take measurements on two separate resistors). Sample at the ideal instant, then ADC convert at your leisure (relatively, because S&H circuits drift). Simultaneity will be determined wholly by the characteristics and matching of your S&H circuits.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

You may still be able to take advantage of sound card technology (simultaneous sampling on 2 channels). You can either use the same sort of chips they use, or modify an existing sound card by adding a diff-amp input as in .

Actually, it may be even simpler, in your case, if you only need DC voltages above ground. The circuit discussed above is to allow bipolar inputs to be referenced to the chip's Vref as 0V, eliminating the AC coupling caps. But if you simply eliminate the caps you should be able to measure 0-3V or so directly. You will need to insure that your measurement ground is the same as the chip/computer ground.

Note that the device mentioned is only about $15 from HDE (via Amazon), so you may want to buy a couple to have a backup in case your initial hacking goes astray. This may still be a lot cheaper than trying to buy the separate ADC and supporting chips, not to mention all the development time.

Best regards,

Bob Masta DAQARTA v7.50 Data AcQuisition And Real-Time Analysis

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