ADM1026
The ADC uses the internal 22.5 kHz clock, which has a toler-
ance of 6%, so the worst-case monitoring cycle time is 290 ms.
The fan speed measurement uses a completely separate
monitoring loop, as described later.
Limit Values
Limit values for analog measurements are stored in the appropri
ate limit registers. In the case of voltage measurements, high and
low limits can be stored so that an interrupt request is generated
if the measured value goes above or below acceptable values. In
the case of temperature, a hot temperature or high limit can be
programmed, and a hot temperature hysteresis or low limit can
be programmed, which is usually some degrees lower. This can
be useful because it allows the system to be shut down when the
hot limit is exceeded, and restarted automatically when it has
cooled down to a safe temperature.
Input Safety
Scaling of the analog inputs is performed on-chip, so external
attenuators are typically not required. However, because the
power supply voltages appear directly at the pins, it is advisable
to add small external resistors (that is, 500 Ω) in series with the
supply traces to the chip to prevent damaging the traces or
power supplies should an accidental short such as a probe
connect two power supplies together.
Analog Monitoring Cycle Time
The analog monitoring cycle begins when a 1 is written to the
start bit (Bit 0), and a 0 to the INT_Clear bit (Bit 2) of the con-
figuration register. INT_Enable (Bit 1) should be set to 1 to
Because the resistors form part of the input attenuators, they
affect the accuracy of the analog measurement if their value
is too high. The worst such accident would be connecting
−12 V to +12 V where there is a total of 24 V difference. With
the series resistors, this would draw a maximum current of
approximately 24 mA.
enable the
output. The ADC measures each analog input in
INT
turn, starting with Remote Temperature Channel 1 and ending
with local temperature. As each measurement is completed, the
result is automatically stored in the appropriate value register.
This round-robin monitoring cycle continues until it is disabled
by writing a 0 to Bit 0 of the configuration register. Because the
ADC is typically left to free-run in this way, the most recently
measured value of any input can be read out at any time.
ANALOG OUTPUT
The ADM1026 has a single analog output from an unsigned
8-bit DAC that produces 0 V to 2.5 V (independent of the refer-
ence voltage setting). The input data for this DAC is contained
in the DAC control register (Address 04h). The DAC control
register defaults to FFh during a power-on reset, which pro-
duces maximum fan speed. The analog output may be amplified
and buffered with external circuitry such as an op amp and a
transistor to provide fan speed control. During automatic fan
speed control, described later, the four MSBs of this register set
the minimum fan speed.
For applications where the monitoring cycle time is important,
it can easily be calculated.
The total number of channels measured is
•
•
•
•
•
•
Five dedicated supply voltage inputs
Ten general-purpose analog inputs
3.3 V MAIN
3.3 V STBY
Local temperature
Suitable fan drive circuits are shown in Figure 36 through
Figure 40. When using any of these circuits, note the following:
Two remote temperature
•
All of these circuits provide an output range from 0 V to
almost +12 V, apart from Figure 36, which loses the base-
emitter voltage drop of Q1 due to the emitter-follower
configuration.
Pins 28 and 27 are measured both as analog inputs AIN8/AIN9 and
as remote temperature input D2+/D2−, irrespective of which
configuration is selected for these pins.
•
•
To amplify the 2.5 V range of the analog output up to 12 V,
the gain of these circuits needs to be about 4.8.
Take care when choosing the op amp to ensure that its
input common-mode range and output voltage swing are
suitable.
The op amp may be powered from the +12 V rail alone
or from 12 V. If it is powered from +12 V, the input
common-mode range should include ground to accom-
modate the minimum output voltage of the DAC, and the
output voltage should swing below 0.6 V to ensure that the
transistor can be turned fully off.
If Pins 28 and 27 are configured as AIN8/AIN9, the measurements
for these channels are stored in Registers 27h and 29h, and the
invalid temperature measurement is discarded. On the other
hand, if Pins 28 and 27 are configured as D2+/D2−, the temper-
ature measurement is stored in Register 29h, and there is no
valid result in Register 27h.
•
As mentioned previously, the ADC performs a conversion every
711 µs on the analog and local temperature inputs and every
2.13 ms on the remote temperature inputs. Each input is
measured 16 times and averaged to reduce noise.
•
If the op amp is powered from −12 V, precautions such as
a clamp diode to ground may be needed to prevent the
base-emitter junction of the output transistor being
reverse-biased in the unlikely event that the output of
the op amp should swing negative for any reason.
The total monitoring cycle time for voltage and temperature
inputs is therefore nominally
(18 × 16 × 0.711) + (2 × 16 × 2.13) = 273 ms
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