AD8571ARMZ datasheet(16/24 Pages) AD

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AD8571/AD8572/AD8574

Rev. E | Page 16 of 24

Therefore,

OSB

OSA

EFF

≈

(14)

Thus, the offset voltages of both the primary and nulling

amplifiers are reduced by the gain factor BA, which takes a typical

input offset voltage from several millivolts down to an effective

input offset voltage of submicrovolts. This autocorrection scheme

makes the AD857x family of amplifiers extremely precise.

HIGH GAIN, CMRR, AND PSRR

Common-mode and power supply rejection are indications of the

amount of offset voltage an amplifier has as a result of a change in

its input common-mode or power supply voltages. As shown in

the Amplification Phase section, the autocorrection architecture

of the AD857x allows it to effectively minimize offset voltages.

The technique also corrects for offset errors caused by common-

mode voltage swings and power supply variations, which results

in superb CMRR and PSRR figures in excess of 130 dB. Because

the autocorrection occurs continuously, these figures can be

maintained across the temperature range of the device (−40°C

to +125°C).

MAXIMIZING PERFORMANCE THROUGH PROPER

LAYOUT

To achieve the maximum performance of the extremely high

input impedance and low offset voltage of the AD857x, care

should be taken in the circuit board layout. The PCB surface

must remain clean and free of moisture to avoid leakage currents

between adjacent traces. Surface coating of the circuit board

reduces surface moisture and provides a humidity barrier, reducing

parasitic resistance on the board. The use of guard rings around

the amplifier inputs further reduces leakage currents. Figure 52

shows how the guard ring should be configured, and Figure 53

shows the top view of how a surface-mount layout can be

arranged. The guard ring does not need to be a specific width,

but it should form a continuous loop around both inputs. By

setting the guard ring voltage equal to the voltage at the non-

inverting input, parasitic capacitance is minimized as well. For

further reduction of leakage currents, components can be mounted

to the PCB using Teflon® standoff insulators.

VOUT

VIN

AD8572

VIN

AD8572

Figure 52. Guard Ring Layout and Connections to

Reduce PCB Leakage Currents

V–

V+

VREF

VIN1

VIN2

GUARD

RING

AD8572

GUARD

RING

Figure 53. Top View of AD8572 SOIC Layout with Guard Rings

Other potential sources of offset error are thermoelectric

voltages on the circuit board. This voltage, also called Seebeck

voltage, occurs at the junction of two dissimilar metals and is

proportional to the junction temperature. The most common

metallic junctions on a circuit board are solder-to-board trace

and solder-to-component lead. Figure 54 shows a cross-section

view of the thermal voltage error sources. When the temperature

of the PCB at one end of the component (TA1) differs from the

temperature at the other end (TA2), the Seebeck voltages are not

equal, resulting in a thermal voltage error.

This thermocouple error can be reduced by using dummy

components to match the thermoelectric error source. Placing

the dummy component as close as possible to its partner ensures

that both Seebeck voltages are equal, thus canceling the thermo-

couple error. Maintaining a constant ambient temperature on the

circuit board further reduces this error. The use of a ground

plane helps distribute heat throughout the board and also

reduces EMI noise pickup.

SURFACE MOUNT

COMPONENT

LEAD

SOLDER

PC BOARD

COPPER

TRACE

TA2

IF TA1 ≠ TA2, THEN

VTS1 + VSC1 ≠ VTS2 + VSC2

TA1

VSC1

VTS1

–

VSC2

VTS2

–

Figure 54. Mismatch in Seebeck Voltages Causes a Thermoelectric Voltage Error

RS SHOULD BE PLACED IN CLOSE PROXIMITY AND

ALIGNMENT TO R1 TO BALANCE SEEBECK VOLTAGES

VOUT

VIN

AD8571/AD8572/

AD8574

AV = 1 + (RF /R1)

RS = R1

Figure 55. Using Dummy Components to Cancel Thermoelectric Voltage Errors

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