AD8228 View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD8228 Datasheet PDF : 24 Pages
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AD8228
References
The output voltage of the AD8228 is developed with respect to
the potential on the reference terminal. Care should be taken to
tie REF to the appropriate local ground.
Input Bias Current Return Path
The input bias current of the AD8228 must have a return path
to common. When the source, such as a thermocouple, cannot
provide a return current path, one should be created, as shown
in Figure 46.
INCORRECT
+VS
CORRECT
+VS
AD8228
REF
–VS
TRANSFORMER
+VS
AD8228
REF
–VS
TRANSFORMER
+VS
AD8228
REF
–VS
THERMOCOUPLE
AD8228
REF
10MΩ
–VS
THERMOCOUPLE
+VS
C
+VS
C
AD8228
fHIGH-PASS
=
1
2πRC
R
AD8228
C
REF
C
REF
R
–VS
–VS
CAPACITIVELY COUPLED
CAPACITIVELY COUPLED
Figure 46. Creating an IBIAS Path
INPUT PROTECTION
All terminals of the AD8228 are protected against ESD (1 kV,
human body model). In addition, the input structure allows for
dc overload conditions of about 3.5 V beyond the supplies.
Input Voltages Beyond the Rails
For larger input voltages, an external resistor should be used in
series with each input to limit current during overload conditions.
The AD8228 can safely handle a continuous 6 mA current. The
limiting resistor can be computed from
R LIMIT
≥ V IN − VSUPPLY
6 mA
− 600 Ω
For applications where the AD8228 encounters extreme overload
voltages, such as cardiac defibrillators, external series resistors
and low leakage diode clamps such as the BAV199L, the FJH1100s,
or the SP720 should be used.
Large Differential Voltages When G = 100
When operating at a gain of 100, large differential input voltages
can cause more than 6 mA of current to flow into the inputs.
This condition occurs when the voltage between +IN and –IN
exceeds 5 V. This is true for differential voltages of either polarity.
The maximum allowed differential voltage can be increased by
adding an input protection resistor in series with each input.
The value of each protection resistor should be
RPROTECT = (VDIFF_MAX − 5 V)/6 mA
RADIO FREQUENCY INTERFERENCE (RFI)
RF rectification is often a problem when amplifiers are used in
applications having strong RF signals. The disturbance can appear
as a small dc offset voltage. High frequency signals can be filtered
with a low-pass RC network placed at the input of the instru-
mentation amplifier, as shown in Figure 47. The filter limits the
input signal bandwidth, according to the following relationship:
1
FilterFrequencyDIFF = 2πR(2CD + CC)
FilterFrequencyCM = 1
2πRCC
where CD ≥ 10 CC.
+15V
0.1µF
10µF
CC
R
4.02kΩ
CD
R
4.02kΩ
CC
1nF
+IN
10nF
1nF
AD8228
VOUT
REF
–IN
0.1µF
10µF
–15V
Figure 47. RFI Suppression
CD affects the difference signal, and CC affects the common-mode
signal. Values of R and CC should be chosen to minimize RFI.
Mismatch between the R × CC at the positive input and the R × CC
at the negative input degrades the CMRR of the AD8228. By using
a value of CD one magnitude larger than CC, the effect of the
mismatch is reduced, and performance is improved.
Rev. 0 | Page 18 of 24
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