ADL5906ACPZN-R7 datasheet(21/32 Pages) AD

21 / 32 page

Data Sheet

ADL5906

Rev. 0 | Page 21 of 32

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

computation. 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).

0.1

100

1000

10000

100000

1000000

100

150

200

250

300

350

100

1000

10000

CRMS (nF)

OUTPUT NOISE (V p-p)

RISE TIME (µs)

FALL TIME (µs)

Figure 50. Output Noise, Rise and Fall Times vs. CRMS Capacitance,

Single-Carrier W-CDMA (TM1-64) at 2.14 GHz with PIN = 0 dBm

Figure 50 also shows how the response time is affected by the

value of CRMS. To measure this, an RF burst at 2.14 GHz at 0 dBm

was applied to the ADL5906. The 10% to 90% rise time and

90% to 10% fall time were then measured.

0.1µF

(SEE TEXT)

(AND TABLE)

(SEE TEXT)

(AND TABLE)

BIAS AND POWER

DOWN CONTROL

NIC

ITGT

LINEAR-IN-dB VGA

(NEGATIVE SLOPE)

ISQR

26pF

VPOS1

VPOS2

GND1

VREF

VTGT

GND2

CRMS

VRMS

VSET

VTEMP

NIC

RFIN–

RFIN+

NIC

EPAD

ADL5906

VREF

2.3V

TEMPERATURE

SENSOR

G = 5

60.4Ω

C10

10nF

RFIN

C12

10nF

R12

R11

2kΩ

R10

3.74kΩ

+5V

0.1µF

100pF

0.1µF

100pF

+5V

TADJ/

PWDN

Figure 51. Basic Connections for Operation in Measurement Mode

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