AD8421 AD [Analog Devices], AD8421 Datasheet - Page 21

• Manufacturer Part Number: AD8421
• Description: 3 nV/?Hz, Low Power
• Manufacturer: AD [Analog Devices]

Data Sheet
REFERENCE TERMINAL
The output voltage of the
the potential on the reference terminal. This can be used to sense
the ground at the load, thereby taking advantage of the CMRR to
reject ground noise or to introduce a precise offset to the signal
at the output. For example, a voltage source can be tied to the REF
pin to level shift the output, allowing the
supply ADC. The REF pin is protected with ESD diodes and
should not exceed either +V
For best performance, maintain a source impedance to the
REF terminal that is below 1 Ω. As shown in Figure 61, the
reference terminal, REF, is at one end of a 10 kΩ resistor.
Additional impedance at the REF terminal adds to this 10 kΩ
resistor and results in amplification of the signal connected to
the positive input. The amplification from the additional R
can be calculated as follows:
Only the positive signal path is amplified; the negative path is
unaffected. This uneven amplification degrades CMRR.
INPUT VOLTAGE RANGE
The 3-op-amp architecture of the
first stage before removing the common-mode voltage in the
difference amplifier stage. Internal nodes between the first and
second stages (Node 1 and Node 2 in Figure 61) experience
a combination of a gained signal, a common-mode signal, and
a diode drop. The voltage supplies can limit the combined signal,
even when the individual input and output signals are not limited.
Figure 10 through Figure 13 show this limitation in detail.
LAYOUT
To ensure optimum performance of the
care must be taken in the design of the board layout. The pins of
the
AD8421
2(10 kΩ + R
V
are arranged in a logical manner to aid in this task.
INCORRECT
AD8421
Figure 63. Pin Configuration Diagram
REF
Figure 62. Driving the Reference Pin
REF
+IN
–IN
R
R
)/(20 kΩ + R
G
G
1
2
3
4
(Not to Scale)
AD8421
AD8421
TOP VIEW
S
or −V
V
REF
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is developed with respect to
)
+
OP1177
–
S
8
7
6
5
by more than 0.3 V.
CORRECT
+V
V
REF
–V
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OUT
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S
S
applies gain in the
REF
to drive a single-
at the PCB level,
REF
Rev. 0 | Page 21 of 28
Common-Mode Rejection Ratio over Frequency
Poor layout can cause some of the common-mode signals to
be converted to differential signals before reaching the in-amp.
Such conversions occur when one input path has a frequency
response that is different from the other. To maintain high CMRR
over frequency, closely match the input source impedance and
capacitance of each path. Place additional source resistance in
the input path (for example, input protection resistors) close to
the in-amp inputs, to minimize the interaction of the resistance
with parasitic capacitance from the PCB traces.
Parasitic capacitance at the gain setting pins (R
CMRR over frequency. If the board design has a component at
the gain setting pins (for example, a switch or jumper), choose
a component such that the parasitic capacitance is as small as
possible.
Power Supplies and Grounding
Use a stable dc voltage to power the instrumentation amplifier.
Noise on the supply pins can adversely affect performance.
Place a 0.1 μF capacitor as close as possible to each supply pin.
Because the length of the bypass capacitor leads is critical at
high frequency, surface-mount capacitors are recommended.
Any parasitic inductance in the bypass ground trace works against
the low impedance that is created by the bypass capacitor. As
shown in Figure 64, a 10 μF capacitor can be used farther away
from the device. For these larger value capacitors, which are
intended to be effective at lower frequencies, the current return
path distance is less critical. In most cases, the 10 μF capacitor
can be shared by other local precision integrated circuits.
A ground plane layer helps to reduce parasitic inductances, which
minimizes voltage drops with changes in current. The area of
the current path is directly proportional to the magnitude of
parasitic inductances and, therefore, the impedance of the path
at high frequency. Large changes in currents in an inductive
decoupling path or ground return create unwanted effects due
to the coupling of such changes into the amplifier inputs.
Because load currents flow from the supplies, the load should be
connected at the same physical location as the bypass capacitor
grounds.
Figure 64. Supply Decoupling, REF, and Output Referred to Local Ground
R
G
+IN
–IN
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+V
–V
S
S
0.1µF
0.1µF
REF
10µF
10µF
LOAD
V
OUT
G
) can also affect
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