ADT7518
Rev. A | Page 19 of 40
DAC Reference Inputs
VREF and this can be increased to 0 V to 2 VREF. Increasing the
output voltage span to 2 VREF can be done by setting D0 = 1 for
DAC A (internal temperature sensor) and D1 = 1 for DAC B
(external temperature sensor) in the DAC configuration register
(Address 1Bh).
There is an input reference pin for the DACs. This reference
input is buffered (see Figure 40).
The advantage with the buffered input is the high impedance it
presents to the voltage source driving it. The user can have an
external reference voltage as low as 1 V and as high as VDD. The
restriction of 1 V is due to the footroom of the reference buffer.
The output voltage is capable of tracking a maximum temp-
erature range of 128癈 to +127癈, but the default setting is
40癈 to +127癈. If the output voltage range is 0 V to VREF-IN
(VREF-IN = 2.25 V), then this corresponds to 0 V representing
40癈, and 1.48 V representing +127癈. This, of course, will
give an upper deadband between 1.48 V and VREF.
The
LDAC
configuration register controls the option to select
between internal and external voltage references. The default
setting is for external reference selected.
Output Amplifier
The internal and external analog temperature offset registers
can be used to vary this upper deadband and, consequently, the
temperature that 0 V corresponds to. Table 6 and Table 7 give
examples of how this is done using a DAC output voltage span
of VREF and 2 VREF, respectively. Simply write in the temperature
value, in twos complement format, at which 0 V is to start. For
example, if using the DAC A output and 0 V to start at 40癈,
program D8h into the internal analog temperature offset reg-
ister (Address 21h). This is an 8-bit register and has a temp-
erature offset resolution of only 1癈 for all device models. Use
the formulas following the tables to determine the value to
program into the offset registers.
The output buffer amplifier can generate output voltages to
within 1 mV of either rail. Its actual range depends on the value
of VREF, gain, and offset error.
If a gain of 1 is selected (Bits 0 to 3 of the DAC configuration
register = 0), the output range is 0.001 V to VREF.
If a gain of 2 is selected (Bits 0 to 3 of the DAC configuration
register = 1), the output range is 0.001 V to 2 V
REF
. Because of
clamping, however, the maximum output is limited to V
DD
0.001 V.
The output amplifier can drive a load of 4.7 k& to GND or VDD,
in parallel with 200 pF to GND or VDD (see Figure 5). The
source and sink capabilities of the output amplifier can be seen
in the plot of Figure 16.
Table 6. Thermal Voltage Output (0 V to VREF)
O/P Voltage (V) Default 癈
Max 癈
Sample 癈
0
40
128
0
0.5
+17
71
+56
1
+73
15
+113
1.12
+87
1
+127
1.47
+127
+39
UDB
1.5
UDB
+42
UDB
2
UDB
+99
UDB
2.25
UDB
+127
UDB
The slew rate is 0.7 V/祍 with a half-scale settling time to
?.5 LSB (at 8 bits) of 6 祍.
Thermal Voltage Output
The ADT7518 can output voltages that are proportional to
temperature. DAC A output can be configured to represent the
temperature of the internal sensor while DAC B output can be
configured to represent the external temperature sensor. Bits C5
and C6 of the Control Configuration 3 register select the temp-
erature proportional output voltage. Each time a temperature
measurement is taken, the DAC output is updated. The output
resolution for the ADT7518 is 8 bits with a 1癈 change corres-
ponding to 1 LSB change. The default output range is 0 V to
Upper deadband has been reached. DAC output is not capable of
increasing. See Fig
.
ure 9
C1
D+
LOW-PASS
FILTER
f
C
= 65kHz
BIAS
DIODE
DD
TO ADC
V
OUT+
V
OUT
REMOTE
SENSING
TRANSISTOR
(2N3906)
OPTIONAL CAPACITOR, UP TO
3nF MAX. CAN BE ADDED TO
IMPROVE HIGH FREQUENCY
NOISE REJECTION IN NOISY
ENVIRONMENTS
D
I
N ?/SPAN> I
I
BIAS
Figure 41. Signal Conditioning for External Diode Temperature Sensor
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