Hooking up a 10K NTC Thermistor to an LTC2440

hello everybody,

disclaimer: not , electronics expert, knowledge limit please bear me

i'm doing research find limit of cooling solution i'm using. i'd measure temperature stability, not accuracy.
i ordered linear dc570a demoboard in hopes hook 10k ntc thermistor easily.

i wrong.  =(

i go interface running using following topic link.

made adjustments , got following sketch

code: [select]

/*
 interface between arduino dm board , linear tech ltc2440 24-bit adc
 nov. 12 2010 john beale

  ltc2440  <---------->  arduino
  11: /cs <- digital pin 10  (ss pin)
   7: mosi <- digital pin 11 (mosi pin)
  12: miso -> digital pin 12 (miso pin)
  13: sclk <- digital pin 13 (sck pin)
  10: /ext - ground
   1: gnd - ground
   2: vdd3 - 5v supply

*/

#include <spi.h>  // include spi library

// set i/o pins used in addition clock, data in, data out
const byte slaveselectpin = 10;  // digital pin 10 /cs
const byte resetpin = 9;  // digital pin 9 /reset
const int nsamples = 5;  // how many adc readings average together

// spi_clock_div16 gives me 1.0 mhz spi clock, 16 mhz crystal on arduino

void setup() {
 
   serial.begin(115200);      // set serial comm pc @ baud rate
   
   pinmode (slaveselectpin, output);
   pinmode (resetpin, output);
   digitalwrite(slaveselectpin,high);  // chip select active low
   digitalwrite(resetpin,high);  // reset active low

   spi.begin(); // initialize spi, covering mosi,miso,sck signals
   spi.setbitorder(msbfirst);  // data clocked in msb first
   spi.setdatamode(spi_mode0);  // sclk idle low (cpol=0), mosi read on rising edge (cphi=0)
   spi.setclockdivider(spi_clock_div16);  // system clock = 16 mhz, chip max = 1 mhz
   
   serial.println("ltc2440 test");
}

#include <math.h>

float vcc = 5.00;                       // used display purposes, if used
// set measured vcc.
float pad = 10000;                       // balance/pad resistor value, set to
// measured resistance of pad resistor
float thermr = 10000;                   // thermistor nominal resistance

float thermistor(long rawadc) {
long resistance;  
float temp;  // dual-purpose variable save space.

resistance=((16777216 * pad / rawadc) - pad);
temp = log(resistance); // saving log(resistance) not calculate  it 4 times later
temp = 1 / (0.000831166375923117 + (0.000256436021065264 * temp) + (0.000000206045827449766 * temp * temp * temp));
temp = temp - 273.15;  // convert kelvin celsius                      

// begin- remove these lines function not display anything
//serial.print("adc: ");
//serial.print(sum);
//serial.print("/1677216");                           // print out raw adc number
//serial.print(", vcc: ");
//serial.print(vcc,2);
//serial.print(", pad: ");
//serial.print(pad/1000,3);
//serial.print(" kohms, volts: ");
//serial.print(((sum*vcc)/1677216.0),3);  
//serial.print(", resistance: ");
//serial.print(resistance);
//serial.print(" ohms, ");
// end- remove these lines function not display anything

// uncomment line function return fahrenheit instead.
//temp = (temp * 9.0)/ 5.0 + 32.0;                  // convert fahrenheit
return temp;                                      // return temperature
}

// =============================================================================
// main loop:
// acquire 'nsamples' readings, convert units of volts, , send out on serial port

void loop() {

int i;
long secs;
float mins;
double volts;
long in;         // incoming serial 32-bit word
long sum = 0;
float temp;
long rawadc;

   for (i=0; i<nsamples; i++) {
     in = spiread();
     in &= 0x1fffffff; // force high 3 bits zero
     in = in>>5;   // truncate lowest 5 bits
     sum += in;
     delay(198);      // (msec). total looptime: +2 msec (overhead comms)
   }

   // volts =  in * 2.5 / 8.388607;  // 0x7fffff = 8388607
   volts = sum * (0.2980232594);  // microvolts
   volts = volts / nsamples;
   rawadc = sum / nsamples;

   mins = (float) millis() / 60000;   // elapsed time in minutes
temp=thermistor(rawadc);       // read adc ,  convert celsius
   serial.print(volts);
   serial.print(", ");
   serial.print(rawadc);
   serial.print(", ");
   serial.println(temp,3);                             // display celsius

} // end main loop


// =================================================================
// spiread() -- read out 4 bytes ltc2440 chip via spi interface
// =================================================================

long spiread(void) {

 long result = 0;
 long b;
 
//  long result2 = 0;// mosi/sdi pin 7 high => 7 hz, best resolution

 digitalwrite(slaveselectpin,low);   // take ss pin low select chip
 delaymicroseconds(1);              // not needed, need 25 nsec delay
 
 b = spi.transfer(0xff);   // b3
 result = b<<8;
 b = spi.transfer(0xff);   // b2
 result |= b;
 result = result<<8;
 b = spi.transfer(0xff);   // b1
 result |= b;
 result = result<<8;
 b = spi.transfer(0xff);   // b0
 result |= b;
 
 // take ss pin high de-select chip:
 digitalwrite(slaveselectpin,high);
 return(result);
}

i readings 25c , rawadc value of 16777216 /2
it not go further no matter how connect thermistor.
any other connection tried doesn't give me correct temperature.

i connected dc570a followed:
from ref+ 10k resistor

from 10k resistor 10k thermistor
from 10k resistor in+

from 10k thermistor in -

in- connected straight gnd

could advice on how full range of 10k thermistor?

a few things:

1) 99.9% of seasoned engineers cannot 24-bit adc work. requires tremendous amount of know-how , art of engineering (layout in particular).

2) 1 approach usable data out of high resolution adc average. has downside.

3) in case, should @ room temperature close half of full scale , varies temperature.

4) do, onboard adc fine.

Arduino Forum > Using Arduino > Sensors > Hooking up a 10K NTC Thermistor to an LTC2440

arduino