AD8340

Rev. 0 | Page 13 of 20

RF OUTPUT AND MATCHING

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

collectors of a transimpedance amplifier which need 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.

50

Ω

DIFFERENTIAL

100pF

1:1

RF

OUTPUT

RFOM

RFOP

120nH

100pF

Figure31.RFOutputInterfacetotheAD8340Showing

CouplingCapacitors,Pull-UpRFChokes,andBalun

Since the output dc levels are at the positive supply, ac coupling

capacitors will usually be 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 will be an imped-

ance 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 may 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 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

Figure32.GainoftheAD8340UsingaSingle-EndedOutput withDifferentDummy

Loads,RL2ontheUnusedOutput

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 be-

tween 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. So

with a common-mode level of 500 mV, IBBP and IBBM will

each swing 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 supe-

rior 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 Pins IFLP to IFLM and

QFLP to QFLM to reduce the ingress of baseband noise and

spurious signal into the control path.