Datasheet

117
8024A–AVR–04/08
ATmega8HVA/16HVA
21. Voltage Reference and Temperature Sensor
21.1 Features
• Accurate Voltage Reference of 1.100V
• Internal Temperature Sensor
• Possibility for Runtime Compensation of Temperature Drift in Both Voltage Reference and On-
chip Oscillators
• External Decoupling for Optimum Noise Performance
• Low Power Consumption
21.2 Overview
A low power band-gap reference provides ATmega8HVA/16HVA with an accurate On-chip volt-
age reference V
REF
of 1.100V. This reference voltage is used as reference for the On-chip
Voltage Regulator, the V-ADC and the CC-ADC. The reference to the ADCs uses a buffer with
external decoupling capacitor to enable excellent noise performance with minimum power con-
sumption. The reference voltage V
REF_P
/V
REF_N
to the CC-ADC is scaled to match the full scale
requirement at the current sense input pins. This configuration also enables concurrent opera-
tion of both V-ADC and CC-ADC.
To guarantee ultra low temperature drift after factory calibration, ATmega8HVA/16HVA features
a two-step calibration algorithm. The first step is performed at T
HOT
°C and the second at room
temperature. By default, Atmel factory calibration is performed at T
HOT
°C, and the result is stored
in the signature row. The value of T
HOT
can also be found in the signature row. See ”Reading the
Signature Row from Software” on page 144 for details. The customer can easily implement the
second calibration step in their test flow. This requires an accurate input voltage and a stable
room temperature. Temperature drift after this calibration is guaranteed by design and charac-
terization to be less than 90 ppm/°C from -10°C to 70°C. The BG Calibration C Register can also
be altered runtime to implement temperature compensation in software. Very high accuracy for
any temperature inside the temperature range can thus be achieved at the cost of extra calibra-
tion steps.
ATmega8HVA/16HVA has an On-chip temperature sensor for monitoring the die temperature. A
voltage Proportional-To-Absolute-Temperature, V
PTAT
, is generated in the voltage reference cir-
cuit and connected to the multiplexer at the V-ADC input. This temperature sensor can be used
for runtime compensation of temperature drift in both the voltage reference and the On-chip
Oscillator. To get the absolute temperature in degrees Kelvin, the measured V
PTAT
voltage must
be scaled with the VPTAT factory calibration value stored in the signature row. See Section
“26.2.5” on page 144. for details.