REV. D

AD8002

–13–

Single-Ended-to-Differential Driver Using an AD8002

The two halves of an AD8002 can be configured to create a

single-ended-to-differential high-speed driver with a –3dB

bandwidth in excess of 200 MHz, as shown in Figure 11. Although

the individual op amps are each current feedback, the overall

architecture yields a circuit with attributes normally associated

with voltage feedback amplifiers, while offering the speed advan-

tages inherent in current feedback amplifiers. In addition, the gain

which is often not possible in a dual op amp differential driver.

50

OUTPUT #1

50

OUTPUT #2

511

1/2

AD8002

1/2

AD8002

OP AMP #1

OP AMP #2

511

511

511

511

Figure 11. Differential Line Driver

The current feedback nature of the op amps, in addition to

enabling the wide bandwidth, provides an output drive of more

than 3 V p-p into a 20

Ω load for each output at 20 MHz. On the

other hand, the voltage feedback nature provides symmetrical

high impedance inputs and allows the use of reactive compo-

nents in the feedback network.

The circuit consists of the two op amps, each configured as a

unity gain follower by the 511

each op amp’s output and inverting input. The output of each op

amp has a 511

Ω R

op amp. Thus, each output drives the other op amp through a

unity gain inverter configuration. By connecting the two amplifi-

ers as cross-coupled inverters, their outputs are freed to be equal

and opposite, assuring zero-output common-mode voltage.

With this circuit configuration, the common-mode signal of the

outputs is reduced. If one output moves slightly higher, the nega-

tive input to the other op amp drives its output to go slightly

lower and thus preserves the symmetry of the complementary

outputs, which reduces the common-mode signal. The common-

mode output signal was measured to be –50 dB at 1 MHz.

Looking at this configuration overall, there are two high imped-

ance inputs (the + inputs of each op amp), two low impedance

outputs, and high open-loop gain. If we consider the two nonin-

verting inputs and just the output of Op Amp #2, the structure

looks like a voltage feedback op amp having two symmetrical,

high-impedance inputs, and one output. The +input to Op Amp

#2 is the noninverting input (it has the same polarity as Output

#2) and the +input to Amplifier #1 is the inverting input (oppo-

site polarity of Output #2).

of the +input of Op Amp #2, a feedback amplifier is formed.

This configuration is just like a voltage feedback amplifier in an

inverting configuration if only Output #2 is considered. The

addition of Output #1 makes the amplifier differential output.

The differential gain of this circuit is:

G

R

R

R

R

F

G

A

B

=×

+









1

and is the same as for an inverting op amp except for the polarity.

If Output #1 is used as the output reference, the gain is posi-

amp in its noninverting configuration.

The resulting architecture offers several advantages. First, the gain

can be changed by changing a single resistor. Changing either

For most types of differential circuits, more than one resistor

must be changed to change gain and still maintain good CMR.

Reactive elements can be used in the feedback network. This is

in contrast to current feedback amplifiers that restrict the use of

reactive elements in the feedback. The circuit described requires

peaking and realize a –3 dB bandwidth of more than 200 MHz.

The peaking exhibited by the circuit is very sensitive to the value

of this capacitor. Parasitics in the board layout on the order of

tenths of picofarads will influence the frequency response and

the value required for the feedback capacitor, so a good lay-

out is essential.

The shunt capacitor type selection is also critical. A good micro-

wave type chip capacitor with high Q was found to yield best

performance. The part selected for this circuit was a muRata

Erie part number MA280R9B.

The distortion was measured at 20 MHz with a 3 V p-p input

and a 100

Ω load on each output. For Output #1 the distortion

is –37 dBc and –41 dBc for the second and third harmonics

respectively. For Output #2 the second harmonic is –35 dBc

and the third harmonic is –43 dBc.

6

–4

–14

1M

10M

1G

100M

–6

–8

–10

–12

–2

0

2

4

FREQUENCY – Hz

OUT+

OUT–

Figure 12. Differential Driver Frequency Response