Introduction
This document discusses the calibration details the DS1925’s temperature sensor which provides an accuracy of +/-0.5C over the operating range of -40C to 85C. The DS1925 temperature sensing solution operates from an advanced principle within the electrical engineering field known as “band-gap temperature sensor design1“ whereby Maxim-factory temperature calibration is performed with a process involving the application of a precision voltage during manufacturing. This differs from previous calibration methods involving the use of temperature chambers in which products are placed and exposed to multiple test temperatures then adjusted for accuracy. Based on applying external precision voltages to the IC containing the temperature sensor circuit, responses are measured by test equipment to determine the required calibration adjustment to achieve the specified accuracy performance. Both simulation results and post-calibrated data from temperature chamber studies confirm this methodology.Temp Sensor Circuit Implementation Details
Fundamentally, the DS1925 uses p-n junctions of a bipolar transistors (BJT) to measure temperature. These BJT p-n junctions are electrically equivalent to a circuit element known as a diode. The electrical characteristics of the BJT diodes have predictable voltage-temperature dependencies that are described with mathematical equations enabling them to be used as temperature sensors. The DS1925 uses two BJT diodes, with voltages Vbe1Figure 1: Transistors Configured as Diodes
A constant current through each diode in Figure 1 produces the temperature dependence of the base-emitter voltage (Vbe
∆Vbe= K*Tq* ln(Ic1Ic2) |
Eq. (1) |
∆Vbe=Vbe1-Vbe2 |
Eq. (2) |
Equation 1 is for an ideal transistor, so a non-ideality factor (η
∆Vbe= K*T*ηq* ln(m) |
Eq. (3) |
Where:
m= Ic1Ic2
K
T
q
With Equation 3, from a known ∆Vbe
Advanced Single-Point Calibration
Unlike traditional calibration techniques requiring multiple temperature points and precise references, this advanced single-point calibration methodology does not require either one. This section presents the principles and methodology used to calibrate the DS1925’s temperature sensor, which does not require a forced IC temperature or a reference temperature sensor to measure the actual IC temperature. Instead, it relies on applying a high-precision external voltage to the device under test (DUT) to determine its temperature followed by adjusting, and therefore calibrating, it’s Vbe- Through application of a precision voltage by test equipment, determine the actual temperature (T
) of the DS1925 IC.
- Adjust and calibrate Vbe
such that it’s value at T
is equal to ideal.
Calibration Step 1
Figure 2 shows the DS1925’s internal analog-to-digital converter (ADC) and the various inputs. Equation 3 has two unknowns: temperature and ∆Vbe
Figure 2: DS1925’s Internal ADC and Inputs
The resultant ADC code from applying an external voltage Vext
ADC Code= ∆VbeVext+∆Vbe |
Eq. (4) |
Since the ADC code was read from the device and the externally applied voltage Vext
is known, we can solve for ∆Vbe
Next, with ∆Vbe
T= ∆Vbe*qK*η* ln(m) |
Eq. (5) |
This temperature value T
Calibration Step 2
With T