AD8051ARZ-REEL Datasheet AD8051ARTZ-REEL7, AD8052ARMZ-REEL7, AD8052ARZ-REEL7 | P16-P18

AD8051/AD8052/AD8054

Rev. J | Page 16 of 24

THEORY OF OPERATION

CIRCUIT DESCRIPTION

The AD8051/AD8052/AD8054 are fabricated on the Analog

Devices, Inc. proprietary eXtra-Fast Complementary Bipolar

(XFCB) process, which enables the construction of PNP and

NPN transistors with similar fTs in the 2 GHz to 4 GHz region.

The process is dielectrically isolated to eliminate the parasitic

and latch-up problems caused by junction isolation. These

features allow the construction of high frequency, low distortion

amplifiers with low supply currents. This design uses a differential

output input stage to maximize bandwidth and headroom (see

Figure 40). The smaller signal swings required on the first stage

outputs (nodes SIP, SIN) reduce the effect of nonlinear currents

due to junction capacitances and improve the distortion per-

formance. This design achieves harmonic distortion of −80 dBc

@ 1 MHz into 100  with V

OUT

= 2 V p-p (gain = +1) on a

single 5 V supply.

The inputs of the device can handle voltages from −0.2 V below

the negative rail to within 1 V of the positive rail. Exceeding

these values do not cause phase reversal; however, the input

ESD devices begin to conduct if the input voltages exceed the

rails by greater than 0.5 V. During this overdrive condition, the

output stays at the rail.

The rail-to-rail output range of the AD8051/AD8052/AD8054

is provided by a complementary common emitter output stage.

High output drive capability is provided by injecting all output

stage predriver currents directly into the bases of the output

devices Q8 and Q36. Biasing of Q8 and Q36 is accomplished by

I8 and I5, along with a common-mode feedback loop (not

shown). This circuit topology allows the AD8051/AD8052 to

drive 45 mA of output current and allows the AD8054 to drive

30 mA of output current with the outputs within 0.5 V of the

supply rails.

I10

R39

I2 I3

Q25

Q51

R23

R27

Q36

I11

R21

SIP

SIN

R15

R26

Q50

Q22

Q21 Q27

Q23

Q31

Q39

Q13

Q24 Q47

Q11

Q40

OUT

01062-045

Figure 40. AD8051/AD8052 Simplified Schematic

AD8051/AD8052/AD8054

Rev. J | Page 17 of 24

APPLICATION INFORMATION

OVERDRIVE RECOVERY

Overdrive of an amplifier occurs when the output and/or input

range is exceeded. The amplifier must recover from this over-

drive condition. As shown in Figure 41, the AD8051/AD8052/

AD8054 recover within 60 ns from negative overdrive and

within 45 ns from positive overdrive.

VOLTS

= ±5V

G = +5

= 2kΩ

V/DIV AS SHOWN

100ns

INPUT 1V/DIV

OUTPUT 2V/DIV

01062-040

Figure 41. Overdrive Recovery

DRIVING CAPACITIVE LOADS

Consider the AD8051/AD8052 in a closed-loop gain of +1 with

= 5 V and a load of 2 k in parallel with 50 pF. Figure 42

and Figure 43 show their frequency and time domain responses,

respectively, to a small-signal excitation. The capacitive load

drive of the AD8051/AD8052/AD8054 can be increased by

adding a low value resistor in series with the load. Figure 44

and Figure 45 show the effect of a series resistor on the capaci-

tive drive for varying voltage gains. As the closed-loop gain is

increased, the larger phase margin allows for larger capacitive

loads with less peaking. Adding a series resistor with 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 the load

capacitance.

FREQUENCY (MHz)

–2

–4

–6

–8

–10

GAIN (dB)

–12

0.1 500100110

01062-041

= 5V

G = +1

= 2kΩ

= 50pF

OUT

= 200mV p-p

Figure 42. AD8051/AD8052 Closed-Loop Frequency Response; C

= 50 pF

2.60

2.55

2.50

2.45

2.40

VOLTS

= 5V

G = +1

= 2kΩ

= 50pF

50mV

100ns

1062-042

Figure 43. AD8051/AD8052 200 mV Step Response; C

= 50 pF

10000

1000

CAPACITIVE LOAD (pF)

100

(V/V)

= 5V

≤ 30%

OVERSHOOT

= 3Ω

= 0Ω

123456

OUT

50Ω

100mV

STEP

01062-043

Figure 44. AD8051/AD8052 Capacitive Load Drive vs. Closed-Loop Gain

1000

100

CAPACITIVE LOAD (pF)

(V/V)

123456

OUT

50Ω

100mV

STEP

= 5V

≤ 30%

OVERSHOOT

= 0Ω

= 10Ω

01062-044

Figure 45. AD8054 Capacitive Load Drive vs. Closed-Loop Gain

AD8051/AD8052/AD8054

Rev. J | Page 18 of 24

LAYOUT CONSIDERATIONS

The specified high speed performance of the AD8051/AD8052/

AD8054 requires careful attention to board layout and component

selection. Proper RF design techniques and low parasitic

component selection are necessary.

The PCB should have a ground plane covering all unused

portions of the component side of the board to provide a low

impedance path. The ground plane should be removed from the

area near the input pins to reduce parasitic capacitance.

Chip capacitors should be used for supply bypassing. One end

should be connected to the ground plane and the other within

3 mm of each power pin. An additional large (4.7 µF to 10 µF)

tantalum electrolytic capacitor should be connected in parallel,

but not necessarily so close, to supply current for fast, large

signal changes at the output.

The feedback resistor should be located close to the inverting

input pin to keep the parasitic capacitance at this node to a

minimum. Parasitic capacitance of less than 1 pF at the inverting

input can significantly affect high speed performance.

Stripline design techniques should be used for long signal traces

(greater than about 25 mm). These should be designed with a

characteristic impedance of 50  or 75  and be properly

terminated at each end.

ACTIVE FILTERS

Active filters at higher frequencies require wider bandwidth op

amps to work effectively. Excessive phase shift produced by

lower frequency op amps can significantly affect active filter

performance.

Figure 46 shows an example of a 2 MHz biquad bandwidth filter

that uses three op amps of an AD8054. Such circuits are

sometimes used in medical ultrasound systems to lower the

noise bandwidth of the analog signal before analog-to-digital

conversion.

Note that the unused amplifier’s inputs should be tied to ground.

AD8054

1kΩ

2kΩ

3kΩ

50pF

BAND-PASS

FILTER OUTPUT

01062-046

Figure 46. 2 MHz Biquad Band-Pass Filter Using AD8054

The frequency response of the circuit is shown in Figure 47.

FREQUENCY (Hz)

–10

–20

–30

–40

GAIN (dB)

10k 100k 1M 10M 100M

01062-047

Figure 47. Frequency Response of 2 MHz Band-Pass Biquad Filter

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24

AD8051ARZ-REEL

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