ADA4500-2

Data Sheet

Rev. B | 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

VIN+VIN–

–AV

OUT

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 VSY = 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

05

4

3

2

1

VCM (V)

ADA4500-2

VSY = 5.0V

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.