AD8318ACPZ-R2 datasheet(13/24 Pages) AD

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Data Sheet

AD8318

Rev. D | Page 13 of 24

VPSI

2kΩ

A = 8.6dB

20kΩ

CURRENT

STAGE

INLO

INHI

OFFSET

COMP

10pF

FIRST

GAIN

STAGE

Figure 26. Input Interface

While the input can be reactively matched, this is typically not

necessary. An external 52.3 Ω shunt resistor (connected on the

signal side of the input coupling capacitors, see Figure 23)

combines with the relatively high input impedance to provide

an adequate broadband 50 Ω match.

Table 4. Input Impedance for Select Frequency

Frequency

S11

Impedance Ω

(MHz)

Real

Imaginary

(Series)

100

+0.918

−0.041

927-j491

450

+0.905

−0.183

173-j430

900

+0.834

−0.350

61-j233

1900

+0.605

−0.595

28-j117

2200

+0.524

−0.616

28-j102

3600

+0.070

−0.601

26-j49

5300

−0.369

−0.305

20-j16

5800

−0.326

−0.286

22-j16

8000

−0.390

−0.062

22-j3

The coupling time constant, 50 × CC/2, forms a high-pass

corner with a 3 dB attenuation at fHP = 1/(2π × 50 × CC), where

C1 = C2 = CC. Using the typical value of 1 nF, this high-pass

corner is ~3.2 MHz. In high frequency applications, fHP must be

as large as possible to minimize the coupling of unwanted low

frequency signals. Likewise, in low frequency applications, a

simple RC network forming a low-pass filter must be added,

generally placed at the generator side of the coupling capacitors,

thereby lowering the required capacitance value for a given

high-pass corner frequency.

OUTPUT INTERFACE

The logarithmic output interface is shown in Figure 27. The

VOUT pin is driven by a PNP output stage. An internal 10 Ω

resistor is placed in series with the emitter follower output and

the VOUT pin. The rise time of the output is limited mainly by

the slew on CLPF. The fall time is an RC limited slew provided

by the load capacitance and the pull-down resistance at VOUT.

There is an internal pull-down resistor of 350 Ω. Any resistive

load at VOUT is placed in parallel with the internal pull-down

resistor and provides additional discharge current.

0.2V

150Ω

200Ω

–

10Ω

VOUT

VPSO

CLPF

CMOP

Figure 27. Output Interface

SETPOINT INTERFACE

The setpoint interface is shown in Figure 28. The VSET input

drives the high impedance (250 kΩ) input of an internal

operational amplifier. The VSET voltage appears across the

internal 3.13 kΩ resistor to generate ISET. When a portion

of VOUT is applied to VSET, the feedback loop forces

−ID × log10(VIN/VINTERCEPT) = ISET

(2)

If VSET = VOUT/X, ISET = VOUT/(X × 3.13 kΩ). The result is

VOUT = (−ID × 3.13 kΩ × X) × log10(VIN/VINTERCEPT).

3.13kΩ

ISET

CMOP

VSET

Figure 28. VSET Interface

The slope is given by −ID × X × 3.13 kΩ = −500 mV × X. For

example, if a resistor divider to ground is used to generate a

VSET voltage of VOUT/2, X = 2. The slope is set to −1 V/decade or

−50 mV/dB.

TEMPERATURE COMPENSATION OF OUTPUT

VOLTAGE

The AD8318 functionality includes the capability to externally

trim the temperature drift. Attaching a ground-referenced

resistor to the TADJ pin alters an internal current, minimizing

intercept drift vs. temperature. As a result, the RTADJ can be

optimized for operation at different frequencies.

2kΩ

ICOMP

~0.4V

TADJ

VINTERNAL

Figure 29. TADJ Interface

RTADJ, nominally 499 Ω for optimal temperature compensation

at 2.2 GHz input frequency, is connected between the TADJ pin

and ground (see Figure 23). The value of this resistor partially

determines the magnitude of an analog correction coefficient

that is employed to reduce intercept drift.

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