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ADT7466ZEVB

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P36P37-P39P40-P42P43-P45P46-P48
ADT7466
Rev. 2 | Page 16 of 48 | www.onsemi.com
Table 8. Single-Channel ADC Conversions
Bits 2:0, Reg. 0x03 Channel Selected
000 AIN1
001 AIN2
010 V
CC
REFERENCE VOLTAGE OUTPUT
The ADT7466 has a reference voltage of 2.25 V, which is
available on Pin 13 of the device. It can be used for scaling and
offsetting the analog inputs to give different voltage ranges. It
can also be used as an excitation voltage for a thermistor when
the analog inputs are configured as thermistor inputs. See the
Temperature Measurement section for more details.
CONFIGURATION OF PIN 11 AND PIN 12
Pin 11 and Pin 12 can be used for analog inputs, thermistor
inputs, or connecting a second remote thermal diode. The
ADT7466 is configured for thermistor connection by default.
The device is configured for the different modes by setting the
appropriate bits in the configuration registers. Bits 6:7 of
Configuration Register 3 (0x02) configure the device for either
analog inputs or thermistor inputs. Bit 7 of Configuration
Register 2 (0x01) configures Pin 11 and Pin 12 for the
connection of a second thermal diode. Bits 2:3 of Interrupt
Status Register 2 (0x11) indicate either an open or short circuit
on Thermal Diode 1 and Diode 2 inputs. Bits 4:5 of Interrupt
Status Register 2 (0x11) indicate either an open or short circuit
on TH1 and TH2 inputs. It is advisable to mask interrupts on
diode open/short alerts when in thermistor monitoring mode
and to mask interrupts on thermistor open/short alerts when in
REM2 mode.
Table 9. A-to-D Output Code vs. V
IN
V
CC
3.3 V V
CC
5 V A
IN
Decimal Binary
<0.0172 <0.026 <0.0088 0 00000000
0.017–0.034 0.026–0.052 0.0088–0.0176 1 00000001
0.034–0.052 0.052–0.078 0.0176–0.0264 2 00000010
0.052–0.069 0.078–0.104 0.0264–0.0352 3 00000011
1.110–1.127 1.667–1.693 0.563–0.572 64 (¼ scale) 01000000
2.220–2.237 3.333–3.359 1.126–1.135 128 (½ scale) 10000000
3.3–3.347 5–5.026 1.689–1.698 192 (¾ scale) 11000000
4.371–4.388 6.563–6.589 2.218–2.226 252 11111100
4.388–4.405 6.589–6.615 2.226–2.235 253 11111101
4.405–4.423 6.615–6.641 2.235–2.244 254 11111110
>4.423 >6.634 >2.244 255 11111111
Table 10. Mode Configuration Summary
Mode
Configuration
Register Settings Limits Alerts
1
Description
Thermistor Mode
Default mode. Mask interrupts on diode NC.
(Set Bits 2:3 of Reg. 0x13.)
TH1 Register 0x02
Bit 7 = 1
Low: Reg 0x14
High: Reg 0x15
OOL: Reg. 0x10, Bit 6
NC: Reg. 0x11, Bit 4
TH2 Register 0x02
Bit 6 = 1
Low: Reg 0x16
High: Reg 0x17
OOL: Reg. 0x10, Bit 5
NC: Reg. 0x11, Bit 5
AIN Mode
Ensure that AFC is not on. (Clear Bits 0:1 of
AFC Configuration Register 1, 0x05.)
AIN1 Register 0x 02
Bit 7 = 0
Low: Reg 0x14
High: Reg 0x15
OOL: Reg. 0x10, Bit 6
AIN2 Register 0x02
Bit 6 = 0
Low: Reg 0x16
High: Reg 0x17
OOL: Reg. 0x10, Bit 5
Remote 2 Diode Mode Register 0x01
Bit 7 = 1
Low: Reg 0x14
High: Reg 0x15
OOL: Reg. 0x10, Bit 6
NC: Reg. 0x11, Bit 3
Mask interrupts on thermistor NC. (Set
Bits 4:5 of Reg. 0x13) and AIN2 (Bit 5 of
Reg. 0x12.)
1
OOL = Out of limit. NC = No connection.
ADT7466
Rev. 2 | Page 17 of 48 | www.onsemi.com
TEMPERATURE MEASUREMENT
The ADT7466 has two dedicated temperature measurement
channels, one for measuring the temperature of an on-chip
band gap temperature sensor, and one for measuring the
temperature of a remote diode, usually located in the CPU. In
addition, the analog input channels, AIN1 and AIN2, can be
reconfigured to measure the temperature of a second diode by
setting Bit 7 of Configuration Register 2 (0x01), or to measure
temperature using thermistors by setting Bit 6 and/or Bit 7 of
Configuration Register 3 (0x02).
SERIES RESISTANCE CANCELLATION
Parasitic resistance, seen in series with the remote diode
between the D+ and D− inputs to the ADT7466, is caused by a
variety of factors including PCB track resistance and track
length. This series resistance appears as a temperature offset in
the sensor’s temperature measurement. This error typically
causes a 1°C offset per ohm of parasitic resistance in series with
the remote diode. The ADT7466 automatically cancels the
effect of this series resistance on the temperature reading, giving
a more accurate result without the need for user characterization
of the resistance. The ADT7466 is designed to automatically
cancel typically 2 kΩ of resistance. This is done transparently to
the user, using an advanced temperature measurement method
described in the following section.
TEMPERATURE MEASUREMENT METHOD
A simple method of measuring temperature is to exploit the
negative temperature coefficient of a diode, by measuring the
base emitter voltage (V
BE
) of a transistor operated at constant
current. Unfortunately, this technique requires calibration to
null out the effect of the absolute value of V
BE
, which varies
from device to device.
The technique used in the ADT7466 measures the change in
V
BE
when the device is operated at three different currents.
Previous devices used only two operating currents, but it is the
third current that allows series resistance cancellation.
Figure 24 shows the input signal conditioning used to measure
the output of a remote temperature sensor. This figure shows
the remote sensor as a substrate transistor, provided for
temperature monitoring on some microprocessors, but it could
also be a discrete transistor. If a discrete transistor is used, the
collector is not grounded, and should be linked to the base. To
prevent ground noise from interfering with the measurement,
the more negative terminal of the sensor is not referenced to
ground but is biased above ground by an internal diode at the
D– input. If the sensor is operating in an extremely noisy
environment, C1 may optionally be added as a noise filter. Its
value should never exceed 1000 pF. See the Layout
Considerations section for more information on C1.
To mea sure Δ V
BE
, the operating current through the sensor is
switched between three related currents. Figure 24 shows N1 × I
and N2 × I as different multiples of the current I. The currents
through the temperature diode are switched between I and
N1 × I, giving ΔV
BE1
, and then between I and N2 × I, giving
ΔV
BE2
. The temperature can then be calculated using the two
ΔV
BE
measurements. This method can also cancel the effect of
series resistance on the temperature measurement. The
resulting ΔV
BE
waveforms are passed through a 65 kHz low-pass
filter to remove noise, and then to a chopper-stabilized
amplifier. This amplifies and rectifies the waveform to produce
a dc voltage proportional to ΔV
BE
. The ADC digitizes this
voltage, and a temperature measurement is produced. To reduce
the effects of noise, digital filtering is performed by averaging
the results of 16 measurement cycles for low conversion rates.
Signal conditioning and measurement of the internal
temperature sensor is performed in the same manner.
USING DISCRETE TRANSISTORS
If a discrete transistor is used, the collector is not grounded and
should be linked to the base. If an NPN transistor is used, the
emitter is connected to the D− input and the base to the D+
input. If a PNP transistor is used, the base is connected to the
D− input and the emitter to the D+ input. Figure 23 shows how
to connect the ADT7466 to an NPN or PNP transistor for
temperature measurement. To prevent ground noise interfering
with the measurement, the more negative terminal of the sensor
is not referenced to ground, but is biased above ground by an
internal diode at the D− input.
04711-023
D+
D–
ADT7466
2N3904
NPN
D+
D–
ADT7466
2N3906
PNP
Figure 23. Connections for NPN and PNP Transistors
ADT7466
Rev. 2 | Page 18 of 48 | www.onsemi.com
04711-024
C1*
D+
BIAS
DIODE
*CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.
V
DD
TO ADC
V
OUT+
V
OUT–
REMOTE
SENSING
T
RANSISTOR
D–
I N1
×
I N2
×
I
I
BIAS
LOW-PASS FILTER
f
C
= 65kHz
Figure 24. Signal Conditioning for Remote Diode Temperature Sensors
Temperature Data Format
The temperature data stored in the temperature data registers
consists of a high byte with an LSB size equal to 1°C. If higher
resolution is required, two additional bits are stored in the
extended temperature registers, giving a resolution of 0.25°C.
The temperature measurement range for both local and remote
measurements is, by default, 0°C to 127°C (binary), so the ADC
output code equals the temperature in degrees Celsius, and half
the range of the ADC is not actually used.
The ADT7466 can also be operated by using an extended
temperature range from −64°C to +191°C. In this case, the
whole range of the ADC is used, but the ADC code is offset by
+64°C, so it does not correspond directly to the temperature.
(0°C = 0100000) .
The user can switch between these two temperature ranges by
setting or clearing Bit 7 in Configuration Register 1. The
measurement range should be switched only once after power-
up, and the user should wait for two monitoring cycles
(approximately 68 ms) before expecting a valid result. Both
ranges have different data formats, as shown in Table 11.
Table 11. Temperature Data Format
Temperature Binary
1
Offset Binary
2
−64°C 0 000 0000 0 000 0000
0°C 0 000 0000 0 100 0000
1°C 0 000 0001 0 100 0001
10°C 0 000 1010 0 100 1010
25°C 0 001 1001 0 101 1001
50°C 0 011 0010 0 111 0010
75°C 0 100 1011 1 000 1011
100°C 0 110 0100 1 010 0100
125°C 0 111 1101 1 011 1101
127°C 0 111 1111 1 011 1111
191°C 0 111 1111 1 111 1111
1
Binary scale temperature measurement returns 0 for all temperatures ≤0°C.
2
Offset binary scale temperature values are offset by +64.
While the temperature measurement range can be set to −64°C
to +191°C for both local and remote temperature monitoring,
the ADT7466 itself should not be exposed to temperatures
greater than those specified in the Absolute Maximum Ratings
table. Furthermore, the device is guaranteed to only operate at
ambient temperatures from −40°C to +125°C. In practice, the
device itself should not be exposed to extreme temperatures,
and may need to be shielded in extreme environments to
comply with these requirements. Only the remote temperature
monitoring diode should be exposed to temperatures above
+120°C and below −40°C. Care should be taken in choosing a
remote temperature diode to ensure that it can function over
the required temperature range.
Nulling Out Temperature Errors
The ADT7466 automatically nulls out temperature
measurement errors due to series resistance, but systematic
errors in the temperature measurement can arise from a
number of sources, and the ADT7466 can reduce these errors.
As CPUs run faster, it is more difficult to avoid high frequency
clocks when routing the D+, D− tracks around a system board.
Even when recommended layout guidelines are followed, there
may still be temperature errors attributed to noise being
coupled onto the D+/D− lines. High frequency noise generally
has the effect of giving temperature measurements that are too
high by a constant amount. The ADT7466 has temperature
offset registers at addresses 0x26 and 0x27 for the remote and
local temperature channels. A one time calibration of the
system 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. The LSB adds 1°C offset to the
temperature reading so the 8-bit register effectively allows
temperature offsets of up to ±128°C with a resolution of 1°C.
This ensures that the readings in the temperature measurement
registers are as accurate as possible.
Table 12. Temperature Offset Registers
Register Description Default
0x24 Thermistor 1/Remote 2 offset 0x00 (0°C)
0x25 Thermistor 2 offset 0x00 (0°C)
0x26 Remote1 temperature offset 0x00 (0°C)
0x27 Local temperature offset 0x00 (0°C)
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ADT7466ZEVB

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