AD8571ARMZ-R2 datasheet(21/24 Pages) AD

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AD8571/AD8572/AD8574

Rev. B | Page 21 of 24

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.

AD8572

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

53.6kΩ

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 AD857x 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 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 AD857x. The output of the amplifier is corrected

until this voltage appears at the inverting input. This creates a

current through R1 that in turn flows through R2. The monitor

output is given by

SENSE

Output

Monitor

⎟

⎠

⎞

⎜

⎝

⎛

(23)

Using the components shown in Figure 67, the monitor output

transfer function is 2.5 V/A.

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

OUT

(24)

For the component values shown in Figure 68, the output

transfer function decreases from V at –2.5 V/A.

0.1µF

V+

Si9433

MONITOR

OUTPUT

1/2

AD8572

100Ω

2.49kΩ

RSENSE

0.1Ω

Figure 67. High-Side Load Current Monitor

V+

RETURN TO

GROUND

1/2 AD8572

V+

VOUT

RSENSE

0.1Ω

100Ω

2.49kΩ

Figure 68. Low-Side Load Current Monitor

PRECISION VOLTAGE COMPARATOR

The AD857x can be operated open-loop and used as a precision

comparator. The AD857x 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.

Care should be taken to ensure the maximum differential

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

refer to the Input Overvoltage Protection section.

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