AD8302 datasheet(16/24 Pages) AD

16 / 24 page

REV. A

AD8302

–16–

Note that by convention, the phase difference is taken in the range

from –180

° to +180°. Since this style of phase detector does not

distinguish between

±90°, it is considered to have an unambiguous

180

° phase difference range that can be either 0° to +180° centered

at +90

° or 0° to –180° centered at –90°.

The basic structure of both output interfaces is shown in Figure 3. It

accepts a setpoint input and includes an internal integrating/averag-

ing capacitor and a buffer amplifier with gain K. External access to

these setpoints provides for several modes of operation and enables

flexible tailoring of the gain and phase transfer characteristics. The

setpoint interface block, characterized by a transresistance RF, gener-

ates a current proportional to the voltage presented to its input pin,

MSET or PSET. A precise offset voltage of 900 mV is introduced

internally to establish the center-point (VCP) for the gain and phase

functions, i.e., the setpoint voltage that corresponds to a gain of 0 dB

and a phase difference of 90

°. This setpoint current is subtracted

from the signal current, IIN, coming from the log amps in the gain

channel or from the phase detector in the phase channel. The result-

ing difference is integrated on the averaging capacitors at either pin

MFLT or PFLT and then buffered by the output amplifier to the

respective output pins, VMAG and VPHS. With this open-loop

arrangement, the output voltage is a simple integration of the differ-

ence between the measured gain/phase and the desired setpoint:

VR I

OUT

=−

() ( )

(6)

where IFB is the feedback current equal to (VSET – VCP)/RF, VSET

is the setpoint input, and T is the integration time constant equal

to RFCAVE/K, where CAVE is the parallel combination of the inter-

nal 1.5 pF and the external capacitor CFLT.

MSET/PSET

20k

VCP = 900mV

1.5pF

CFLT

MFLT/PFLT

VMAG/VPHS

IIN = ILA OR IPD

IFB

–

Figure 3. Simplified Block Diagram of the Output Interface

BASIC CONNECTIONS

Measurement Mode

The basic function of the AD8302 is the direct measurement of gain

and phase. When the output pins, VMAG and VPHS, are connected

directly to the feedback setpoint input pins, MSET and PSET, the

default slopes and center points are invoked. This basic connection

shown in Figure 4 is termed the measurement mode. The current

from the setpoint interface is forced by the integrator to be equal to

the signal currents coming from the log amps and phase detector.

The closed loop transfer function is thus given by:

OUT

() +

()

(7)

The time constant T represents the single-pole response to the enve-

lope of the dB-scaled gain and the degree-scaled phase functions. A

small internal capacitor sets the maximum envelope bandwidth to

approximately 30 MHz. If no external CFLT is used, the AD8302

can follow the gain and phase envelopes within this bandwidth. If

longer averaging is desired, CFLT can be added as necessary accord-

ing to T (ns) = 3.3

× CAVE (pF). For best transient response with

minimal overshoot, it is recommended that 1 pF minimum value

external capacitors be added to the MFLT and PFLT pins.

COMM

MFLT 14

INPA

VMAG

213

OFSA

MSET

312

VPOS

VREF

411

OFSB

PSET

510

INPB

VPHS

COMM

PFLT

AD8302

VMAG

VPHS

VINA

VINB

Figure 4. Basic Connections in Measurement Mode with

30 mV/dB and 10 mV/Degree Scaling

In the low frequency limit, the gain and phase transfer functions

given in Equations 4 and 5 become:

VR I

MAG

F SLP

INA

INB

()+

log

(8a)

VR I

MAG

F SLP

INA

INB

()

−

()+

/20

(8b)

VR I

PHS

INA

INB

()− ()

()+

–

||–

Φ ΦΦ

(9)

which are illustrated in Figure 5. In Equation 8b, PINA and PINB are

the power in dBm equivalent to VINA and VINB at a specified refer-

ence impedance. For the gain function, the slope represented by

RF ISLP is 600 mV/decade or, dividing by 20 dB/decade, 30 mV/dB.

With a center point of 900 mV for 0 dB gain, a range of –30 dB to

+30 dB covers the full-scale swing from 0 V to 1.8 V. For the phase

function, the slope represented by RFIΦ is 10 mV/degree. With a

center point of 900 mV for 90

°, a range of 0° to 180° covers the

full-scale swing from 1.8 V to 0 V. The range of 0

° to –180° covers

the same full-scale swing but with the opposite slope.

1.8V

900mV

30mV/dB

VCP

MAGNITUDE RATIO – dB

–30

+30

1.8V

900mV

PHASE DIFFERENCE – Degrees

+10mV/DEG

–10mV/DEG

VCP

–180

–90

180

Figure 5. Idealized Transfer Characteristics for the Gain

and Phase Measurement Mode

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