ADA4522-4ARZ-R7 datasheet(29/32 Pages) AD

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Data Sheet

ADA4522-1/ADA4522-2/ADA4522-4

Rev. E | Page 29 of 32

performance of the ultralow offset voltage of the ADA4522-1/

ADA4522-2/ADA4522-4.

SOLDER

COMPONENT

LEAD

COPPER

TRACE

VSC1

VTS1

TA1

SURFACE-MOUNT

COMPONENT

PC BOARD

TA2

VSC2

VTS2

IF TA1 ≠ TA2, THEN

VTS1 + VSC1 ≠ VTS2 + VSC2

Figure 85. Mismatch in Seebeck Voltages Causes Seebeck Voltage Error

In Figure 85, VSC1 and VSC2 are the Seebeck voltages due to solder to

component at Junction 1 and Junction 2, respectively. VTS1 and

VTS2 are the Seebeck voltages due to solder to trace at Junction 1

and Junction 2. TA1 and TA2 are the temperatures of Junction 1

and Junction 2, respectively.

To minimize these thermocouple effects, orient resistors so

that heat sources warm both ends equally. Where possible, it

is recommended that the input signal paths contain matching

numbers and types of components to match the number and type

of thermocouple junctions. For example, dummy components,

such as zero value resistors, can be used to match the thermo-

electric error source (real resistors in the opposite input path).

Place matching components in close proximity and orient them

in the same manner to ensure equal Seebeck voltages, thus

cancelling thermal errors. Additionally, use leads that are of

equal length to keep thermal conduction in equilibrium. Keep

heat sources on the PCB as far away from amplifier input circuitry

as is practical.

It is highly recommended to use a ground plane. A ground

plane helps distribute heat throughout the board, maintain a

constant temperature across the board, and reduce EMI noise

pickup.

COMPARATOR OPERATION

An op amp is designed to operate in a closed-loop configuration

with feedback from its output to its inverting input. In contrast to

op amps, comparators are designed to operate in an open-loop

configuration and to drive logic circuits. Although op amps are

different from comparators, occasionally an unused section of a

dual op amp is used as a comparator to save board space and

cost; however, this is not recommended for the ADA4522-1/

ADA4522-2/ADA4522-4.

Figure 86 and Figure 87 show the ADA4522-1/ADA4522-2/

ADA4522-4 configured as a comparator, with 10 kΩ resistors in

series with the input pins. Any unused channels are configured as

buffers with the input voltage kept at the midpoint of the power

supplies. The ADA4522-1/ADA4522-2/ADA4522-4 have input

devices that are protected from large differential input voltages by

Diode D5 and Diode D6, as shown in Figure 72. These diodes

consist of substrate PNP bipolar transistors, and conduct whenever

the differential input voltage exceeds approximately 600 mV;

however, these diodes also allow a current path from the input

to the lower supply rail, resulting in an increase in the total

supply current of the system. Both comparator configurations yield

the same result. At 30 V of power supply, ISY+ remains at 1.55 mA

per dual amplifier, but ISY− increases close to 2 mA in magni-

tude per dual amplifier.

ADA4522-1/

ADA4522-2/

ADA4522-4

10kΩ

ISY+

+VSY

VOUT

–VSY

ISY–

Figure 86. Comparator Configuration A

10kΩ

ISY+

+VSY

VOUT

–VSY

ISY–

ADA4522-1/

ADA4522-2/

ADA4522-4

Figure 87. Comparator Configuration B

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

VSY (V)

ISY+

ISY–

Figure 88. Supply Current (ISY) per Dual Amplifier vs. Supply Voltage (VSY)

(ADA4522-1/ADA4522-2/ADA4522-4 as a Comparator)

Note that 10 kΩ resistors are used in series with the input of the

op amp. If smaller resistor values are used, the supply current of the

system increases much more. For more details on op amps as

comparators, see the AN-849 Application Note, Using Op Amps

as Comparators.

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