AD8235 View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD8235 Datasheet PDF : 20 Pages
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AD8235
Board Thickness
Typical board thicknesses used in the industry range from
0.4 mm to 1.6 mm and are most applicable for the AD8235. The
thickness selected depends on the required robustness of the
populated system assembly. The thinner board results in smaller
shear stress range, creep shear strain range, and creep strain
energy density range in the solder joints under the thermal
loading. Therefore, the thinner build-up board leads to longer
thermal fatigue life of solder joints [John H. Lau and S.W. Ricky
Lee]1
Grounding
The output voltage of the AD8235 is developed with respect to
the potential on the reference terminal, REF. To ensure the most
accurate output, the trace from the REF pin should either be
connected to the AD8235 local ground (see Figure 39) or
connected to a voltage that is referenced to the AD8235 local
ground (Figure 36).
REFERENCE TERMINAL
The reference terminal, REF, is at one end of a 210 kΩ resistor
(see Figure 35). The output of the instrumentation amplifier
is referenced to the voltage on the REF terminal; this is useful
when the output signal needs to be offset to voltages other than
common. For example, a voltage source can be tied to the REF
pin to level-shift the output so that the AD8235 can interface
with an ADC. The allowable reference voltage range is a function
of the gain, common-mode input, and supply voltages. The REF
pin should not exceed either +VS or −VS by more than 0.5 V.
For best performance, especially in cases where the output is not
measured with respect to the REF terminal, source impedance to
the REF terminal should be kept low because parasitic resistance
can adversely affect CMRR and gain accuracy. Figure 38
demonstrates how an op amp is configured to provide a low
source impedance to the REF terminal when a midscale
reference voltage is desired.
INCORRECT
CORRECT
AD8235
REF
V
AD8235
V
REF
+
OP AMP
–
Figure 38. Driving the REF Pin
POWER SUPPLY REGULATION AND BYPASSING
The AD8235 has high power supply rejection ration (PSRR).
However, for optimal performance, a stable dc voltage should be
used to power the instrumentation amplifier. Noise on the supply
pins can adversely affect performance. As in all linear circuits,
bypass capacitors must be used to decouple the amplifier.
A 0.1 μF capacitor should be placed close to each supply pin.
A 10 μF tantalum capacitor can be used farther away from the
part (see Figure 39). In most cases, it can be shared by other
precision integrated circuits.
+VS
SDN 0.1µF
10µF
+IN
AD8235
–IN
REF
VOUT
LOAD
–VS
0.1µF
10µF
–VS
Figure 39. Supply Decoupling, REF, and Output Referred to Ground
1John H. Lau and S.W. Ricky Lee, “Effects of Build-Up Printed Circuit Board
Thickness on the Solder Joint Reliability of a Wafer Level Chip Scale Package
(WLCSP),” IEEE Transactions on Components and Packaging Technologies,
Vol.25, No.1, March 2002, pages 3-14.
Rev. 0 | Page 16 of 20
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