Data Sheet ADT6401/ADT6402
Rev. C | Page 9 of 12
THEORY OF OPERATION
CIRCUIT INFORMATION
The ADT6401/ADT6402 are 11-bit digital temperature sensors
with a 12
th
bit acting as the sign bit. An on-board temperature
sensor generates a voltage precisely proportional to absolute
temperature, which is compared to an internal voltage reference
and input to a precision digital modulator. The 12-bit output
from the modulator is input into a digital comparator, where it
is compared with a pin-selectable trip level. The output trip pin
is activated if the temperature measured is greater than, or less
than, the pin-selectable trip level. Overall accuracy for the
ADT6401/ ADT6402 is ±6°C (maximum) from −45°C to
+115°C.
The on-board temperature sensor has excellent accuracy and
linearity over the entire rated temperature range without needing
correction or calibration by the user. The ADT6401 has active
low, open-drain output structures that can sink current. The
ADT6402 has active high, push-pull output structures that can
sink and source current. On power-up, the output becomes
active when the first conversion is completed, which typically
takes 30 ms.
The sensor output is digitized by a first-order, ∑-∆ modulator,
also known as the charge balance type analog-to-digital converter
(ADC). This type of converter utilizes time domain oversampling
and a high accuracy comparator to deliver 11 bits of effective
accuracy in an extremely compact circuit.
CONVERTER DETAILS
The Σ-Δ modulator consists of an input sampler, a summing
network, an integrator, a comparator, and a 1-bit digital-to-
analog converter (DAC). Similar to the voltage-to-frequency
converter, this architecture creates a negative feedback loop and
minimizes the integrator output by changing the duty cycle of
the comparator output in response to input voltage changes.
The comparator samples the output of the integrator at a much
higher rate than the input sampling frequency; this is called
oversampling. Oversampling spreads the quantization noise
over a much wider band than that of the input signal, improving
overall noise performance and increasing accuracy.
PIN-SELECTABLE TRIP POINT AND HYSTERESIS
The temperature trip point and hysteresis values for the
ADT6401/ADT6402 are selected using Pin S0, Pin S1, and
Pin S2. These three pins can be connected to V
CC
, tied to GND,
or left floating. The ADT6401/ADT6402 decode the inputs on
S0, S1, and S2 to determine the temperature trip point and
hysteresis value, as outlined in Table 4.
The ADT6401 overtemperature/undertemperature output is
intended to interface to reset inputs of microprocessors. The
ADT6402 is intended for driving circuits of applications, such
as fan control circuits.
Table 4. Selecting Trip Points and Hysteresis
1
S2 S1 S0
Temperature
Trip Point Hysteresis
0 0 1 +55°C 2°C
0 0 Float +65°C 2°C
0 1 0 +75°C 2°C
0 1 1 +85°C 2°C
0 1 Float +95°C 2°C
0 Float 0 +105°C 2°C
0 Float 1 +115°C 2°C
0 Float Float +55°C 10°C
1 0 0 +65°C 10°C
1 0 1 +75°C 10°C
1 1 0 +95°C 10°C
1 1 1 +105°C 10°C
1 1 Float +115°C 10°C
1 Float 0 +5°C 2°C
1 Float 1 −5°C 2°C
1 Float Float −15°C 2°C
Float 0 0 −25°C 2°C
Float 0 1 −35°C 2°C
Float 0 Float −45°C 2°C
Float 1 0 +5°C 10°C
Float 1 1 −5°C 10°C
Float 1 Float −15°C 10°C
Float Float 0 −25°C 10°C
Float Float Float −45°C 10°C
1
0 = pin tied to GND, 1 = pin tied to V
CC
, Float = pin left floating.
