AD8032BRZ Datasheet AD8031ARTZ-REEL7, AD8032ARMZ-REEL7, AD8031ARTZ-R2 | P13-P15

Data Sheet AD8031/AD8032

Rev. G | Page 13 of 20

THEORY OF OPERATION

The AD8031/AD8032 are single and dual versions of high

speed, low power, voltage feedback amplifiers featuring an

innovative architecture that maximizes the dynamic range

capability on the inputs and outputs. The linear input common-

mode range exceeds either supply voltage by 200 mV, and the

amplifiers show no phase reversal up to 500 mV beyond supply.

The output swings to within 20 mV of either supply when

driving a light load; 300 mV when driving up to 5 mA.

Fabricated on Analog Devices, Inc. eXtra Fast Complementary

Bipolar (XFCB) process, the amplifier provides an impressive

80 MHz bandwidth when used as a follower and a 30 V/µs slew

rate at only 800 µA supply current. Careful design allows the

amplifier to operate with a supply voltage as low as 2.7 V.

INPUT STAGE OPERATION

A simplified schematic of the input stage appears in Figure 43.

For common-mode voltages up to 1.1 V within the positive

supply (0 V to 3.9 V on a single 5 V supply), tail current I2

flows through the PNP differential pair, Q13 and Q17. Q5 is cut

off; no bias current is routed to the parallel NPN differential

pair, Q2 and Q3. As the common-mode voltage is driven within

1.1 V of the positive supply, Q5 turns on and routes the tail

current away from the PNP pair and to the NPN pair. During

this transition region, the input current of the amplifier changes

magnitude and direction. Reusing the same tail current ensures

that the input stage has the same transconductance, which

determines the gain and bandwidth of the amplifier, in both

regions of operation.

Switching to the NPN pair as the common-mode voltage is

driven beyond 1 V within the positive supply allows the amplifier

to provide useful operation for signals at either end of the

supply voltage range and eliminates the possibility of phase

reversal for input signals up to 500 mV beyond either power

supply. Offset voltage also changes to reflect the offset of the

input pair in control. The transition region is small, approximately

180 mV. These sudden changes in the dc parameters of the

input stage can produce glitches that adversely affect distortion.

OVERDRIVING THE INPUT STAGE

Sustained input differential voltages greater than 3.4 V should

be avoided as the input transistors can be damaged. Input clamp

diodes are recommended if the possibility of this condition

exists.

The voltages at the collectors of the input pairs are set to

200 mV from the power supply rails. This allows the amplifier

to remain in linear operation for input voltages up to 500 mV

beyond the supply voltages. Driving the input common-mode

voltage beyond that point will forward bias the collector junction of

the input transistor, resulting in phase reversal. Sustaining this

condition for any length of time should be avoided because it is

easy to exceed the maximum allowed input differential voltage

when the amplifier is in phase reversal.

Q13

Q17

Q10

Q14

Q15

Q11

Q16

Q18

OUTPUT STAGE,

OMMON-MOD

FEEDBACK

2kΩ

25µA

2kΩ

5µA

90µA

1.1V

50kΩ

850Ω

01056-043

Figure 43. Simplified Schematic of AD8031 Input Stage

AD8031/AD8032 Data Sheet

Rev. G | Page 14 of 20

OUTPUT STAGE, OPEN-LOOP GAIN AND

DISTORTION vs. CLEARANCE FROM POWER

SUPPLY

The AD8031 features a rail-to-rail output stage. The output

transistors operate as common-emitter amplifiers, providing the

output drive current as well as a large portion of the amplifier’s

open-loop gain.

DIFFERENTIAL

FROM

T S

Q37

Q47

Q51

Q27

Q68

Q49

Q38

25µA

5pF

µA

25µA

300Ω

01056-044

Figure 44. Output Stage Simplified Schematic

The output voltage limit depends on how much current the

output transistors are required to source or sink. For applications

with low drive requirements (for instance, a unity gain follower

driving another amplifier input), the AD8031 typically swings

within 20 mV of either voltage supply. As the required current

load increases, the saturation output voltage increases linearly as

LOAD

× R

where:

LOAD

is the required load current.

is the output transistor collector resistance.

For the AD8031, the collector resistances for both output

transistors are typically 25 Ω. As the current load exceeds the

rated output current of 15 mA, the amount of base drive current

required to drive the output transistor into saturation reaches its

limit, and the amplifier’s output swing rapidly decreases.

The open-loop gain of the AD8031 decreases approximately

linearly with load resistance and depends on the output voltage.

Open-loop gain stays constant to within 250 mV of the positive

power supply, 150 mV of the negative power supply, and then

decreases as the output transistors are driven further into

saturation.

The distortion performance of the AD8031/AD8032 amplifiers

differs from conventional amplifiers. Typically, the distortion

performance of the amplifier degrades as the output voltage

amplitude increases.

Used as a unity gain follower, the output of the AD8031/

AD8032 exhibits more distortion in the peak output voltage

region around V

− 0.7 V. This unusual distortion characteristic is

caused by the input stage architecture and is discussed in detail

in the Input Stage Operation section,

OUTPUT OVERDRIVE RECOVERY

Output overdrive of an amplifier occurs when the amplifier

attempts to drive the output voltage to a level outside its normal

range. After the overdrive condition is removed, the amplifier

must recover to normal operation in a reasonable amount of

time. As shown in Figure 45, the AD8031/AD8032 recover

within 100 ns from negative overdrive and within 80 ns from

positive overdrive.

50Ω

100

ns1V

= ±2.5V

= ±2.

1kΩ T

O GND

= R

= 2k

Ω

01056-045

Figure 45. Overdrive Recovery

Data Sheet AD8031/AD8032

Rev. G | Page 15 of 20

DRIVING CAPACITIVE LOADS

Capacitive loads interact with an op amp’s output impedance to

create an extra delay in the feedback path. This reduces circuit

stability and can cause unwanted ringing and oscillation. A

given value of capacitance causes much less ringing when the

amplifier is used with a higher noise gain.

The capacitive load drive of the AD8031/AD8032 can be

increased by adding a low valued resistor in series with the

capacitive load. Introducing a series resistor tends to isolate the

capacitive load from the feedback loop, thereby diminishing its

influence. Figure 46 shows the effects of a series resistor on the

capacitive drive for varying voltage gains. As the closed-loop

gain is increased, the larger phase margin allows for larger

capacitive loads with less overshoot. Adding a series resistor at

lower closed-loop gains accomplishes the same effect. For large

capacitive loads, the frequency response of the amplifier is

dominated by the roll-off of the series resistor and capacitive load.

1000

100

CAPACITIVE LOAD (pF)

CLOSED-LOOP GAIN (V/V)

0 1 2 3 4 5

= 5Ω

= 0Ω

= 20Ω

= 0Ω, 5Ω

= 5V

200mV STEP

WITH 30% OVERSHOOT

OUT

01056-046

Figure 46. Capacitive Load Drive vs. Closed-Loop Gain

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