Data Sheet

AD8340

RF OUTPUT AND MATCHING

The RF outputs of the AD8340, RFOP, and RFOM, are open

collectors of a transimpedance amplifier that needs to be pulled

up to the positive supply, preferably with RF chokes, as shown

in Figure 31. The nominal output impedance looking into each

individual output pin is 25 Ω. Consequently, the differential

output impedance is 50 Ω.

50Ω

DIFFERENTIAL

100pF

1:1

RF

OUTPUT

RFOM

RFOP

RT

RT

120nH

100pF

VP

GM

±ISIG

Figure 31. RF Output Interface to the AD8340 Showing

Coupling Capacitors, Pull-Up RF Chokes, and Balun

Because the output dc levels are at the positive supply, ac

coupling capacitors are usually needed between the AD8340

outputs and the next stage in the system.

A 1:1 RF broadband output balun, such as the ETC1-1-13

(M/A-COM), converts the differential output of the AD8340

into a single-ended signal. Note that the loss and balance of the

balun directly impact the apparent output power, noise floor,

and gain/phase errors of the AD8340. In critical applications,

narrow-band baluns with low loss and superior balance are

recommended.

If the output is taken in a single-ended fashion directly into a

50 Ω load through a coupling capacitor, there is an impedance

mismatch. This can be resolved with a 1:2 balun to convert the

single-ended 25 Ω output impedance to 50 Ω. If loss-of-signal

swing is not critical, a 25 Ω back termination in series with the

output pin can also be used. The unused output pin must still be

pulled up to the positive supply. The user can load it through a

coupling capacitor with a dummy load to preserve balance. The

gain of the AD8340 when the output is single-ended varies

slightly with the dummy load value, as shown in Figure 32.

FREQUENCY (MHz)

–0.5

–1.0

–5.5

1000

–3.5

–4.0

–4.5

–5.0

–2.5

–3.0

–1.5

–2.0

–6.0

900

800

700

RL2 = SHORT

RL2

= 50Ω

RL2 = OPEN

Figure 32. Gain of the AD8340 Using a Single-Ended Output with Different

Dummy Loads, RL2 on the Unused Output

The RF output signal can be disabled by raising the DSOP pin

to the positive supply. The shutdown function provides >40 dB

attenuation of the input signal even at full gain. The interface

to DSOP is high impedance, and the shutdown and turn-on

response times are <100 ns. If the disable function is not

needed, the DSOP should be tied to ground.

DRIVING THE I-Q BASEBAND CONTROLS

The I and Q inputs to the AD8340 set the gain and phase

between input and output. These inputs are differential and

should normally have a common-mode level of 0.5 V. However,

when differentially driven, the common mode can vary from

250 mV to 750 mV while still allowing full gain control. Each

input pair has a nominal input swing of ±0.5 V differential

around the common-mode level. The maximum gain of unity

is achieved if the differential voltage is equal to +500 mV or

−500 mV. Therefore, with a common-mode level of 500 mV,

IBBP and IBBM each swings between 250 mV and 750 mV.

The I and Q inputs can also be driven with a single-ended

signal. In this case, one side of each input should be tied to a

low noise 0.5 V voltage source (a 0.1 µF decoupling capacitor

located close to the pin is recommended), while the other input

swings from 0 V to 1 V. Differential drive generally offers

superior even-order distortion and lower noise than single-

ended drive.

The bandwidth of the baseband controls exceeds 200 MHz even

at full-scale baseband drive. This allows for very fast gain and

phase modulation of the RF input signal. In cases where lower

modulation bandwidths are acceptable or desired, external filter

capacitors can be connected across Pin IFLP to Pin IFLM, and

across Pin QFLP to Pin QFLM, to reduce the ingress of base-

band noise and spurious signal into the control path.

The 3 dB bandwidth is set by choosing CFLT according to the

following equation:

pF

5

.

0

nF

10

kHz

45

dB

3

+

×

≈

EXTERNAL

C

f

This equation has been verified for values of CFLT from 10 pF to

0.1 µF (bandwidth settings of approximately 4.5 kHz to 43 MHz).

Rev. C | Page 13 of 20