Datasheet

AD8235
Rev. 0 | Page 17 of 20
R
R
1
f
HIGH-PASS
=
2RC
TRANSFORMER
AC-COUPLED
REF
C
+V
S
–V
S
AD8235
REF
–V
S
+
V
S
AD8235
TRANSFORMER
AC-COUPLED
REF
C
+V
S
–V
S
AD8235
REF
–V
S
+
V
S
AD8235
0
8211-038
Figure 40. Creating an I
BIAS
Path
INPUT BIAS CURRENT RETURN PATH
The AD8235 input bias current is extremely small at less than
50 pA. Nonetheless, the input bias current must have a return
path to common. When the source, such as a transformer,
cannot provide a return current path, one should be created
(see Figure 40).
INPUT PROTECTION
All terminals of the AD8235 are protected against ESD. In addition,
the input structure allows for dc overload conditions a diode drop
above the positive supply and a diode drop below the negative
supply. Voltages beyond a diode drop of the supplies cause the
ESD diodes to conduct and enable current to flow through the
diode. Therefore, an external resistor should be used in series
with each of the inputs to limit current for voltages above +V
S
.
In either scenario, the AD8235 safely handles a continuous 6 mA
current at room temperature.
For applications where the AD8235 encounters extreme
overload voltages, as in cardiac defibrillators, external series
resistors and low leakage diode clamps, such as BAV199Ls,
FJH1100s, or SP720s, should be used.
RF INTERFERENCE
RF rectification is often a problem in applications where there are
large RF signals. The problem appears as a small dc offset voltage.
The AD8235, by its nature, has a 3.1 pF gate capacitance, C
G
, at
each input. Matched series resistors form a natural low-pass filter
that reduces rectification at high frequency (see Figure 41). The
relationship between external, matched series resistors and the
internal gate capacitance is expressed as
G
DIFF
RCπ
FilterFreq
2
1
=
G
CM
RCπ
FilterFreq
2
1
=
AD8235
V
OUT
–V
S
C
G
C
G
–V
S
+
V
REF
S
–V
S
R
R
+IN
–IN
0.1µF 10µF
0.1µF 10µF
SDN
8211-039
0
Figure 41. RFI Filtering Without External Capacitors