AD8551/AD8552/AD8554
R
2
A High Accuracy Thermocouple Amplifier
Figure 60 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
temperature 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 mea-
suring tip immersed in a zero-degree ice bath, R6 should be
adjusted until the output is at 0 V.
R
1
V2
V1
V
OUT
AD855x
R
3
R
4
R
R
R
R
R
R
4
2
2
IF
=
, THEN V
=
OUT
3
1
1
Figure 58. Using the AD855x as a Difference Amplifier
In an ideal difference amplifier, the ratio of the resistors are set
exactly equal to:
Using the values shown in Figure 60, 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.
R2 R4
R1 R3
AV
=
=
(19)
Which sets the output voltage of the system to:
VOUT = A V1−V2
(20)
+5.000V
(
)
V
2
REF02EZ
4
6
+12V
R
9
0.1F
Due to finite component tolerance the ratio between the four
resistors will not be exactly equal, and any mismatch results in a
reduction of common-mode rejection from the system. Referring
to Figure 58, the exact common-mode rejection ratio can be ex-
pressed as:
124k⍀
R
R
5
1
10.7k⍀ 40.2k⍀
+5V
1N4148
D1
10F
+
0.1F
R
R
453⍀
2
8
2.74k⍀
–
–
+
8
K-TYPE
THERMOCOUPLE
40.7V/؇C
2
R1R4 + 2R2R4 + R2R3
1
+
CMRR =
(21)
R
6
2R1R4 − 2R2R3
AD8551
200⍀
4
3
R
53.6⍀
R
4
5.62k⍀
3
0V TO 5.00V
(0؇C TO 500؇C)
In the 3 op amp instrumentation amplifier configuration shown
in Figure 59, the output difference amplifier is set to unity gain
with all four resistors equal in value. If the tolerance of the resis-
tors used in the circuit is given as δ, the worst-case CMRR of
the instrumentation amplifier will be:
Figure 60. A Precision K-Type Thermocouple Amplifier
with Cold-Junction Compensation
Precision Current Meter
1
2δ
CMRRMIN
=
Because of its low input bias current and superb offset voltage at
single supply voltages, the AD855x 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 AD8552 provides a simple
method to monitor both current supply and return paths for
load or fault detection.
(22)
AD8554-A
V2
R
R
R
R
V
R
OUT
G
Figure 61 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.
AD8554-C
R
R
R
V1
TRIM
AD8554-B
2R
(V1 ؊ V2)
V
= 1 +
OUT
R
G
Figure 59. A Discrete Instrumentation Amplifier
Configuration
The 0.1 Ω resistor creates a voltage drop to the noninverting
input of the AD855x. 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:
Thus, using 1% tolerance resistors would result in a worst-case
system CMRR of 0.02, or 34 dB. Therefore either high precision
resistors or an additional trimming resistor, as shown in Figure
59, should be used to achieve high common-mode rejection. The
value of this trimming resistor should be equal to the value of R
multiplied by its tolerance. For example, using 10 kΩ resistors
with 1% tolerance would require a series trimming resistor equal to
100 Ω.
RSENSE
Monitor Output = R2 ×
× IL
(23)
R1
Using the components shown in Figure 61, the Monitor Output
transfer function is 2.5 V/A.
–16–
REV. 0
ADI [ ADI ]
