AD8224ACPZ-R7 datasheet(23/29 Pages) AD

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AD8224

Data Sheet

Rev. D | Page 22 of 28

Table 13. Gains Achieved Using 1% Resistors

1% Standard Table Value of RG (Ω)

Calculated Gain

49.9 k

1.990

12.4 k

4.984

5.49 k

9.998

2.61 k

19.93

1.00 k

50.40

499

100.0

249

199.4

100

495.0

49.9

991.0

The AD8224 defaults to G = 1 when no gain resistor is used.

The tolerance and gain drift of the RG resistor should be added

to the AD8224 specifications to determine the total gain

accuracy of the system. When the gain resistor is not used,

gain error and gain drift are kept to a minimum.

REFERENCE TERMINAL

The output voltage of the AD8224 is developed with respect to

the potential on the reference terminal. This is useful when the

output signal needs to be offset to a precise midsupply level. For

example, a voltage source can be tied to the REF1 pin or the

REF2 pin to level-shift the output so that the AD8224 can drive

a single-supply ADC. Pin REFx is protected with ESD diodes

and should not exceed either +VS or −VS by more than 0.5 V.

For best performance, source impedance to the REF terminal

should be kept below 1 Ω. As shown in Figure 56, the reference

terminal, REF, is at one end of a 20 kΩ resistor. Additional

impedance at the REF terminal adds to this 20 kΩ resistor and

results in amplification of the signal connected to the positive

input. The amplification from the additional RREF can be

computed by





REF



kΩ

Only the positive signal path is amplified; the negative path is

unaffected. This uneven amplification degrades the CMRR of

the amplifier.

INCORRECT

AD8224

VREF

CORRECT

AD8224

OP2177

–

VREF

CORRECT

AD8224

–

VREF

Figure 57. Driving the Reference Pin

LAYOUT

The AD8224 is a high precision device. To ensure optimum

performance at the PCB level, care must be taken in the design

of the board layout. The AD8224 pinout is arranged in a logical

manner to aid in this task.

Package Considerations

The AD8224 is available in two version s of the 16-lead, 4 mm ×

4 mm LFCSP package: with or without an exposed pad. Blindly

copying the footprint from another 4 mm × 4 mm LFCSP part

is not recommended because it may not have the same thermal

pad size and leads. Refer to the Outline Dimensions section to

verify that the PCB symbol has the correct dimensions.

Hidden Paddle Package

The AD8224 is available in an LFCSP package with a hidden

paddle. It is the preferred package for the AD8224. Unlike

chip scale packages where the pad limits routing capability,

this package allows routes and vias directly underneath the

chip, so that the full space savings of the small LFCSP can be

realized. Although the package has no metal in the center of

the part, the manufacturing process does leave a very small

section of exposed metal at each of the package corners, shown

in Figure 58 as well as Figure 69 in the Outline Dimensions

section. This metal is connected to +VS through the part.

Because of a possibility of a short, vias should not be placed

underneath these exposed metal tabs.

HIDDEN

PADDLE

BOTTOM VIEW

EXPOSED LEAD

FRAME TABS

NOTES

1. EXPOSED LEAD FRAME TABS AT THE FOUR CORNERS

OF THE PACKAGE ARE INTERNALLY CONNECTED TO

+VS. REFER TO THE OUTLINE DIMENSIONS PAGE, FOR

FURTHER INFORMATION ON PACKAGE AVAILABILITY.

Figure 58. Hidden Paddle Package: Bottom View

Exposed Pad Package

The AD8224 4 mm × 4 mm LFCSP is also available with an

exposed thermal pad package version. This pad is connected

internally to +VS. The pad can either be left unconnected or

connected to the positive supply rail. Space between the leads

and thermal pad should be kept as wide as possible for the best

bias current performance. To maintain the AD8224 ultralow

bias current performance, the thermal pad area can be reduced

to extend the gap between the leads and the pad. The exposed

pad package also has exposed lead frame tabs at the corners of

the package, similar to those of the hidden paddle package,

which are internally connected to +VS. Do not place vias

underneath these metal tabs.

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