AD8571/AD8572/AD8574

–12–

REV. 0

AD8572

AD8572

AD8572

Figure 46. Guard Ring Layout and Connections to Reduce

PC Board Leakage Currents

V

V+

GUARD

RING

AD8572

GUARD

RING

Figure 47. 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 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

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

COMPONENT

LEAD

SOLDER

PC BOARD

COPPER

TRACE

+

+

+

+

Figure 48. Mismatch in Seebeck Voltages Causes a

Thermoelectric Voltage Error

AD857x

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 AD857x 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 AD857x

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

that are susceptible to 1/f noise.

Random Autozero Correction Eliminates Intermodulation

Distortion

The AD857x can be used as a conventional op amp for gains up

to 1 MHz. The autozero correction frequency of the device

continuously varies, based on a pseudo-random generator with a

uniform distribution from 2 kHz to 4 kHz. The randomization

of the autocorrection clock creates a continuous randomization

of intermodulation distortion (IMD) products, which show up

as simple broadband noise at the output of the amplifier. This

noise naturally combines with the amplifier’s voltage noise in a

root-squared-sum fashion, resulting in an output free of IMD.

Figure 50a shows the spectral output of an AD8572 with the

amplifier configured for unity gain and the input grounded. Figure

50b shows the spectral output with the amplifier configured for a

gain of 60 dB.