AD8029 데이터 시트보기 (PDF) - Analog Devices
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AD8029 Datasheet PDF : 20 Pages
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AD8029/AD8030/AD8040
THEORY OF OPERATION
DISABLE
AD8029 ONLY
IN–
TO DISABLE
CIRCUITRY
RTH
+VS –1.2V
ITH
–VS
SPD
ITAIL
Q9
R1 R2 R3 R4
Q5
Q6
Q1
Q2
Q7
Q8
Q3
IN+
Q4
R5 R6 R7 R8
+VS
MTOP
OUTPUT
BUFFER
Q10
CMT
CMB
VOUT
MBOT
Q11
OUT
IN
COM
Figure 50. Simplified Schematic
–VS
03679-0-051
The AD8029 (single), AD8030 (dual), and AD8040 (quad) are
rail-to-rail input and output amplifiers fabricated using Analog
Devices’ XFCB process. The XFCB process enables the AD8029/
AD8030/AD8040 to operate on 2.7 V to 12 V supplies with a
120 MHz bandwidth and a 60 V/µs slew rate. A simplified sche-
matic of the AD8029/AD8030/AD8040 is shown in Figure 50.
INPUT STAGE
For input common-mode voltages less than a set threshold
(1.2 V below VCC), the resistor degenerated PNP differential pair
(comprising Q1 toQ4) carries the entire ITAIL current, allowing
the input voltage to go 200 mV below –VS. Conversely, input
common-mode voltages exceeding the same threshold cause
ITAIL to be routed away from the PNP differential pair and into
the NPN differential pair through transistor Q9. Under this
condition, the input common-mode voltage is allowed to rise
200 mV above +VS while still maintaining linear amplifier
behavior. The transition between these two modes of operation
leads to a sudden, temporary shift in input stage transconduc-
tance, gm, and dc parameters (such as the input offset voltage
VOS), which in turn adversely affect the distortion performance.
The SPD block shortens the duration of this transition, thus
improving the distortion performance. As shown in Figure 50,
the input differential pair is protected by a pair of two series
diodes, connected in anti-parallel, which clamp the differential
input voltage to approximately ±1.5 V.
OUTPUT STAGE
The currents derived from the PNP and NPN input differential
pairs are injected into the current mirrors MBOT and MTOP, thus
establishing a common-mode signal voltage at the input of the
output buffer.
The output buffer performs three functions:
1. It buffers and applies the desired signal voltage to the
output devices, Q10 and Q11.
2. It senses the common-mode current level in the output
devices.
3. It regulates the output common-mode current by
establishing a common-mode feedback loop.
The output devices Q10 and Q11 work in a common-emitter
configuration, and are Miller-compensated by internal
capacitors, CMT and CMB.
The output voltage compliance is set by the output devices’
collector resistance RC (about 25 Ω), and by the required load
current IL. For instance, a light equivalent load (5 kΩ) allows the
output voltage to swing to within 40 mV of either rail, while
heavier loads cause this figure to deteriorate as RC × IL.
Rev. A | Page 15 of 20
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