I need to read the pressure from a transducer that puts out 4-20 mA. I was planning on putting an ADC across a precision 250 Ohm resistor to read the voltages from 1v to 5v. I need atleast 16 bits of resolution. I have been searching data pages for a good 16bit ADC but most ratiometric ones are limited to 2.5v. I use an AD7730 24 bit for some projects but it is limited to 80 mv I believe. Do I need to use a Voltage divider to get the voltages low enough or are there any 16 bit ADC that can handle the 1-5V? I dont need very fast output, say less than 20 Hz.
4-20 ma ADC IC needed
Nov 15, 2007
5 Replies
Why not use a 100 ohm resistor and a 2.5 volt ADC? AD7793 would work, and has an internal reference.
John
No, don't do that. It is much simpler to stick with that 250 ohm resistor...
There should be, but, if not, you can always go custom...The important thing here is to avoid off-value resistors or voltage dividers.
LOL. What a card you are. ;-)
Best regards, Spehro Pefhany
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The serial AD7680 does 16 bits at upto 5V.
Precision resistors don't benefit from heating; your peak dissipation in that resistor would be a tenth watt, which isn't (IMHO) low enough. Remember, a 0.1% resistor has temperature dependence up to six bits of your 16-bit converter!
I'd also be uncomfortable at changing the voltage bias on the transducer by four volts; an op amp, couple of resistors, and a pass transistor makes a better receiver, and holds the sense terminal at pseudoground. Such a receiver is inverting, and a second (also inverting) gain/offset stage can condition the signal for your A/D converter.
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