Incorrect value coming from remote XBEE. ADC input max?

[jaymer]

I have a simple MCP9700A hooked up to a remote XBEE Series 2.
Actually 3 thermistors.
2 work fine, but one is in a "oven" type environment and although I can PHYSICALLY measure the voltage coming from the MCP as 1.53 mv, for some reason the XBEE is only sending a value of "3" and "255" as the MSB and LSB,.. which equates to a 1000 mv and thus 158 degrees F. My oven is between 200 & 225, well within the limits of the thermistor.

OK, so the MCP is working perfectly, but why would the XBEE ignore the input voltage? Basically truncatating it. Is there something with the ADC on the XBEE. Did I miss a configuration in XCTU? Its too tidy to be sending EXACTLY x03FF.

thx

[adafruit_support_bill]

0x3FF does sound suspicious. I'd suspect a configuration problem. I'm not up on the Series 2 modules. But the Digi folks would certainly know: http://www.digi.com/support/forum/

[jaymer]

I think the answer is that the XBee only has a 10-bit ADC.

I'd be happy with cutting my input voltage coming from the thermistor. If there is a way to halve it, then I'll double it in my python program.

[adafruit_support_bill]

But your input voltage is only 1.53 mV as it is. You won't max out the 10-bit range until your voltage exceeds VREF. Have you connected the VREF pin?

http://www.digi.com/support/kbase/kbase ... tl?id=3522

The Series 1 XBee (802.15.4 and DigiMesh 2.4), XBee-Pro 900, and XBee-Pro 868 have a VREF pin that must be connected in order to use ADC samples. Typically this is connected to VCC so that your ADC range is between 0 and 3.3V. Even if you are not using an XBee that requires a VREF, it is recommended to connect VREF to VCC in order to provide compatibility with other XBees.

[jaymer]

I'm Xbee Series 2
I'm using your XBee adapter board.
I have a 5v power supply feeding my project board.
I soldered leads into the adapter board for 3 inputs coming from the thermistor output pins.

thx

[adafruit_support_bill]

The Series 2 has an internal 1.2v analog reference. Is your sensor output 1.53mV or 1.53V?

[jaymer]

Volts
Not mV. At 225f I'm already over 1000mv

At 1.53mv I'd be dead. 500mv is freezing. The data sheet for the mcp9700 shows way over a volt for 100c.
I can't remember, but my ambient readings on a mcp9700 in the project board is 650ish I think.

I could post my python code, but that's not the issue since the hex bytes observed from the XBee stop rising after x03FF

Thx for your help

[adafruit_support_bill]

If it is volts, than you are over-range. You can use a voltage divider to bring it back in range.
http://en.wikipedia.org/wiki/Voltage_divider

[jaymer]

ok, then i'm def. over range.

This is a common cheapie MCP9700 and a stock xbee and adapter board.
I guess since most people use it for temps under 100F they never run into this problem... its only a problem for me over 158F.

I'll divide the voltage - that should work perfectly.

thx
jaymer...