Ordered the wrong ADC and need help...
MarkS
Posts: 342
I was following the LM34 temperature sensor and ADC from the Industrial Supply text, but wanted a 12-bit ADC instead of the 8-bit ADC0831 that is shown in the text. I did a little research and found the ADC LTC1286, which matches the 0831. With that in mind, this is what the circuit would look like:
Pretty much the same as shown in the IC text.
However, I accidentally ordered the LTC1298. The pins are quite a bit different. After reading the data sheet, to the best of my ability, I came up with this circuit:
It seems like it should work, but I'd like some input before I blow something up. Also, the ADC requires 5v inputs to operate properly. How do I interface this to the Prop? I can just use a resistor for the DOUT pin, but how do I raise the 3.3v from the Prop to 5v required by the ADC? Would pull up resistors work? I'm not to clear on this after reading the sticky. Is this correct:
Pretty much the same as shown in the IC text.
However, I accidentally ordered the LTC1298. The pins are quite a bit different. After reading the data sheet, to the best of my ability, I came up with this circuit:
It seems like it should work, but I'd like some input before I blow something up. Also, the ADC requires 5v inputs to operate properly. How do I interface this to the Prop? I can just use a resistor for the DOUT pin, but how do I raise the 3.3v from the Prop to 5v required by the ADC? Would pull up resistors work? I'm not to clear on this after reading the sticky. Is this correct:
Comments
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There are a couple of things...
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As long as you don't need the differential input that the LTC1286 offers ...edit: I see that the LTC1298 has a differential mode·
and as long as your VREF can be the same as VCC
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...Then the LTC1298 should work just fine.· On the LTC1298, VREF "IS" VCC and you shouldn't adjust it.
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The input threshold of the LTC1298 is 2V, so CS(Pin 1), DIN(Pin 5),·and CLK(Pin 7) should be just fine with a direct connection from the Propeller.
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For the DOUT(Pin 6), an equal balanced·voltage divider would be fine.· The·input Threshold of the Propeller is about·1.65V so 2.5V output from the divider will be plenty.
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1K 1K Dout (0V to 5V) >---/\/\---o---/\/\---< GND | o----------> To Prop (0V to 2.5)·Reference:
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1155,C1001,C1158,P1445,D2760
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Post Edited (Beau Schwabe (Parallax)) : 1/24/2008 4:00:24 PM GMT
I find the digital temperature sensors are ALOT easier to use.
Something like the MAX6626 has an I2C interface and a resolution of 1/16 of a degree.
Bean.
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www.iElectronicDesigns.com
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Post Edited (MarkS) : 1/24/2008 6:40:06 PM GMT
With the LTC1298, you don't set the span.· The VCC (Which is also the VREF) of the LTC1298 can only tolerate 4.5V to 5.5V and still maintain accuracy.
The only way to adjust the span would be to scale the result in software.· Since you have 12-bits (1.2mV resolution) compared to 8-bits (19.5mV resolution), software scaling may not be an issue in this case.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
http://www.sensorguys.com/STSWDO.html
The TO92 is there and also encased in a Stainless tube..(maybe overkill for your app)
The Smartec sensors are very hard to beat..
Way more accurate and cheaper··· just measure the duty cycle of the sensor
apply some mathematics and your up and running..
Cheers· Ron Mel OZ
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