Datasheet

ADA4500-2 Data Sheet
Rev. A | Page 20 of 24
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
VDD
VSS
VDD
VSS
BIAS6
BIAS5
BIAS4
BIAS3
M1 M2
V
IN
+ V
IN
–
–A
V
OUT
10617-102
CHARGE
PUMP
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.
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
–300
–240
–180
–120
–60
0
60
120
180
240
0 54321
V
OS
(µV)
V
CM
(V)
ADA4500-2
V
SY
= 5.0V
10617-108
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 E
2
PROM's," IEEE Journal of Solid-State Circuits, Vol. SC-18, No.5, pp.532-538, October 1983.