ADA4522-4ARZ-R7 datasheet(30/33 Pages) AD

30 / 33 page

Data Sheet

ADA4522-1/ADA4522-2/ADA4522-4

APPLICATIONS INFORMATION

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Rev. G | 30 of 33

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 recom-

mended that the input signal paths contain matching numbers and

types of components to match the number and type of thermocou-

ple junctions. For example, dummy components, such as zero

value resistors, can be used to match the thermoelectric 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 canceling 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 dif-

ferent 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 sup-

plies. 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 magnitude per dual amplifier.

Figure 86. Comparator Configuration A

Figure 87. Comparator Configuration B

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.

USE OF LARGE SOURCE RESISTANCE

The ADA4522-1/ADA4522-2/ADA4522-4 are designed to work with

low value source resistance. Note that the amplifier has an ultralow

voltage noise density of 6 nV/√Hz. A 1 kΩ resistor contributes 4

nV/√Hz; therefore, placing a 1 kΩ resistor at the input increases

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