AD8571/AD8572/AD8574

–16–

REV. 0

A High Accuracy Thermocouple Amplifier

Figure 58 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

until the output is at 0 V.

Using the values shown in Figure 58, 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.

AD8571

3

8

4

0V TO 5V

(0 C TO 500 C)

5V

0.1 F

+

10 F

REF02EZ

0.1 F

12V

2

6

4

++

––

D1

1N4148

5V

K-TYPE

THERMOCOUPLE

40.7 V/ C

1

200

5.62k

124k

53.6

40.2k

453

10.7k

2

2.74k

Figure 58. 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 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 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 59 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 AD857x. 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 59, the Monitor Output

transfer function is 2.5 V/A.

Figure 60 shows the low-side monitor equivalent. In this circuit,

the input common-mode voltage to the AD8572 will be at or near

ground. Again, a 0.1

Ω resistor provides a voltage drop proportional

to the return current. The output voltage is given as:

VV

R

R

RI

OUT

SENSE

L

=+ −

×

×









2

1

(24)

For the component values shown in Figure 60, the output transfer

function decreases from V at –2.5 V/A.

8

1

4

3

3V

0.1 F

0.1

V+

G

S

D

2

M1

Si9433

MONITOR

OUTPUT

3V

1/2

AD8572

100

2.49k

Figure 59. A High-Side Load Current Monitor

V+

RETURN TO

GROUND

1/2 AD8572

V+

Q1

2.49k

100

0.1

Figure 60. A 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 volt-

age of the device is not exceeded. For more information, please

refer to the section on Input Overvoltage Protection.