AD8275
THEORY OF OPERATION
The AD8275 level translates 10 V signals at its inputs to 4 V
at its output. It does this by attenuating the input signal by 5.
A subtractor network performs the attenuation, the level shifting,
and the differential-to-single-ended conversion. One benefit of
the subtractor topology is that it can accept input signals
beyond its supply voltage. The subtractor is composed of tightly
matched resistors. By integrating the resistors and trimming the
resistor ratios, the AD8275 achieves 80 dB CMRR and 0.024%
gain error.
The AD8275 employs a balanced, high gain, linear output stage
that adaptively generates current as required, eliminating the
dynamic errors found in other amplifiers. This is useful when
driving SAR ADCs, which can deliver kickback current into the
output of the amplifier. The result is a design that achieves low
distortion, consistent bandwidth, and high slew rate.
BASIC CONNECTION
The basic configurations for the AD8275 are shown in
Figure 33 and Figure 34. In Figure 33, REF1 and REF2 are
tied together. A voltage, VREF, applied to the tied REF1 and
REF2 pins, sets the output voltage level to VREF. For example,
in Figure 33, if VREF = 2 V and the inputs are tied to ground,
the output remains at 2 V.
+V
S
50kΩ
10kΩ
INPUT
ESD
–IN
SENSE
+V
S
–V
+V
S
–V
+V
+V
S
S
S
S
7kΩ
7kΩ
+5V
OUT
0.1µF
–V
S
2.5V
7
–V
S
+V
S
–V
S
–V
S
+V
S
50kΩ
–IN
10kΩ
V
5
6
2
INN
20kΩ
REF2
REF1
SENSE
OUT
–V
S
V
OUT
+V
S
20kΩ
INPUT
ESD
V
+IN
REF
50kΩ
+IN
20kΩ
20kΩ
50kΩ
3
8
1
V
INP
–V
REF2
REF1
S
Figure 31. AD8275 Simplified Schematic
AD8275 –V
S
4
To achieve a wider input voltage range, the AD8275 uses an
internal 2.5 V voltage bias tied to –VS and two 7 kΩ resistors, as
shown in Figure 31. The resistors help to set the common mode
of the internal amplifier. The benefit of this circuit is that it
extends the input range without causing crossover distortion
typical of amplifiers that have rail-to-rail complementary
transistor inputs. The input range of the internal op amp is
+VS − 0.9 V to −VS + 1.35 V.
(V ) – (V
INP
)
INN
V
=
+ V
REF
OUT
5
Figure 33. Basic Configuration 1: Shared Reference
In contrast, Figure 34 shows REF1 tied to ground and REF2
tied to VREF. In this example, the two 20 kΩ resistors serve as a
resistor divider, and VREF is divided by 2. For example, if both
inputs of the AD8275 are grounded and VREF = 5 V, the output
is 2.5 V.
600
400
200
+5V
0.1µF
7
+V
S
0
50kΩ
–IN
10kΩ
V
5
6
2
INN
SENSE
OUT
–200
–400
–600
V
OUT
V
REF
50kΩ
+IN
20kΩ
20kΩ
3
8
1
V
INP
REF2
REF1
–10
–8
–6
–4
–2
0
2
4
6
8
10
COMMON-MODE VOLTAGE (V)
AD8275 –V
S
4
Figure 32. AD8275 Does Not Have Crossover Distortion Typical of Rail-to-Rail
Input Amplifiers
(V ) – (V
INP
)
V
+ 0V
REF
2
INN
V
=
+
OUT
5
Figure 34. Basic Configuration 2: Split Reference
Rev. A | Page 11 of 16
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