ADA4500-2_16 Datasheet, PDF (21/25 Pages) Analog Devices – 10 MHz, 14.5 nV/√Hz, Rail-to-Rail I/O, Zero Input Crossover Distortion Amplifier

ADA4500-2
The ADA4500-2 solves the crossover distortion problem by using
an on-chip charge pump in its input structure to power the input
differential pair (see Figure 61). The charge pump creates a
supply voltage higher than the voltage of the supply, allowing
the input stage to handle a wide range of input signal voltages
without using a second differential pair. With this solution, the
input voltage can vary from one supply voltage to the other with
no distortion, thereby restoring the full common-mode dynamic
range of the op amp.
VCP
BIAS6
CHARGE
PUMP
VDD
VDD
BIAS5
V_IN+
V_IN–
M1 M2
BIAS4
–A_V
OUT
BIAS3
VSS
VSS
Figure 61. ADA4500-2 Input Structure
Some charge pumps are designed to run in an open-loop
configuration. Disadvantages of this design include: a large ripple
voltage on the output, no output regulation, slow start-up, and a
large power-supply current ripple. The charge pump in this op
amp uses a feedback network that includes a controllable clock
driver and a differential amplifier. This topology results in a low
ripple voltage; a regulated output that is robust to line, load, and
process variations; a fast power-on startup; and lower ripple on
the power supply current.¹ The charge pump ripple does not
show up on an oscilloscope; however, it can be seen at a high
frequency on a spectrum analyzer. The charge pump clock speed
adjusts between 3.5 MHz (when the supply voltage is 2.7 V) to
5 MHz (at V_SY = 5 V). The noise and distortion are limited only by
the input signal and the thermal or flicker noise.
Data Sheet
Figure 62 shows the elimination of the crossover distortion in
the ADA4500-2. This solution improves the CMRR performance
tremendously. For example, if the input varies from rail to rail
on a 5 V supply rail, using a part with a CMRR of 70 dB minimum,
an input-referred error of 1581 µV is introduced. The ADA4500-2,
with its high CMRR of 90 dB minimum (over its full operating
temperature) reduces distortion to a maximum error of 158 µV
with a 5 V supply. The ADA4500-2 eliminates crossover distortion
without unnecessary circuitry complexity and increased cost.
300
ADA4500-2
240 ^VSY ^{= 5.0V}
180
120
60
0
–60
–120
–180
–240
–300
0
1
2
3
4
5
V_CM (V)
Figure 62. Charge Pump Design Eliminates Crossover Distortion
OVERLOAD RECOVERY
When the output is driven to one of the supply rails, the
ADA4500-2 is in an overload condition. The ADA4500-2 recovers
quickly from the overload condition. Typical op amp recovery
times can be in the tens of microseconds. The ADA4500-2 typically
recovers from an overload condition in 1 µs from the time the
overload condition is removed until the output is active again.
This is important in, for example, a feedback control system. The
fast overload recovery of the ADA4500-2 greatly reduces loop
delay and increases the response time of the control loop (see
Figure 41 to Figure 44).
1 _{Oto, D.H.; Dham, V.K.; Gudger, K.H.; Reitsma, M.J.; Gongwer, G.S.; Hu, Y.W.; Olund, J.F.; Jones, H.S.; Nieh, S.T.K.; "High-Voltage Regulation and Process}
Considerations for High-Density 5 V-Only E²PROM's," IEEE Journal of Solid-State Circuits, Vol. SC-18, No.5, pp.532-538, October 1983.
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