Data Sheet

AD8002

Rev. E | Page 17 of 21

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.

Conversely, the voltage feedback nature provides symmetrical

high impedance inputs and allows the use of reactive

components in the feedback network.

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

unity-gain follower by the 511 Ω RA feedback resistors between

the output and inverting input of each op amp. The output of

each op amp has a 511 Ω RB resistor to the inverting input of

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

through a unity-gain inverter configuration. By connecting

the two amplifiers as cross-coupled inverters, the outputs of the

amplifiers are freed to be equal and opposite, assuring zero

output common-mode voltage.

Using this circuit configuration, the common-mode signal of

the outputs is reduced. If one output increases slightly, the

negative input to the other op amp drives its output slightly

lower and thus preserves the symmetry of the complementary

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

mode output signal was measured as −50 dB at 1 MHz.

Looking at this configuration overall, there are two high

impedance inputs (the +IN1, +IN2 of each op amp), two

low impedance outputs, and a high open-loop gain. The two

noninverting inputs and the output of the Op Amp 2 structure

looks like a voltage feedback op amp having two symmetrical,

high impedance inputs and one output. The +IN1, +IN2 to

Op Amp 2 is the noninverting input (it has the same polarity as

OUT2) and the +IN1, +IN2 to Op Amp 1 is the inverting input

(opposite polarity of Output 2).

With a feedback resistor, RF, an input resistor, RG, and the

grounding of the +IN1, +IN2 of Op Amp 2, a feedback

amplifier is formed. This configuration is similar to a voltage

feedback amplifier in an inverting configuration if only OUT2

is considered. The addition of OUT1 makes the amplifier a

differential output.

The differential gain of this circuit is













+

×

=

B

A

G

F

R

R

R

R

G

1

where:

RF/RG is the gain of the overall op amp configuration and is the

same as for an inverting op amp except for the polarity. If OUT1

is used as the output reference, the gain is positive.

1 + RA/RB is the noise gain of each individual op amp in its

noninverting configuration.

The resulting architecture offers several advantages. First, the

gain can be changed by changing a single resistor. Changing

either RF or RG changes the gain as in an inverting op amp

circuit. For most types of differential circuits, more than one

resistor must be changed to change gain and still maintain good

common-mode rejection (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 op amp. The circuit described

requires about 0.9 pF of capacitance in shunt across RF to optimize

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 influences the frequency response

and the value required for the feedback capacitor, thus a good

layout is essential.

The shunt capacitor type selection is also critical. A good

microwave type chip capacitor with high Q was found to yield

best performance. The device selected for this circuit was a

Murata Erie MA280R9B.

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

and a 100 Ω load on each output. For OUT1, the distortion is

−37 dBc and −41 dBc for the second and third harmonics,

respectively. For OUT2, the second harmonic is −35 dBc and

the third harmonic is −43 dBc.

6

–4

–14

1G

10M

1M

100M

–6

–8

–10

–12

–2

0

2

4

FREQUENCY (Hz)

CC = 0.9pF

OUT+

OUT–

Figure 56. Differential Driver Frequency Response