Data Sheet

ADRF5720

Rev. 0 | Page 13 of 19

THEORY OF OPERATION

The ADRF5720 incorporates a 6-bit fixed attenuator array that

offers an attenuation range of 31.5 dB in 0.5 dB steps. An

integrated driver provides both serial and parallel mode control

of the attenuator array (see Figure 32).

Note that when referring to a single function of a multifunction

pin in this section, only the portion of the pin name that is relevant

is mentioned. For full pin names of the multifunction pins, refer

to the Pin Configuration and Function Descriptions section.

POWER SUPPLY

The ADRF5720 requires a positive supply voltage applied to the

VDD pin and a negative supply voltage applied to the VSS pin.

Bypassing capacitors are recommended on the supply lines to

filter high frequency noise.

The power-up sequence is as follows:

1. Connect GND.

2. Power up VDD.

3. Power up VSS.

4. Apply the digital control inputs. The relative order of the

digital control inputs is not important. However, powering

the digital control inputs before the VDD supply can

inadvertently forward bias and damage the internal ESD

protection structures.

5. Apply an RF input signal to ATTIN or ATTOUT.

The power-down sequence is the reverse order of the power-up

sequence.

RF INPUT AND OUTPUT

Both RF ports (ATTIN and ATTOUT) are dc-coupled to 0 V.

No dc blocking is required at the RF ports when the RF line

potential is equal to 0 V.

The RF ports are internally matched to 50 Ω. Therefore,

external matching components are not required. For wideband

applications, use impedance matching to improve insertion loss,

return loss, and attenuation accuracy performance at high

frequencies. See the Impedance Matching section.

The ADRF5720 supports bidirectional operation at a lower

power level. The power handling of the ATTIN and ATTOUT

ports are different. Therefore, the bidirectional power handling is

defined by the ATTOUT port. Refer to the RF input power

specifications in Table 1.

SERIAL OR PARALLEL MODE SELECTION

The ADRF5720 can be controlled in either serial or parallel mode

by setting the PS pin to high or low, respectively (see Table 6).

Table 6. Mode Selection

PS

Control Mode

Low

Parallel

High

Serial

Table 7. Truth Table

Attenuation State (dB)

D5

D4

D3

D2

D1

D0

Low

Low

Low

Low

Low

Low

0 (reference)

Low

Low

Low

Low

Low

High

0.5

Low

Low

Low

Low

High

Low

1.0

Low

Low

Low

High

Low

Low

2.0

Low

Low

High

Low

Low

Low

4.0

Low

High

Low

Low

Low

Low

8.0

High

Low

Low

Low

Low

Low

16.0

High

High

High

High

High

High

31.5

1

Any combination of the control voltage input states shown in Table 7 provides an attenuation equal to the sum of the bits selected.

Figure 32. Simplified Circuit Diagram

D

Q

D

Q

D

Q

D

Q

D

Q

PARALLEL OR SERIAL SELECT

6-BIT OR 8-BIT LATCH

D

Q

D

D0

SERIN

CLK

PS

LE

D1

D2

D3

D4

D5

Q

D

Q

RF

INPUT

RF

OUTPUT

SEROUT

0.5dB

1dB

2dB

4dB

8dB

16dB