AD8551/AD8552/AD8554
COMPONENT
LEAD
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.
V
V
SC2
SC1
+
SOLDER
SURFACE MOUNT
COMPONENT
+
؊
؊
V
V
TS2
TS1
+
+
؊
؊
PC BOARD
T
T
A2
A1
COPPER
TRACE
IF T
V
fi
T
, THEN
A1
A2
V
V
OUT
+ V
fi
V
+ V
OUT
TS1
SC1
TS2 SC2
V
V
IN
IN
AD8552
AD8552
Figure 48. Mismatch in Seebeck Voltages Causes a
Thermoelectric Voltage Error
R
F
V
IN
V
OUT
R
1
AD8552
V
OUT
V
IN
AD855x
R
= R
1
S
Figure 46. Guard Ring Layout and Connections to Reduce
PC Board Leakage Currents
A
= 1 + (R /R )
F
1
V
NOTE: R SHOULD BE PLACED IN CLOSE PROXIMITY AND
S
ALIGNMENT TO R TO BALANCE SEEBECK VOLTAGES
1
V+
R
R
2
Figure 49. Using Dummy Components to Cancel
Thermoelectric Voltage Errors
1
AD8552
R
R
1
2
V
IN1
V
IN2
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.
GUARD
RING
GUARD
RING
V
REF
V
REF
V؊
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.
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.
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.
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.
–12–
REV. 0
ADI [ ADI ]
