Data Sheet AD8551/AD8552/AD8554
Rev. F | Page 21 of 24
A HIGH ACCURACY THERMOCOUPLE AMPLIFIER
Figure 68 shows a K-type thermocouple amplifier configuration
with cold junction compensation. Even from a 5 V supply, the
AD8551 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, R
6
should be adjusted until the
output is at 0 V.
Using the values shown in Figure 68, the output voltage tracks
temperature at 10 mV/°C. For a wider range of temperature
measurement, R
9
can be decreased to 62 kΩ. This creates a
5 mV/°C change at the output, allowing measurements of up
to 1000°C.
3
2
7
4
5V
+
REF02EZ
12V
2
6
4
D1
1N4148
5.000V
1
–
+
AD8551
0.1µF
0.1µF
10µF
K-TYPE
THERMOCOUPLE
40.7µV/°C
0V TO 5.00V
(0°C TO 500°C)
R
4
5.62kΩ
R
6
200Ω
R
3
53.6Ω
R
2
2.74kΩ
R
1
10.7kΩ
R
5
40.2kΩ
R
8
124kΩ
R
7
453Ω
01101-068
Figure 68. 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 AD8551/AD8552/AD8554 are
excellent amplifiers 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 AD8552
provides a simple method to monitor both current supply and
return paths for load or fault detection.
Figure 69 shows a high-side current monitor configuration. In
this configuration, 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 AD8551/AD8552/AD8554. The output of the
amplifier is corrected until this voltage appears at the inverting
input. This creates a current through R
1
, which in turn flows
through R
2
. The monitor output is given by
L
1
SENSE
2
I
R
R
ROutput
Monitor ×






ï£

×
=
(23)
Using the components shown in Figure 69, the monitor output
transfer function is 2.5 V/A.
Figure 70 shows the low-side monitor equivalent. In this circuit,
the input common-mode voltage to the AD8552 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
(
)






ï£

×
×−+=
L
SENSE
OUT
IR
R
R
VV
1
2
(24)
For the component values shown in Figure 70, the output
transfer function decreases from V+ at −2.5 V/A.
8
1
4
3
3V
V+
G
S
D
2
3V
1/2
AD8552
MONITOR
OUTPUT
M1
Si9433
R
1
100Ω
R
2
2.49kΩ
R
SENSE
0.1Ω
I
L
0.1µF
01101-069
Figure 69. A High-Side Load Current Monitor
V+
1/2 AD8552
V+
Q1
RETURN TO
GROUND
V
OUT
R
2
2.49kΩ
R
1
100Ω
R
SENSE
0.1Ω
01101-070
Figure 70. A Low-Side Load Current Monitor
PRECISION VOLTAGE COMPARATOR
The AD8551/AD8552/AD8554 can be operated open-loop and
used as a precision comparator. The AD8551/AD8552/AD8554
have 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. Ensure the maximum differential voltage of the
device is not exceeded. For more information, refer to the Input
Overvoltage Protection section.
