AD8551ARM-R2 datasheet(21/24 Pages) AD

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AD8551/AD8552/AD8554

Rev. C | Page 21 of 24

HIGH ACCURACY THERMOCOUPLE AMPLIFIER

SENSE

Output

Monitor

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

(23)

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, R6 should be adjusted until the

output is at 0 V.

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

()

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

SENSE

OUT

(24)

Using the values shown in Figure 68, 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.

For the component values shown in Figure 70, the output

transfer function decreases from V+ at −2.5 V/A.

V+

1/2

AD8552

MONITOR

OUTPUT

Si9433

100Ω

2.49kΩ

RSENSE

0.1Ω

0.1µF

REF02EZ

12V

1N4148

5.000V

–

AD8551

0.1µF

10µF

K-TYPE

THERMOCOUPLE

40.7µV/°C

0V TO 5.00V

(0°C TO 500°C)

5.62kΩ

200Ω

53.6Ω

2.74kΩ

10.7kΩ

40.2kΩ

124kΩ

453Ω

Figure 69. A High-Side Load Current Monitor

Figure 68. A Precision K-Type Thermocouple Amplifier with

Cold Junction Compensation

V+

1/2 AD8552

V+

RETURN TO

GROUND

VOUT

2.49kΩ

100Ω

RSENSE

0.1Ω

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 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.

Figure 70. A Low-Side Load Current Monitor

PRECISION VOLTAGE COMPARATOR

The AD855x can be operated open-loop and used as a precision

comparator. The AD855x 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. Ensure the maximum differential

voltage of the device is not exceeded. For more information,

refer to the Input Overvoltage Protection section.

The 0.1 Ω resistor creates a voltage drop to the noninverting

input of the AD855x. The output of the amplifier 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

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