AD8008AR-REEL View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AD8008AR-REEL

Ultralow Distortion High Speed Amplifiers

Analog Devices 

AD8007/AD8008

THEORY OF OPERATION

The AD8007 (single) and AD8008 (dual) are current feedback

amplifiers optimized for low distortion performance. A simplified

conceptual diagram of the AD8007 is shown in Figure 3. It closely

resembles a classic current feedback amplifier comprised of a

complementary emitter-follower input stage, a pair of signal mir-

rors, and a diamond output stage. However, in the case of the

AD8007/AD8008, several modifications have been made to greatly

improve the distortion performance over that of a classic current

feedback topology.

I1 –

+VS

M1

– I3

D1

IN+

D2

Q1

IDI

IN–

Q2

CJ1

+VS

HiZ

–VS

CJ2

Q3

IDO

Q4

I2 –

M2

RG

– I4

RF

Q5

OUT

Q6

–VS

Figure 3. Simplified Schematic of AD8007

The signal mirrors have been replaced with low distortion, high

precision mirrors. They are shown as “M1” and “M2” in Figure 3.

Their primary function from a distortion standpoint is to greatly

reduce the effect of highly nonlinear distortion caused by capaci-

tances CJ1 and CJ2. These capacitors represent the collector-to-base

capacitances of the mirrors’ output devices.

A voltage imbalance arises across the output stage, as measured

from the high impedance node “HiZ” to the output node “Out.”

This imbalance is a result of delivering high output currents and

is the primary cause of output distortion. Circuitry is included

to sense this output voltage imbalance and generate a compensating

current “IDO.” When injected into the circuit, IDO reduces the

distortion that would be generated at the output stage. Similarly,

the nonlinear voltage imbalance across the input stage (measured

from the noninverting to the inverting input) is sensed, and a cur-

rent “IDI” is injected to compensate for input-generated distortion.

The design and layout are strictly top-to-bottom symmetric in

order to minimize the presence of even-order harmonics.

USING THE AD8007/AD8008

Supply Decoupling for Low Distortion

Decoupling for low distortion performance requires careful

consideration. The commonly adopted practice of returning the

high frequency supply decoupling capacitors to physically sepa-

rate (and possibly distant) grounds can lead to degraded

even-order harmonic performance. This situation is shown in

Figure 4 using the AD8007 as an example. Note that for a sinu-

soidal input, each decoupling capacitor returns to its ground a

quasi-rectified current carrying high even-order harmonics.

RF

499⍀

GND 1

RG

499⍀

0.1␮F

+VS

10␮F

+

RS AD8007

200⍀

IN

–VS

0.1␮F

10␮F

+

OUT

GND 2

Figure 4. High Frequency Capacitors Returned

to Physically Separate Grounds (Not Recommended)

The decoupling scheme shown in Figure 5 is preferable. Here,

the two high frequency decoupling capacitors are first tied

together at a common node, and are then returned to the

ground plane through a single connection. By first adding the

two currents flowing through each high frequency decoupling

capacitor, one is ensuring that the current returned into the

ground plane is only at the fundamental frequency.

RF

499⍀

RG

+VS

499⍀

RS

200⍀

IN

10␮F

+

0.1␮F

AD8007

0.1␮F

–VS

10␮F +

OUT

Figure 5. High Frequency Capacitors Returned

to Ground at a Single Point (Recommended)

Whenever physical layout considerations prevent the decoupling

scheme shown in Figure 5, the user can connect one of the high

frequency decoupling capacitors directly across the supplies and

connect the other high frequency decoupling capacitor to ground.

This is shown in Figure 6.

–14–

REV. C

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