AD8551ARM-R2 datasheet(16/24 Pages) AD

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AD8551/AD8552/AD8554

Rev. C | Page 16 of 24

earn the reputation of being among the most precise amplifiers

available on the market.

HIGH GAIN, CMRR, 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 previous section, the autocorrection architecture

of the AD855x allows it to quite effectively minimize offset volt-

ages. The technique also corrects for offset errors caused by

common-mode voltage swings and power supply variations.

This results in superb CMRR and PSRR figures in excess of

130 dB. Because the autocorrection occurs continuously, these

figures can be maintained across the entire temperature range

of the device, from −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 AD855x, care is

needed in laying out the circuit board. The PC board surface

must remain clean and free of moisture to avoid leakage cur-

rents 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 cur-

rents. Figure 52 shows proper guard ring configuration, and

Figure 53 shows the top view of a surface-mount layout. 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 noninverting

input, parasitic capacitance is minimized as well. For further

reduction of leakage currents, components can be mounted to

the PC board using Teflon standoff insulators.

AD8552

VOUT

VIN

Figure 52. Guard Ring Layout and Connections to Reduce

PC Board Leakage Currents

V+

AD8552

V–

VREF

VIN2

GUARD

RING

GUARD

RING

VIN1

Figure 53. Top View of AD8552 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 temperature of the junction. 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 of the thermal voltage error sources. If the

temperature of the PC board at one end of the component (TA1)

is different from the temperature at the other end (TA2), the

resulting 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

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 reduces

EMI noise pickup.

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 54. Mismatch in Seebeck Voltages Causes

Thermoelectric Voltage Error

AD8551/

AD8552/

AD8554

AV = 1 + (RF/R1)

NOTES

1. RS SHOULD BE PLACED IN CLOSE PROXIMITY AND

ALIGNMENT TO R1 TO BALANCE SEEBECK VOLTAGES.

RS = R1

VIN

VOUT

Figure 55. Using Dummy Components to Cancel

Thermoelectric Voltage Errors

1/f NOISE CHARACTERISTICS

Another advantage of auto-zero amplifiers is their ability to

cancel flicker noise. Flicker noise, also known as 1/f noise, is

noise inherent in the physics of semiconductor devices, and it

increases 3 dB for every octave decrease in frequency. The 1/f

corner frequency of an amplifier is the frequency at which the

flicker noise is equal to the broadband noise of the amplifier.

At lower frequencies, flicker noise dominates, causing higher

degrees of error for sub-Hertz frequencies or dc precision

applications.

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