Data Sheet

ADL5390

Rev. A | Page 13 of 23

RF OUTPUT AND MATCHING

The RF/IF outputs of the ADL5390, RFOP and RFOM, are open

collectors of a transimpedance amplifier that need to be pulled

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

in Figure 30. The nominal output impedance looking into each

individual output pin is 25 Ω. Consequently, the differential

output impedance is 50 Ω.

50



DIFFERENTIAL

10pF

1:1

RF

OUTPUT

RFOM

RFOP

120nH

10pF

0.1

F

Figure 30. RF Output Interface to the ADL5390 Showing

Coupling Capacitors, Pull-Up RF Chokes, and Balun

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

capacitors are usually needed between the ADL5390 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 ADL5390

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 ADL5390. 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 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 may load it through a

coupling capacitor with a dummy load to preserve balance. The

mismatched gain of the ADL5390 when the output is single-ended

varies slightly with dummy load value, as shown in Figure 31.

10

–10

–4

–2

0

2

4

6

8

10

100

1000

10000

FREQUENCY (MHz)

Figure 31. Gain of the ADL5390 Using a Single-Ended Output with Different

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

to the positive supply. The output disable function provides

>40 dB attenuation of the input signal, even at full gain. The

interface to DSOP is high impedance and the output disable

and output enable response times are <100 ns. If the output

disable function is not needed, the DSOP should be tied to

ground.

DRIVING THE I-Q BASEBAND GAIN CONTROLS

The I and Q gain control inputs to the ADL5390 set the gain for

each channel. 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 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 gain control 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. Low speed, single-ended

drive can easily be achieved using 12-bit voltage output DACs

such as AD8303 (serial SPI® interface) or AD8582 (parallel

interface) DACs. A reference voltage should also be supplied.

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 modulation

of the RF input signal. In cases where lower modulation band-

widths are acceptable or desired, external filter capacitors can

be connected across Pins IFLP to IFLM and Pins QFLP to

QFLM to reduce the ingress of baseband noise and spurious

signal into the control path.