AD8223 Analog Devices, AD8223 Datasheet - Page 14

• Manufacturer Part Number: AD8223
• Description: Single Supply, Rail-to-Rail, Low Cost Instrumentation Amplifier
• Manufacturer: Analog Devices

AD8223
REFERENCE TERMINAL
The output voltage of the AD8223 is developed with respect to
the potential on the reference terminal. This is useful when the
output signal needs to be offset to a precise midsupply level. For
example, a voltage source can be tied to the REF pin to level-
shift the output so that the AD8223 can drive a single-supply
ADC. The REF pin is protected with ESD diodes and should
not exceed either +V
For best performance, source impedance to the REF terminal
should be kept below 5 Ω. As shown in Figure 29, the reference
terminal, REF, is at one end of a 50 kΩ resistor. Additional
impedance at the REF terminal adds to this resistor and results
in poorer CMRR performance.
INPUT PROTECTION
Internal supply referenced clamping diodes allow the input,
reference, output, and gain terminals of the AD8223 to safely
withstand overvoltages of 0.3 V above or below the supplies.
This is true for all gains, and for power-on and power-off. This
last case is particularly important because the signal source and
amplifier may be powered separately.
If the overvoltage is expected to exceed this value, the current
through these diodes should be limited to about 10 mA using
external current limiting resistors. This is shown in Figure 31.
The size of this resistor is defined by the supply voltage and the
required overvoltage protection.
V
V
OVER
OVER
V
REF
INCORRECT
R
R
LIM
LIM
AD8223
Figure 30. Driving the Reference Pin
1 = 10mA MAX
S
Figure 31. Input Protection
or −V
R
G
S
by more than 0.3 V.
V
REF
+
–
+
OP2177
–
CORRECT
+V
–V
AD8223
S
S
R
AD8223
LIM
=
V
OVER
10mA
– V
OUTPUT
S
+ 0.7V
Rev. PrA | Page 14 of 20
RF INTERFERENCE
RF rectification is often a problem when amplifiers are used in
applications where there are strong RF signals. The disturbance
can appear as a small dc offset voltage. High frequency signals
can be filtered with a low-pass, R-C network placed at the input
of the instrumentation amplifier, as shown in Figure 32. The
filter limits the input signal bandwidth according to the follow-
ing relationship:
where C
Figure 32 shows an example where the differential filter
frequency is approximately 400 Hz, and the common-mode
filter frequency is approximately 40 kHz. The typical dc offset
shift over frequency is less than 1.5 μV and the circuit’s RF
signal rejection is better than 71 dB.
The resistors were selected to be large enough to isolate the
circuit’s input from the capacitors, but not large enough to
significantly increase the circuit’s noise. Values of R and C
should be chosen to minimize RFI. Mismatch between the R × C
at the positive input and the R × C
the CMRR of the AD8223. Because of their higher accuracy and
stability, COG/NPO type ceramic capacitors are recommended
for the C
as critical.
FilterFreq
FilterFreq
D
C
≥ 10C
capacitors. The dielectric for the C
4.02kΩ
4.02kΩ
R
R
Diff
CM
C
.
=
=
Preliminary Technical Data
2
C
1nF
C
47nF
C
1nF
2
Figure 32. RFI Suppression
C
D
C
π
π
RC
R
1
(2
499Ω
C
C
1
R1
0.1µF
0.1µF
D
+IN
–IN
+
C
+
–
C
AD8223
C
)
+15V
–15V
at negative input degrades
REF
10µF
10µF
D
capacitor is not
V
OUT
+
+
C
C