Datasheet

AD8551/AD8552/AD8554 Data Sheet
Rev. F | Page 16 of 24
AD8551/AD8552/AD8554 allows it to quite effectively
minimize offset voltages. 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 AD8551/
AD8552/AD8554, care is needed in laying out the circuit board.
The PC board 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 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
AD8552
AD8552
V
OUT
V
OUT
V
OUT
V
IN
V
IN
V
IN
01101-052
Figure 52. Guard Ring Layout and Connections to Reduce
PC Board Leakage Currents
V+
AD8552
V–
R
2
R
1
R
1
R
2
V
REF
V
REF
V
IN2
GUARD
RING
GUARD
RING
V
IN1
01101-053
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 (T
A1
) is different from
the temperature at the other end (T
A2
), the resulting Seebeck
voltages are not 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 ensures both
Seebeck voltages are equal, thus canceling the thermocouple error.
Maintaining a constant ambient temperature on the circuit board
further reduces this error. The use of a ground plane helps distrib-
ute heat throughout the board and reduces EMI noise pickup.
SOLDER
+
+
+
+
COMPONENT
LEAD
COPPER
TRACE
V
SC1
V
TS1
T
A1
SURFACE-MOUNT
COMPONENT
PC BOARD
T
A2
V
SC2
V
TS2
IF T
A1
≠ T
A2
, THEN
V
TS1
+ V
SC1
≠ V
TS2
+ V
SC2
01101-054
Figure 54. Mismatch in Seebeck Voltages Causes
Thermoelectric Voltage Error
AD8551/
AD8552/
AD8554
A
V
= 1 + (R
F
/R
1
)
NOTES
1. R
S
SHOULD BE PLACED IN CLOSE PROXIMITY AND
ALIGNMENT TO R
1
TO BALANCE SEEBECK VOLTAGES.
R
S
= R
1
R
1
R
F
V
IN
V
OUT
01101-055
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.
Because the AD8551/AD8552/AD8554 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 AD8551/AD8552/AD8554 to have lower noise near
dc than standard low noise amplifiers that are susceptible to 1/f
noise.
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