AD8224 Analog Devices, AD8224 Datasheet - Page 20

• Manufacturer Part Number: AD8224
• Description: Precision, Dual-Channel, JFET Input Rail-to-Rail Instrumentation Amplifier
• Manufacturer: Analog Devices

AD8224
Table 11. Gains Achieved Using 1% Resistors
1% Standard Table Value of R
49.9 k
12.4 k
5.49 k
2.61 k
1.00 k
499
249
100
49.9
The AD8224 defaults to G = 1 when no gain resistor is used.
The tolerance and gain drift of the R
to the AD8224 specifications to determine the total gain
accuracy of the system. When the gain resistor is not used,
gain error and gain drift are kept to a minimum.
REFERENCE TERMINAL
The output voltage of the AD8224 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 REF1 or REF2 pin
to level-shift the output so that the AD8224 can drive a single-
supply ADC. Pin REFx is protected with ESD diodes and
should not exceed either +V
For best performance, source impedance to the REF terminal
should be kept below 1 Ω. As shown in Figure 52 the reference
terminal, REF, is at one end of a 20 kΩ resistor. Additional
impedance at the REF terminal adds to this 20 kΩ resistor and
results in amplification of the signal connected to the positive
input. The amplification from the additional R
computed by
Only the positive signal path is amplified; the negative path is
unaffected. This uneven amplification degrades the amplifier’s
CMRR.
V
REF
INCORRECT
AD8224
2
40
(
20
kΩ
kΩ
+
+
R
R
REF
REF
Figure 53. Driving the Reference Pin
)
V
REF
+
OP2177
–
CORRECT
S
AD8224
or −V
G
(Ω)
S
G
by more than 0.5 V.
resistor should be added
V
Calculated Gain
1.990
4.984
9.998
19.93
50.40
100.0
199.4
495.0
991.0
REF
REF
+
AD8224
–
CORRECT
can be
AD8224
Rev. 0 | Page 20 of 28
LAYOUT
The AD8224 is a high precision device. To ensure optimum
performance at the PC board level, care must be taken in the
design of the board layout. The AD8224 pinout is arranged in
a logical manner to aid in this task.
Package Considerations
The AD8224 is available in a 16-lead, 4 mm × 4 mm LFCSP.
Blindly copying the footprint from another 4 mm × 4 mm
LFCSP part is not recommended; it may not have the same
thermal pad size and leads. Refer to the Outline Dimensions
section to verify that the PCB symbol has the correct dimensions.
Space between the leads and thermal pad should be kept as
wide as possible for the best bias current performance. To
maintain the AD8224 ultralow bias current performance, the
thermal pad area can be reduced to extend the gap between the
leads and the pad.
Thermal Pad
The AD8224 4 mm × 4 mm LFCSP comes with a thermal pad.
This pad is connected internally to +V
left unconnected or connected to the positive supply rail.
To preserve maximum pin compatibility with other dual
instrumentation amplifiers, such as the AD8222, leave the pad
unconnected. This can be done by not soldering the paddle at
all or by soldering the part to a landing that is a not connected
to any other net. For high vibration applications, a landing is
recommended.
Because the AD8224 dissipates little power, heat dissipation is
rarely an issue. If improved heat dissipation is desired (for example,
when driving heavy loads), connect the thermal pad to the
positive supply rail. For the best heat dissipation performance,
the positive supply rail should be a plane in the board. See
the Thermal Resistance section for more information.
Common-Mode Rejection over Frequency
The AD8224 has a higher CMRR over frequency than typical
in-amps, which gives it greater immunity to disturbances, such
as line noise and its associated harmonics. A well-implemented
layout is required to maintain this high performance. Input
source impedances should be matched closely. Source resistance
should be placed close to the inputs so that it interacts with as
little parasitic capacitance as possible.
Parasitics at the RGx pins can also affect CMRR over frequency.
The PCB should be laid out so that the parasitic capacitances at
each pin match. Traces from the gain setting resistor to the RGx
pins should be kept short to minimize parasitic inductance.
Reference
Errors introduced at the reference terminal feed directly to
the output. Take care to tie the REFx pins to the appropriate
local ground.
S
. The pad can either be