Measuring Temperature with the

文章摘要：Measuring Temperature with the MAX1358 Data Acquisition System Abstract: This application note explains how to use the internal and external temperature sensors on the MAX1358 data acquisition system. The temperature circuits for the / are identical, therefore any reference to the MAX1358 also appli

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Measuring Temperature with the MAX1358 Data Acquisition System

Abstract: This application note explains how to use the internal and external temperature sensors on the MAX1358 data acquisition system. The temperature circuits for the / are identical, therefore any reference to the MAX1358 also applies to the MAX1359 and MAX1360. A step-by-step approach guides the user through the setup, measurement, and calculation of the temperature.

Introduction

The data acquisition system can measure temperature using an internal or external transistor PN junction. Figure 1 shows the internal temperature circuit and the external circuit. A constant current is supplied by the current source to produce a voltage (V_BE) across the transistor. The current source can be programmed to produce up to four currents. For each current the voltage drop across the transistor is measured using the integrated ADC. The ADC has inputs for TEMP+, TEMP-, AIN1, AIN2, and AGND. These measured values are used in an equation to determine the junction temperature.


Figure 1. MAX1358 internal/external temperature measurement circuit.

Internal Four-Current Method

The internal four-current method requires eight measurements to be used in the temperature-measurement equation. Equation 1 below is used for four-current measurement.


Equation 1. Four-current temperature measurement equation.

Substituting for I₁, I₂, I₃, and I₄ in the denominator:



Where:
T_MEAS = temperature in Kelvin
q = electron Charge = 1.60219 × 10^-19 Coulombs
NV_BE1 = ADC reading with I₁ as current source
NV_BE2 = ADC reading with I₂ as current source
NV_BE3 = ADC reading with I₃ as current source
NV_BE4 = ADC reading with I₄ as current source
V_REF = ADC reference voltage = 1.251V (typ)
n = diode ideality = 1.000 (typ)
k = Boltzmann's constant = 1.3807 × 10^-23 Joules/Kelvin
I₁ = Current source low setting (4µA)
I₂ = Current source high setting (60µA)
I₃= Current source high setting (64µA)
I₄ = Current source high setting (120µA)
NV_R1 = ADC reading with I₁ as current source
NV_R2 = ADC reading with I₂ as current source
NV_R3 = ADC reading with I₃ as current source
NV_R4 = ADC reading with I₄ as current source
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