AD8571/AD8572/AD8574 Data Sheet
Rev. F | Page 22 of 28
HIGH ACCURACY THERMOCOUPLE AMPLIFIER
Figure 66 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 0°C ice bath, R6 should be adjusted until the
output is at 0 V.
Using the values shown in Figure 66, the output voltage tracks
temperature at 10 mV/°C. For a wider range of temperature
measurement, R9 can be decreased to 62 kΩ. This creates a
5 mV/°C change at the output, allowing measurements of up
to 1000°C.
AD8571
3
7
4
0V TO 5V
(0°C TO 500°C)
5V
0.1µF
10µF
REF02EZ
0.1µF
12V
2
6
4
+
–
+
–
D1
1N4148
5V
K-TYPE
THERMOCOUPLE
40.7µV/°C
6
2
R2
2.74kΩ
R6
200Ω
R3
53.6Ω
R4
5.62kΩ
R1
10.7kΩ
R5
40.2kΩ
R9
124kΩ
R8
453Ω
+
01104-066
Figure 66. 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 AD8571/AD8572/AD8574 are
excellent amplifiers for precision current monitoring. Its rail-to-
rail input allows the amplifier to be used as either a high-side
or a 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 67 shows a high-side current monitor configuration.
Here, the input common-mode voltage of the amplifier is at or
near the positive supply voltage. The rail-to-rail input of the
amplifier 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 AD8571/AD8572/AD8574. The output of the
amplifier is corrected until this voltage appears at the inverting
input, which creates a current through R1 that in turn flows
through R2. The monitor output is given by
Monitor Output = R2 × (R
SENSE
/R1) × I
L
(23)
Using the components shown in Figure 67, the monitor output
transfer function is 2.49 V/A.
8
1
4
3
V+
0.1µF
I
L
G
S
D
2
M1
Si9433
MONITOR
OUTPUT
V+
1/2
AD8572
R1
100Ω
R2
2.49kΩ
R
SENSE
0.1Ω
01104-067
LOAD
+
–
Figure 67. High-Side Load Current Monitor
Figure 68 shows the low-side monitor equivalent. In this circuit,
the input common-mode voltage to the AD8572 is at or near
ground. Again, a 0.1 Ω resistor provides a voltage drop propor-
tional to the return current. The output voltage is given as
××−=
+
L
SENSE
IR
R
R
VOutputMonitor
1
2
(24)
For the component values shown in Figure 68, the monitor
output transfer function is V+ − 2.49 V/A.
01104-068
V+
2
3
1/2 AD8572
V+
MONITOR
OUTPUT
Q1
R
SENSE
0.1Ω
R1
100Ω
R2
2.49kΩ
I
L
LOAD
V+
Figure 68. Low-Side Load Current Monitor
PRECISION VOLTAGE COMPARATOR
The AD8571/AD8572/AD8574 can be operated open loop and
used as a precision comparator. The AD8571/AD8572/AD8574
have less than 50 µV of offset voltage when they run in this
configuration. The slight increase of offset voltage stems from
the fact that the autocorrection architecture operates with the
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 that the maximum differential
voltage of the device is not exceeded. For more information, see
the Input Overvoltage Protection section.
