Datasheet

AD8236
Rev. 0 | Page 16 of 20
R
R
1
f
HIGH-PASS
=
2RC
TRANSFORMER
AC-COUPLED
REF
C
–V
S
+V
S
AD8236
REF
–V
S
+
V
S
AD8236
TRANSFORMER
AC-COUPLED
REF
C
–V
S
+V
S
AD8236
REF
–V
S
+
V
S
AD8236
08000-139
Figure 38. Creating an I
BIAS
Path
INPUT BIAS CURRENT RETURN PATH
The AD8236 input bias current is extremely small at less than
10 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 38).
INPUT PROTECTION
All terminals of the AD8236 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 AD8236 safely
handles a continuous 6 mA current at room temperature.
For applications where the AD8236 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 AD8236, 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 39). The
relationship between external, matched series resistors and the
internal gate capacitance is expressed as
G
DIFF
RC
FilterFreq
π2
1
=
G
CM
RC
FilterFreq
π2
1
=
AD8236
V
OUT
REF
–V
S
C
G
C
G
–V
S
+
V
S
–V
S
R
R
+IN
–IN
0.1µF 10µF
0.1µF 10µF
08000-140
Figure 39. RFI Filtering Without External Capacitors