Datasheet

AD8426
Rev. 0 | Page 22 of 28
REFERENCE TERMINAL
The output voltage of the AD8426 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 AD8426 can drive a single-supply
ADC. The REF pin is protected with ESD diodes and should
not exceed either +V
S
or −V
S
by more than 0.3 V.
For the best performance, source impedance to the REF
terminal should be kept below 2 Ω. As shown in Figure 62,
the reference terminal, REF, is at one end of a 50 k resistor.
Additional impedance at the REF terminal adds to this 50 k
resistor and results in amplification of the signal connected to
the positive input. The amplification from the additional R
REF
can be computed by 2 × (50 k + R
REF
)/100 k + R
REF
.
Only the positive signal path is amplified; the negative path is
unaffected. This uneven amplification degrades the CMRR of
the amplifier.
CORRECT
AD8426
OP1177
+
–
CORRECT
AD8426
AD8426
+
–
REF REF
INCORRECT
V
REF
V
REF
V
REF
AD8426
REF
09490-156
Figure 62. Driving the Reference Pin
INPUT VOLTAGE RANGE
The 3-op-amp architecture of the AD8426 applies gain in
the first stage before removing common-mode voltage in the
difference amplifier stage. In addition, the input transistors in
the first stage shift the common-mode voltage up one diode
drop. Therefore, internal nodes between the first and second
stages (Node 1 and Node 2 in Figure 61) experience a combina-
tion of gained signal, common-mode signal, and a diode drop.
This combined signal can be limited by the voltage supplies even
when the individual input and output signals are not limited.
Figure 9 to Figure 15 and Figure 18 show the allowable common-
mode input voltage ranges for various output voltages and
supply voltages.
Equation 1 to Equation 3 can be used to understand the inter-
action of the gain (G), common-mode input voltage (V
CM
),
differential input voltage (V
DIFF
), and reference voltage (V
REF
).
The values for the constants (V
−LIMIT
, V
+LIMIT
, and V
REF_LIMIT
)
at different temperatures are shown in Table 12. These three
equations, along with the input and output voltage range speci-
fications in Table 2 and Table 5, set the operating boundaries
of the part.
LIMIT
S
DIFF
CM
VV
GV
V
−
+−>
×
−
2
(1)
LIMIT
S
DIFF
CM
VV
GV
V
+
−+<
×
+
2
(2)
REF_LIMIT
S
REF
CM
DIFF
VV
VV
GV
−+<
⎟
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎜
⎝
⎛
++
×
2
2
(3)
Table 12. Input Voltage Range Constants for Various
Temperatures
Temperature V
−LIMIT
(V) V
+LIMIT
(V) V
REF_LIMIT
(V)
−40°C −0.55 +0.8 +1.3
+25°C −0.35 +0.7 +1.15
+85°C −0.15 +0.65 +1.05
+125°C −0.05 +0.6 +0.9
The common-mode input voltage range shifts upward with temp-
erature. At cold temperatures, the part requires extra headroom
from the positive supply, whereas operation near the negative
supply has more margin. Conversely, at hot temperatures, the part
requires less headroom from the positive supply but is subject
to the worst-case conditions for input voltages near the negative
supply.
A typical part functions up to the boundaries described in this
section. However, for best performance, designing with a few
hundred millivolts of extra margin is recommended. As signals
approach the boundary, internal transistors begin to saturate,
which can affect frequency and linearity performance.