AD8551/AD8552/AD8554

–16–

REV. 0

V2

V1

AD855x

IF

=

(V1

V2)

Figure 58. Using the AD855x as a Difference Amplifier

In an ideal difference amplifier, the ratio of the resistors are set

exactly equal to:

A

R

R

R

R

2

1

4

3

(19)

Which sets the output voltage of the system to:

VA V

V

OUT

V

=−

()

12

(20)

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:

CMRR

R R

RR

RR

RR

R R

=

++

−

1

4

24

23

14

2

3

2

22

(21)

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:

CMRR

1

2

δ

(22)

R

R

R

R

AD8554-C

V2

R

R

V1

AD8554-B

AD8554-A

2R

(V1

V2)

Figure 59. A Discrete Instrumentation Amplifier

Configuration

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 k

Ω resistors

with 1% tolerance would require a series trimming resistor equal to

100

Ω.

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-

adjusted until the output is at 0 V.

Using the values shown in Figure 60, the output voltage will

track temperature at 10 mV/

°C. For a wider range of tempera-

Ω. This will

create a 5 mV/

°C change at the output, allowing measurements

of up to 1000

°C.

AD8551

3

2

8

4

0V TO 5.00V

(0 C TO 500 C)

+5V

0.1 F

+

10 F

124k

453

40.2k

10.7k

2.74k

REF02EZ

0.1 F

+12V

2

6

4

++

––

D1

1N4148

53.6

5.62k

+5.000V

K-TYPE

THERMOCOUPLE

40.7 V/ C

200

1

Figure 60. 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 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.

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.

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

is given by:

Monitor Output

R

R

R

I

SENSE

L

=×









×

2

1

(23)

Using the components shown in Figure 61, the Monitor Output

transfer function is 2.5 V/A.