AD8551ARM datasheet(12/20 Pages) AD

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AD8551/AD8552/AD8554

–12–

REV. 0

be a specific width, but it should form a continuous loop around

both inputs. By setting the guard ring voltage equal to the volt-

age 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.

VOUT

VIN

AD8552

VOUT

VIN

AD8552

VOUT

VIN

AD8552

Figure 46. Guard Ring Layout and Connections to Reduce

PC Board Leakage Currents

V+

VREF

VIN1

VIN2

GUARD

RING

AD8552

VREF

GUARD

RING

Figure 47. 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 48 shows a cross-section diagram view

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 Seebeck voltages will not

be equal, resulting in a thermal voltage error.

This thermocouple error can be reduced by using dummy com-

ponents to match the thermoelectric error source. Placing the

dummy component as close as possible to its partner will ensure

both Seebeck voltages are equal, thus canceling the thermo-

couple error. Maintaining a constant ambient temperature on

the circuit board will further reduce this error. The use of a

ground plane will help distribute heat throughout the board and

will also reduce EMI noise pickup.

SURFACE MOUNT

COMPONENT

LEAD

SOLDER

PC BOARD

COPPER

TRACE

VSC2

VTS2

TA2

TA1

VSC1

VTS1

IF TA1 ﬁ TA2, THEN

VTS1 + VSC1 ﬁ VTS2 + VSC2

Figure 48. Mismatch in Seebeck Voltages Causes a

Thermoelectric Voltage Error

VOUT

VIN

AD855x

AV = 1 + (RF/R1)

RS = R1

NOTE: RS SHOULD BE PLACED IN CLOSE PROXIMITY AND

ALIGNMENT TO R1 TO BALANCE SEEBECK VOLTAGES

Figure 49. Using Dummy Components to Cancel

Thermoelectric Voltage Errors

1/f Noise Characteristics

Another advantage of autozero amplifiers is their ability to cancel

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

ent in the physics of semiconductor devices and 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.

Because the AD855x amplifiers are self-correcting op amps,

they do not have increasing flicker noise at lower frequencies.

In essence, low frequency noise is treated as a slowly varying

offset error and is greatly reduced as a result of autocorrection.

The correction becomes more effective as the noise frequency

approaches dc, offsetting the tendency of the noise to increase

exponentially as frequency decreases. This allows the AD855x

to have lower noise near dc than standard low-noise amplifiers

that are susceptible to 1/f noise.

Intermodulation Distortion

The AD855x can be used as a conventional op amp for gain/

bandwidth combinations up to 1.5 MHz. The autozero correc-

tion frequency of the device is fixed at 4 kHz. Although a trace

amount of this frequency will feed through to the output, the

amplifier can be used at much higher frequencies. Figure 50

shows the spectral output of the AD8552 with the amplifier

configured for unity gain and the input grounded.

The 4 kHz autozero clock frequency appears at the output with

less than 2

µV of amplitude. Harmonics are also present, but at

reduced levels from the fundamental autozero clock frequency.

The amplitude of the clock frequency feedthrough is proportional

to the closed-loop gain of the amplifier. Like other autocorrection

amplifiers, at higher gains there will be more clock frequency

feedthrough. Figure 51 shows the spectral output with the ampli-

fier configured for a gain of 60 dB.

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