adt7476a Analog Devices, Inc., adt7476a Datasheet - Page 20

• Manufacturer Part Number: adt7476a
• Description: Dbcool Remote Thermal Controller And Voltage Monitor
• Manufacturer: Analog Devices, Inc.

ADT7476A
To measure ΔV
currents of I and N x I. The resulting waveform passes through
a 65 kHz low-pass filter to remove noise and through a chopper-
stabilized amplifier. The amplifier performs the amplification
and rectification of the waveform to produce a dc voltage
proportional to ΔV
give a temperature output in 10-bit, twos complement format.
To further reduce the effects of noise, digital filtering is performed
by averaging the results of 16 measurement cycles.
A remote temperature measurement takes nominally 38 ms.
The results of remote temperature measurements are stored in
10-bit, twos complement format, as illustrated in Table 10. The
extra resolution for the temperature measurements is held in
the Extended Resolution Register 2 (0x77). This gives
temperature readings with a resolution of 0.25°C.
Noise Filtering
For temperature sensors operating in noisy environments,
previous practice placed a capacitor across the D+ pin and
the D− pin to help combat the effects of noise. However,
large capacitances affect the accuracy of the temperature
measurement, leading to a recommended maximum capacitor
value of 1000 pF.
This capacitor reduces the noise but does not eliminate it, which
makes using the sensor difficult in a very noisy environment. In
most cases, a capacitor is not required because differential
inputs by their very nature have a high immunity to noise.
FACTORS AFFECTING DIODE ACCURACY
Remote Sensing Diode
The ADT7476A is designed to work with substrate transistors
built into processors or with discrete transistors. Substrate
transistors are generally PNP types with the collector connected
to the substrate. Discrete types can be either PNP or NPN
transistors connected as a diode (base-shorted to the collector).
If an NPN transistor is used, the collector and base are
connected to D+ and the emitter to D−. If a PNP transistor is
used, the collector and base are connected to D− and the
emitter is connected to D+.
To reduce the error due to variations in both substrate and
discrete transistors, a number of factors should be taken into
consideration:
•
The ideality factor, n
deviation of the thermal diode from ideal behavior. The
ADT7476A is trimmed for an n
following equation to calculate the error introduced at a
temperature T (°C), when using a transistor whose n
does not equal 1.008 (see the processor’s data sheet for
the n
ΔT = (n
f
values):
f
− 1.008) × (273.15 K + T)
BE
, the sensor switches between operating
BE
. This voltage is measured by the ADC to
f
, of the transistor is a measure of the
f
value of 1.008. Use the
f
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•
If a discrete transistor is used with the ADT7476A, the best
accuracy is obtained by choosing devices according to the
following criteria:
•
•
•
•
Transistors, such as 2N3904, 2N3906, or equivalents in SOT-23
packages, are suitable devices to use.
Nulling Out Temperature Errors
As CPUs run faster, it is more difficult to avoid high frequency
clocks when routing the D+/D– traces around a system board.
Even when recommended layout guidelines are followed, some
temperature errors can still be attributable to noise coupled
onto the D+/D– lines. Constant high frequency noise usually
attenuates, or increases, temperature measurements by a linear,
constant value.
The ADT7476A has temperature offset registers (0x70 and
0x72) for the Remote 1 and Remote 2 temperature channels. By
doing a one-time calibration of the system, the user can determine
the offset caused by system board noise and null it out using the
offset registers. The offset registers automatically add a twos
complement 8-bit reading to every temperature measurement.
Changing Bit 1 of Configuration Register 5 (0x7C) changes the
resolution and therefore, the range of the temperature offset as
either having a −63°C to +127°C range with a resolution of 1°C
or having a −63°C to +64°C range with a resolution of 0.5°C.
This temperature offset can be used to compensate for linear
temperature errors introduced by noise.
To factor this in, the user can write the ΔT value to the
offset register. The ADT7476A then automatically adds it
to or subtracts it from the temperature measurement.
Some CPU manufacturers specify the high and low current
levels of the substrate transistors. The high current level of
the ADT7476A, I
I
the current levels specified by the CPU manufacturer, it
could be necessary to remove an offset. The CPU’s data sheet
advises whether this offset needs to be removed and how to
calculate it. This offset can be programmed to the offset
register. It is important to note that if more than one offset
must be considered, then the algebraic sum of these offsets
must be programmed to the offset register.
Base-emitter voltage greater than 0.25 V at 11 μA, at
the highest operating temperature.
Base-emitter voltage less than 0.95 V at 180 μA,
at the lowest operating temperature.
Base resistance less than 100 Ω.
Small variation in the current gain, h
to 150) that indicates tight control of V
LOW
, is 11 μA. If the ADT7476A current levels do not match
HIGH
, is 180 μA, and the low level current,
FE
, (approximately 50
BE
characteristics.