Data Sheet
ADA4528-1
Rev. A | Page 17 of 20
SOLDER
+
COMPONENT
LEAD
COPPER
TRACE
VSC1
VTS1
TA1
SURFACE-MOUNT
COMPONENT
PC BOARD
TA2
VSC2
VTS2
IF TA1 ≠ TA2, THEN
VTS1 + VSC1 ≠ VTS2 + VSC2
09
43
7-
15
4
PRINTED CIRCUIT BOARD LAYOUT
The
ADA4528-1 is a high precision device with ultralow offset
voltage and noise. Therefore, care must be taken in the design of
the printed circuit board (PCB) layout to achieve optimum
To avoid leakage currents, keep the surface of the board clean
and free of moisture. Coating the board surface creates a barrier
to moisture accumulation and reduces parasitic resistance on
the board.
Figure 61. Mismatch in Seebeck Voltages Causes
Seebeck Voltage Error
Properly bypassing the power supplies and keeping the supply
traces short minimizes power supply disturbances caused by
output current variation. Connect bypass capacitors as close
as possible to the device supply pins. Stray capacitances are a
concern at the outputs and the inputs of the amplifier. It is
recommended that signal traces be kept at a distance of at
least 5 mm from supply lines to minimize coupling.
To minimize these thermocouple effects, orient resistors so that
heat sources warm both ends equally. Where possible, the input
signal paths should contain matching numbers and types of com-
ponents to match the number and type of thermocouple junctions.
For example, dummy components, such as zero value resistors,
can be used to match the thermoelectric error source (real resistors
in the opposite input path). Place matching components in close
proximity and orient them in the same manner to ensure equal
Seebeck voltages, thus cancelling thermal errors. Additionally,
use leads that are of equal length to keep thermal conduction in
equilibrium. Keep heat sources on the PCB as far away from
amplifier input circuitry as is practical.
A potential source of offset error is the Seebeck voltage on the
circuit board. The Seebeck voltage occurs at the junction of two
dissimilar metals and is a function of 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 61shows a cross section of a surface-mount component soldered
to a PCB. A variation in temperature across the board (where TA1 ≠
TA2) causes a mismatch in the Seebeck voltages at the solder joints,
thereby resulting in thermal voltage errors that degrade the per-
formance of the ultralow offset voltage of the
ADA4528-1.It is highly recommended to use a ground plane. A ground
plane helps distribute heat throughout the board, maintains a
constant temperature across the board, and reduces EMI noise
pickup.