AD8571ARM View Datasheet(PDF) - Analog Devices
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AD8571ARM Datasheet PDF : 19 Pages
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AD8571/AD8572/AD8574
A High Accuracy Thermocouple Amplifier
Figure 58 shows a K-type thermocouple amplifier configuration
with cold-junction compensation. Even from a 5 V supply, the
AD8571 can provide enough accuracy to achieve a resolution
of better than 0.02°C from 0°C to 500°C. D1 is used as a tempera-
ture measuring device to correct the cold-junction error from
the thermocouple and should be placed as close as possible to
the two terminating junctions. With the thermocouple measuring
tip immersed in a zero-degree ice bath, R6 should be adjusted
until the output is at 0 V.
Using the values shown in Figure 58, the output voltage will
track temperature at 10 mV/°C. For a wider range of tempera-
ture measurement, R9 can be decreased to 62 kΩ. This will
create a 5 mV/°C change at the output, allowing measurements
of up to 1000°C.
12V
0.1F
REF02EZ 5V
2
6
4
R1
10.7k⍀
1N4148
D1
R5
40.2k⍀
K-TYPE
THERMOCOUPLE
40.7V/؇C
––
++
R2
2.74k⍀
R8
453⍀ 2
R6
3
200⍀
R4
5.62k⍀
R3
53.6⍀
R9
124k⍀
5V
10F
+
0.1F
8
1
4 AD8571
0V TO 5V
(0؇C TO 500؇C)
Figure 58. A Precision K-Type Thermocouple Amplifier
with Cold-Junction Compensation
Precision Current Meter
Because of its low input bias current and superb offset voltage at
single supply voltages, the AD857x is an excellent amplifier for
precision current monitoring. Its rail-to-rail input allows the
amplifier to be used as either a high-side or low-side current
monitor. Using both amplifiers in the AD8572 provides a simple
method to monitor both current supply and return paths for
load or fault detection.
Figure 59 shows a high-side current monitor configuration. Here,
the input common-mode voltage of the amplifier will be at or near
the positive supply voltage. The amplifier’s rail-to-rail input provides
a precise measurement, even with the input common-mode voltage
at the supply voltage. The CMOS input structure does not draw any
input bias current, ensuring a minimum of measurement error.
The 0.1 Ω resistor creates a voltage drop to the noninverting
input of the AD857x. The amplifier’s output is corrected until
this voltage appears at the inverting input. This creates a current
through R1, which in turn flows through R2. The Monitor Output
is given by:
Monitor
Output
=
R2
×
RSENSE
R1
×
IL
(23)
Using the components shown in Figure 59, the Monitor Output
transfer function is 2.5␣ V/A.
Figure 60 shows the low-side monitor equivalent. In this circuit,
the input common-mode voltage to the AD8572 will be at or near
ground. Again, a 0.1 Ω resistor provides a voltage drop proportional
to the return current. The output voltage is given as:
VOUT
=V
+
−
R2
R1
× RSENSE
×
I
L
(24)
For the component values shown in Figure 60, the output transfer
function decreases from V at –2.5 V/A.
RSENSE
0.1⍀
3V
IL
V+
3V
0.1F
R1
100⍀
3
8
1/2
1
AD8572
2
4
S
M1
G
Si9433
MONITOR
OUTPUT
D
R2
2.49k⍀
Figure 59. A High-Side Load Current Monitor
V+
VOUT
R2
2.49k⍀
Q1
V+
R1
100⍀
RSENSE
0.1⍀
1/2 AD8572
RETURN TO
GROUND
Figure 60. A Low-Side Load Current Monitor
Precision Voltage Comparator
The AD857x can be operated open-loop and used as a precision
comparator. The AD857x has less than 50 µV of offset voltage
when run in this configuration. The slight increase of offset
voltage stems from the fact that the autocorrection architecture
operates with lowest offset in a closed-loop configuration, that
is, one with negative feedback. With 50 mV of overdrive, the
device has a propagation delay of 15 µs on the rising edge and
8 µs on the falling edge.
Care should be taken to ensure the maximum differential volt-
age of the device is not exceeded. For more information, please
refer to the section on Input Overvoltage Protection.
–16–
REV. 0
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