ADL5902ACPZ-R7 datasheet(17/28 Pages) AD

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Data Sheet

ADL5902

Rev. B | Page 17 of 28

SMALL SIGNAL LOOP RESPONSE

The ADL5902 uses a VGA in a loop to force a squared RF signal

to be equal to a squared dc voltage. This nonlinear loop can be

simplified and solved for a small signal loop response. The low-

pass corner pole is given by

FreqLP ≈ 1.83 × ITGT/(CLPF)

(12)

where:

ITGT is in amperes.

CLPF is in farads.

FreqLP is in hertz.

ITGT is derived from VTGT; however, ITGT is a squared value of

VTGT multiplied by a transresistance, namely

ITGT = gm × VTGT2

(13)

gm is approximately 18.9 μs; therefore, with VTGT equal to the

typically recommended 0.8 V, ITGT is approximately 12 μA. The

value of this current varies with temperature; therefore, the small

signal pole varies with temperature. However, because the RF

squaring circuit and dc squaring circuit track with temperature,

there is no temperature variation contribution to the absolute value

of VOUT.

For CW signals,

FreqLP ≈ 67.7 × 10−6/(CLPF)

(14)

However, signals with large crest factors include low pseudo-

random frequency content that must be either filtered out or

sampled and averaged out (see the Choosing a Value for CLPF

section for more information).

TEMPERATURE SENSOR INTERFACE

The ADL5902 provides a temperature sensor output with a scaling

factor of the output voltage of approximately 4.9 mV/°C. The

output is capable of sourcing 4 mA and sinking 50 μA maximum

at 25°C. An external resistor can be connected from TEMP to

COMM to provide additional current sink capability. The

typical output voltage at 25°C is approximately 1.4 V.

TEMP

VPOS

COMM

INTERNAL

VPAT

12kΩ

4kΩ

Figure 38. TEMP Interface Simplified Schematic

VREF INTERFACE

The VREF pin provides an internally generated voltage reference

for the user. The VREF voltage is a temperature stable 2.3 V

reference that is capable of sourcing 4 mA and sinking 50 μA

maximum. An external resistor can be connected from VREF to

COMM to provide additional current sink capability. The voltage

on this pin can drive the TADJ/PWDN and VTGT pins.

INTERNAL

VOLTAGE

16kΩ

VREF

VPOS

COMM

Figure 39. VREF Interface Simplified Schematic

TEMPERATURE COMPENSATION INTERFACE

While the ADL5902 has a highly stable measurement output

with respect to temperature using proprietary techniques, for

optimal performance, the output temperature drift must be

compensated for using the TADJ pin. The absolute value of

compensation varies with frequency and VTGT. Table 4 shows the

recommended voltages for VTADJ to maintain a temperature drift

error of typically ±0.5 dB or better over the intended temperature

range (−40°C < TA < +85°C) when driven single-ended and

VTGT = 0.8 V.

Table 4. Recommended VTADJ for Selected Frequencies

Frequency

VTADJ (V) R9 in Figure 54 (Ω) R12 in Figure 54 (Ω)

100 MHz

0.5

1430

402

700 MHz

0.4

1430

301

900 MHz

0.4

1430

301

1.9 GHz

0.4

1430

301

2.14 GHz

0.4

1430

301

2.6 GHz

0.45

1430

348

3.5 GHz

0.5

1430

402

5.8 GHz

0.95

1430

1007

The values in Table 4 are chosen to give the best drift

performance at the high end of the usable dynamic range

over the −40°C to +85°C temperature range. There is often a

trade off in setting values, and optimizing for one area of the

dynamic range can mean less than optimal drift performance at

other input amplitudes.

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