Datasheet

ADP1053 Data Sheet
Rev. A | Page 26 of 84
The RTD1 and RTD2 value registers (Register 0xFED7 and
Register 0xFED8, respectively) are updated every 10 ms. The
ADP1053 stores every ADC sample for 10 ms and then outputs
the average value at the end of the 10 ms period.
The RTD1 and RTD2 ADCs have an input range of 0 V to
1.6 V and a resolution of 12 bits, which means that the LSB size
is 1.6 V/4096 = 390.6 µV. The valid input range is 1.28 V, which
means that the maximum ADC output code is limited to
1.28 V/390.6 µV = 3277.
The output of the RTD ADC is linearly proportional to the vol-
tage on the RTDx pin. However, thermistors exhibit a nonlinear
function of resistance vs. temperature. Therefore, the user must
perform postprocessing on the RTD ADC reading to accurately
read the temperature.
By connecting an external resistor (R
EXT
) in parallel with the
NTC thermistor (TH), a constant current can be used to
achieve linearization (see Figure 24).
DAC
ADC
RTD
TH
R
EXT
10241-023
Figure 24. Temperature Measurement Using Thermistor
An internal, precision current source of 10 µA, 20 µA, 30 µA, or
40 µA can be selected. This current source can be fine-tuned by
means of an internal DAC to compensate for thermistor accuracy
(see the Calibrating for Accuracy section).
The user can select the output current source using Bits[7:6]
of the RTD1 and RTD2 current source settings registers
(Register 0xFE80 and Register 0xFE81, respectively).
The ADP1053 implements a linearization scheme based on a
preselected combination of external components and current
selection for best performance when linearizing measured
temperatures in the industrial range.
For more information about the required thermistor and
selecting and trimming the precision current sources, see
the Temperature Linearization Scheme section.
Optionally, the user can process the RTD reading and perform
postprocessing in the form of a lookup table or polynomial
equation to match the specific NTC thermistor used.
With the internal current source set to 46 µA, the equation to
calculate the ADC code at a specified NTC value (R
X
) is given
by the following formula:
ADC CODE = 46 µA × R
X
/1.6 × 4096
For example, at 60°C, the NTC at the RTDx pin is 21.82 kΩ.
RTD ADC CODE = 46 µA × 21.82 kΩ/1.6 × 4096 = 2570
TEMPERATURE LINEARIZATION SCHEME
The ADP1053 implements a linearization scheme based on a pre-
selected combination of thermistor (100 kΩ), external resistor
(16.5 kΩ, 1%), and the 46 µA current source for best performance
when linearizing measured temperatures in the industrial range.
The required NTC thermistor should have a resistance of 100 kΩ,
1%, such as the NCP15WF104F03RC (beta = 4250, 1%). It is
recommended that 1% tolerance be used for both the resistor
and beta value.
Calibrating for Accuracy
Register 0xFE80 and Register 0xFE81 set the value of the current
source on the RTD1 and RTD2 pins, respectively. Bits[7:6] set
the value of the current source to 10 µA, 20 µA, 30 µA, or 40 µA.
Bits[5:0] can be used to fine-tune the current value. By fine-tuning
the internal current source, component tolerance can be compen-
sated for and errors can be minimized. One LSB in Bits[5:0] =
156.25 nA. A decimal value of 1 adds 156.25 nA to the current
source set by Bits[7:6]; a decimal value of 63 adds 9.84375 µA.
There is no negative adjustment to the current source.
To calibrate the part, a known reference value can be used, such
as the RTDx ADC code at 25°C. For an ideal thermistor with a
resistance of R
0
, the ADC code reading should be the value
derived from the following equation:
ADC CODE = 46 µA × (R
EXT
//R
0
)/390.6 µV
This fine-tuning adjusts the output current slightly to null out
any inaccuracies on the thermistor (for example, tolerance on
R
0
causing error curves to shift accordingly).
Reading the Linearized Temperature
The PMBus READ_TEMPERATURE_1 and READ_TEMPER-
ATURE_2 commands (Command 0x8D and Command 0x8E)
return the current temperature for RTD1 and RTD2 according
to an internal linearization scheme. See Table 1 for the specified
accuracy of these measurements.
As per the PMBus specification, the temperature reading result
is a word in the following format:
X = Y × 2
N
where:
X is the temperature value in °C.
Y is the twos complement mantissa (Bits[10:0]). Bit 10 is
the sign bit, which is always equal to 0.
N is the twos complement integer exponent (Bits[15:11]).
In the ADP1053, N is always equal to 0. The register value represents
temperature readings in degrees Celsius (°C). The temperature
reading result is represented in 8-bit decimal format in °C.
Note that in the PMBus read format implemented in the ADP1053,
the lowest possible temperature that can be read is 0°C. Reading
Bits[9:0] gives the actual positive temperature in °C. To read the
actual unconverted temperature, the user can read the ADC code
from Register 0xFED7 and Register 0xFED8.