AD8571ARMZ datasheet(22/28 Pages) AD

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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

0V TO 5V

(0°C TO 500°C)

0.1µF

10µF

REF02EZ

0.1µF

12V

–

1N4148

K-TYPE

THERMOCOUPLE

40.7µV/°C

2.74kΩ

200Ω

53.6Ω

5.62kΩ

10.7kΩ

40.2kΩ

124kΩ

453Ω

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 × (RSENSE/R1) × IL

(23)

Using the components shown in Figure 67, the monitor output

transfer function is 2.49 V/A.

V+

0.1µF

Si9433

MONITOR

OUTPUT

V+

1/2

AD8572

100Ω

2.49kΩ

RSENSE

0.1Ω

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













−

SENSE

Output

Monitor

(24)

For the component values shown in Figure 68, the monitor

output transfer function is V+ − 2.49 V/A.

V+

1/2 AD8572

V+

MONITOR

OUTPUT

RSENSE

0.1Ω

100Ω

2.49kΩ

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.

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