Datasheet

ADT7411
Rev. B | Page 15 of 36
ADC Transfer Function
The output coding of the ADT7411 analog inputs is straight
binary. The designed code transitions occur midway between
successive integer LSB values (that is, 1/2 LSB, 3/2 LSB). The
LSB is V
DD
/1024 or Int V
REF
/1024, Int V
REF
= 2.25 V. The ideal
transfer characteristic is shown in Figure 24.
111...111
111...110
111...000
011...111
+V
REF
– 1LSB0V 1/2 LSB
ANALOG INPUT
ADC CODE
1LSB = INT V
REF
/1024
1LSB = V
DD
/1024
000...010
000...001
000...000
0
2882-023
Figure 24. Transfer Function
To work out the voltage on any analog input channel, the
following method is used.
1 LSB = Reference (V)/1024
Convert the value read back from the AIN value register into
de
cimal.
AIN Voltage = AIN Value (d)
× LSB Size
where d is t
he decimal.
Example:
Internal reference used. Therefore, V
REF
= 2.25 V.
AIN Value = 51
2d
1 LSB Size = 2.2
5 V/1024 = 2.197 × 10
−3
AIN Voltage = 512 × 2.197 × 10
−3
= 1.125 V
Analog Input ESD Protection
Figure 26 shows the input structure that provides ESD protec-
tion on any of the analog input pins. The diode provides the
main ESD protection for the analog inputs. Care must be taken
that the analog input signal never drops below the GND rail by
more than 200 mV. If this happens, the diode becomes forward
biased and starts conducting current into the substrate. The
4 pF capacitor is the typical pin capacitance and the resistor is a
lumped component made up of the on resistance of the
multiplexer switch.
BIAS
DIODE
INTERNAL
SENSE
TRANSISTOR
V
DD
TO ADC
V
OUT+
V
OUT–
I N × I I
BIAS
02882-024
Figure 25. Top Level Structure of Int
ernal Temperature Sensor
4pF
A
IN
100Ω
02882-025
Figure 26. Equivalent Analog Input ESD Circuit
AIN Interrupts
The measured results from the AIN inputs are compared with
the AIN V
HIGH
(greater than comparison) and V
LOW
(less than or
equal to comparison) limits. An interrupt occurs if the AIN
inputs exceed or equal the limit registers. These voltage limits
are stored in on-chip registers. Note that the limit registers are
eight bits long while the AIN conversion result is 10 bits long.
If the voltage limits are not masked out, any out-of-limit
comparisons generate flags that are stored in the Interrupt
Status 1 register (Address 00h) and one or more out-of-limit
results will cause the INT/
INT
output to pull either high or
low, depending on the output polarity setting. It is good design
practice to mask out interrupts for channels that are of no
concern to the application.
Figure 27 shows the interrupt
s
tructure for the ADT7411. It shows a block diagram
representation of how the various measurement channels
affect the INT/
INT
pin.
FUNCTIONAL DESCRIPTION—MEASUREMENT
Temperature Sensor
The ADT7411 contains an ADC with special input signal
conditioning to enable operation with external and on-chip
diode temperature sensors. When the ADT7411 is operating in
single-channel mode, the ADC continually processes the
measurement taken on one channel only. This channel is
preselected by Bit C0 to Bit C3 in the Control Configuration 2
register (Address 19h). When in round robin mode, the analog
input multiplexer sequentially selects the V
DD
input channel,
on-chip temperature sensor to measure its internal temperature,
the external temperature sensor, or an AIN channel, and then
the rest of the AIN channels. These signals are digitized by the
ADC and the results stored in the various value registers.