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Xbee and Analog Output K-Type Thermocouple Amplifier - AD849
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Xbee and Analog Output K-Type Thermocouple Amplifier - AD849

by james_wh_gibson on Mon Jul 31, 2017 5:53 pm

Parts being used:

https://www.adafruit.com/product/1778 - Analog Output K-Type Thermocouple Amplifier - AD8495 Breakout
https://www.adafruit.com/product/270 - Thermocouple Type-K Glass Braid Insulated - K                     
XBee ZB module

I am having a strange issue when I try and read the XBee ZB module pin value (AI) coming from the Thermocouple Amplifier.

Every time I read the value at the pin I see "1023". I have tried using AI0, AI1, and AI2 and all of these have been set to 2 so I can read the analog value, but still all read 1023.

The reason why "1023" is not right is because it is a fixed value, ie when I heat up or cool off the Thermocouple the value does not change. Also from the voltage conversion formula it looks like the maximum allowable value:

To convert the A/D reading to mV, do the following: AD(mV)= (ADIO reading/0x3FF)*1200mV 
The reading in the sample frame represent voltage inputs of 1144.9 and 342.5mV for ADIO0 and ADIO1 respectively.
Reference : http://ftp1.digi.com/support/documentat ... zigbee.pdf - Page 28 

0x3FF == 1023

Some other information:
- I am powering the AD8495 with 3.3 volts

After speaking to Digi Support they suggest the thermocouple (AD8495 ) may not be providing output voltage with in prescribed limits of XBee ZB module i.e. under 1.2 volts.

Is this the case?

james_wh_gibson
 
Posts: 2
Joined: Mon Jul 31, 2017 5:43 pm

Re: Xbee and Analog Output K-Type Thermocouple Amplifier - A

by adafruit_support_bill on Mon Jul 31, 2017 6:05 pm

When powered with 3.3v, the output range of the AD8495 is 0v-3.3v. Since the XBee has an input range of only 0v-1.2v, you are probably just overloading the input.

You could use a voltage divider to reduce the input voltage to the XBee: https://en.wikipedia.org/wiki/Voltage_divider

Using a Z1 value of 20K and a Z2 value of 10K, you would change a 0v-3.3v range at Vin to a 0v-1.1v range at Vout.

Image

adafruit_support_bill
 
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Re: Xbee and Analog Output K-Type Thermocouple Amplifier - A

by james_wh_gibson on Mon Jul 31, 2017 8:32 pm

Thanks. This seems to have solved the problem.

james_wh_gibson
 
Posts: 2
Joined: Mon Jul 31, 2017 5:43 pm

Re: Xbee and Analog Output K-Type Thermocouple Amplifier - A

by adafruit_support_bill on Mon Jul 31, 2017 8:37 pm

Good to hear. Thanks for the follow-up.

adafruit_support_bill
 
Posts: 62902
Joined: Sat Feb 07, 2009 10:11 am

Please be positive and constructive with your questions and comments.