What Is the DS1925 Calibration Process?

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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 design 1 “ 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 Vbe1 and Vbe2 , as shown in Figure 1.

Figure 1: Transistors Configured as Diodes 
 
 A constant current through each diode in Figure 1 produces the temperature dependence of the base-emitter voltage ( Vbe ) described by Equation 1. Since the bias current sources are different, the results are two distinct base-emitter voltages. The ∆Vbe is the difference of the base-emitter voltage measurements from the two excitation currents.

∆Vbe= K*T q * ln⁡( Ic1 Ic2 ) 
 Eq. (1)

Where:

∆Vbe=Vbe1-Vbe2 
 Eq. (2)

Equation 1 is for an ideal transistor, so a non-ideality factor ( η ) is needed for real world usage. Applying this adjustment, Equation 3 is the fundamental BJT base-emitter diode voltage equation.

∆Vbe= K*T*η q * ln⁡( m ) 
 Eq. (3)

Where: 
 m= Ic1 Ic2 (constant and known by design) 
 K = Boltzmann's constant 
 T = circuit operating temperature in Kelvin 
 q = constant charge on an electron 
 
 With Equation 3, from a known ∆Vbe the temperature T of the DS1925 sensor, and therefore the DS1925 operating condition, can be precisely determined.

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 to an ideal characteristic 2 . Once performed at temperature T , this calibration allows Vbe to be precisely known across all operating temperatures . There are two steps to the calibration process:

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 . We can configure the DS1925 ADC to select Vbe 1 & 2 as its references and apply a precision external voltage ( Vext ).

Figure 2: DS1925’s Internal ADC and Inputs

The resultant ADC code from applying an external voltage Vext is:

ADC Code= ∆Vbe Vext+∆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 in Equation 4. 
 
 Next, with ∆Vbe known, solving for temperature in Equation 3 gives:

T= ∆Vbe*q K*η* ln⁡( m ) 
 Eq. (5)

This temperature value T determined from Equation 5 is the precise operating temperature of the DS1925 at this test step and used for the next step in the calibration process. 
 
 Calibration Step 2 
 With T known, a precision ∆Vbe is applied externally ( ∆Vext