ADL5906SCPZN-R7 datasheet(20/30 Pages) AD

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Data Sheet

ADL5906

THEORY OF OPERATION

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Rev. B | 20 of 30

forms of modulation with respect to the ideal CW line. This method

for calculating error is accurate, assuming that each device is

calibrated at room temperature.

In the second plot format, the VRMS voltage at a given input ampli-

tude and temperature is subtracted from the corresponding VRMS

at 25°C and then divided by the 25°C slope to obtain an error in

decibels. This type of plot does not provide any information on the

linear-in-dB performance of the device; it merely shows the decibel

equivalent of the deviation of VRMS over temperature, given a

calibration at 25°C. When calculating error from any one particular

calibration point, this error format is accurate. It is accurate over

the full range shown on the plot assuming that enough calibration

points are used. Figure 12 shows this plot type.

The error calculations for Figure 34 are similar to those for the VRMS

plots. The slope and intercept of the VTEMP function vs. temperature

are determined and applied as follows:

Error (°C) = (VTEMP − Slope × (Temp − TZ))/Slope

(10)

where:

VTEMP is the voltage at the TEMP pin at that temperature.

Slope is, typically, 4.8 mV/°C.

Temp is the ambient temperature of the ADL5906 in degrees

Celsius.

TZ is the x-axis intercept expressed in degrees Celsius (the temper-

ature that would result in a VTEMP of 0 V if this were possible).

MEASUREMENT MODE BASIC CONNECTIONS

The basic connections circuit for ADL5906 is shown in Figure 51.

The ADL5906 requires a single supply of nominally 5 V. The supply

is connected to the VPOS1 and VPOS2 supply pins. Decouple

each of these pins using two capacitors with values equal or similar

to those shown in Figure 51. Place these capacitors as close as

possible to the VPOS pins. The three no connect pins (NIC) are not

internally connected. Leave these pins unconnected.

An external 60.4 Ω resistor combines with the relatively high RF

input impedance of the ADL5906 to provide a broadband 50 Ω

match. Place an ac coupling capacitor between this resistor and

RFIN+. AC-couple the RFIN− input to ground using the same value

capacitor. To operate down to 10 MHz, the coupling capacitors must

be at least 100 pF.

The ADL5906 is placed in measurement mode by connecting the

VRMS pin to the VSET pin. In measurement mode, the output

voltage is proportional to the log of the rms input signal level.

SETTING VTADJ

As described in the Theory of Operation section, the output temper-

ature drift can be compensated by applying a voltage to the TADJ

pin. The compensating voltage varies with frequency. The voltage

for the TADJ pin can be easily derived from a resistor divider

connected to the VREF pin. Table 4 shows the recommended VTADJ

voltages for operation from −55°C to +125°C, along with resistor

divider values. Resistor values are chosen so that they neither pull

too much current from the VREF pin (IOUTMAX = 4 mA) nor are

so large that the maximum bias current at a VTADJ = 1 V (14 µA)

affects the resulting voltage.

The VTADJ function provides temperature compensation of the out-

put slope of the ADL5906. The Using VTEMP to Improve Intercept

Temperature Drift section describes how the temperature stability of

the ADL5906 can be further improved.

Recommended VTADJ Voltages

Table 4.

Frequency

VTADJ (V)

R9 (Ω)

R12 (Ω)

10 MHz to 2.14 GHz

0.35

1500

270

2.6 GHz

0.4

1500

316

3.5 GHz

0.45

1500

365

5.8 GHz

1.0

1540

1200

8 GHz

1.0

1540

1200

10 GHz

1.0

1540

1200

SETTING VTGT

As described in the Theory of Operation section, setting the voltage

on VTGT to 0.8 V represents a compromise between achieving

excellent rms accuracy and maximizing dynamic range. The voltage

on VTGT can be derived from the VREF pin using a resistor divider,

as shown Figure 51. Like the resistors chosen to set the VTADJ

voltage, the resistors setting VTGT must have reasonable values

that do not pull too much current from VREF or cause bias current

errors. In addition, note the combined current that VREF must

deliver to generate the VTADJ and VTGT voltages. The values shown

in Figure 51 and Table 4 result in a maximum VREF current of

1.7 mA. This current is well below the maximum specified VREF

current of 4 mA.

CHOOSING A VALUE FOR CRMS

CRMS provides the averaging function for the internal rms compu-

tation. Using the minimum value for CRMS allows the quickest

response time to a pulsed waveform but leaves significant output

noise on the output voltage signal. By the same token, a large

filter capacitor reduces output noise but at the expense of response

time.

In applications where response time is not critical, a relatively large

capacitor can be placed on the CRMS pin. In Figure 51, a value

of 0.1 µF is used. For most signal modulation schemes, this value

ensures excellent rms measurement compliance and low residual

output noise. There is no maximum capacitance limit for CRMS.

Figure 50 shows how output noise varies with CRMS when the

ADL5906 is driven by a single-carrier W-CDMA signal (Test Model

TM1-64, peak envelope power = 10.56 dB, bandwidth = 3.84 MHz).

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