MAT Sensor Input - Block B

NOTE: Modified 03/30/05


        This circuit is designed to work with either standard thermistor sensing element or an precision LM34 temperature sensor.  Using the LM34 is the most precise way to go.  Can't be much simpler than reading the 10mv/degF with the ADC.  Leaving the op-amp gain set to two (2) the LM34 will read up to 250 degF at about 4 ADC bits per degF.  Changing the gain to one (1) would allow higher temperatures to be read but the LM34 is only specified to a maximum of 300 degF so there isn't much point.  Besides if I had an inlet temperature of 250 degF I'd be looking for some fresh air.  So using this device we have a nice linear solution that will read precisely between 0 degF and 250 degF.  I probably will replace the GM/Delphi thermistor element with an LM34.

        When using a typical GM/Delphi thermistor the 1k ohm pull-up resistor (R100) must be in circuit.  A 1k works out to be the best compromise between thermistor self heating and the most bits per degree in the typical temperature operating range.  These thermistors are very non-linear verses temperature with not much actual resistance variation in our expected operating range.  I looked into using a log amp to try get more bits per degree.  It may have done job except that I suspected I'd probably run into noise issues.  The logger needed so much gain in the nominal operating range because of the low thermistor resistance that the slightest bit of noise was going to be amplified so much that trying to recover the original signal was going to be more trouble than it was worth.  It was at this point I decided to go to the LM34.




      The above graph shows the relationship between ADC sensitivity verses temperature (degF) with a 1k pull up using a GM thermistor sensing element.  The 10 bit ADC used in the MPC555 really helps out over the older 8 bit ADCs in some other processors.  The below table is expected ADC input voltage and value verses temperature:

Vref = 5   Series Resistance = 1000
degC   degF     Res      Vout    ADC   C/bit  F/bit
-40    -40    99326.0   4.9502  1013     --    --
-35    -31    71332.0   4.9309  1009   1.25   2.25
-30    -22    51791.0   4.9053  1004   1.00   1.80
-25    -13    37994.0   4.8718   997   0.71   1.29
-20     -4    28146.0   4.8284   988   0.56   1.00
-15      5    21044.0   4.7732   977   0.45   0.82
-10     14    15873.0   4.7037   963   0.36   0.64
 -5     23    12073.0   4.6175   945   0.28   0.50
  0     32     9256.0   4.5125   924   0.24   0.43
  5     41     7153.0   4.3867   898   0.19   0.35
 10     50     5572.0   4.2392   868   0.17   0.30
 15     59     4373.0   4.0694   833   0.14   0.26
 20     68     3457.0   3.8782   794   0.13   0.23
 25     77     2752.0   3.6674   751   0.12   0.21
 30     86     2205.0   3.4399   704   0.11   0.19
 35     95     1778.0   3.2001   655   0.10   0.18
 40    104     1443.0   2.9533   604   0.10   0.18
 45    113     1177.0   2.7033   553   0.10   0.18
 50    122      965.0   2.4555   502   0.10   0.18
 55    131      796.0   2.2160   453   0.10   0.18
 60    140      660.0   1.9880   407   0.11   0.20
 65    149      551.0   1.7763   363   0.11   0.20
 70    158      462.0   1.5800   323   0.12   0.23
 75    167      389.0   1.4003   286   0.14   0.24
 80    176      329.0   1.2378   253   0.15   0.27
 85    185      279.0   1.0907   223   0.17   0.30
 90    194      238.1   0.9616   196   0.19   0.33
 95    203      203.9   0.8468   173   0.22   0.39
100    212      175.3   0.7458   152   0.24   0.43
105    221      151.3   0.6571   134   0.28   0.50
110    230      131.0   0.5791   118   0.31   0.56
115    239      113.9   0.5113   104   0.36   0.64
120    248       99.4   0.4521    92   0.42   0.75
125    257       87.0   0.4002    81   0.45   0.82
130    266       76.4   0.3549    72   0.56   1.00
135    275       67.3   0.3153    64   0.62   1.12
140    284       59.4   0.2803    57   0.71   1.29
145    293       52.6   0.2499    51   0.83   1.50
150    302       46.7   0.2231    45   0.83   1.50