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Data Sheet

ADL5920

Rev. B | Page 19 of 26

VREF INTERFACE

The VREF pin provides an internally generated voltage reference

for the user. The VREF voltage is temperature stable and is

capable of sourcing 4 mA and sinking 50 µA maximum. To

provide additional current sink capability, connect an external

resistor from VREF to GNDx. The voltage on this pin can drive

the PWDN/TADJS, TADJI, VTGT, and VOCM pins.

VPOSx

GNDx

VREF

18kΩ

INTERNAL

VOLTAGE

Figure 39. VREF Interface Simplified Schematic

VDIFF OUTPUT INTERFACE

The ADL5920 contains a differential output stage (see Figure 40)

that converts the detector output voltages of VRMSF and VRMSR

to a differential voltage (VDIFF+ − VDIFF−) with two differential

amplifiers that each have a gain of one half. The differential gain

from VRMSF minus VRMSR to VDIFF+ − VDIFF− is therefore

equal to one, that is,

VDIFF+ − VDIFF− = VRMSF – VRMSR

(6)

The VOCM pin sets the output common-mode voltage of VDIFF.

Because the difference voltage can be as large as 2 V to 2.5 V

depending on directivity and frequency, VOCM must be high

enough (at least 1.25 V for |VRMSF − VRMSR| = 2.5 V) such

that the negative swinging output voltage is not limited at ground.

A voltage of midsupply (2.5 V) is optimal for VOCM. The

VOCM pin must be driven by a low impedance because the

current flowing in and out of this pin can be up to ±2 mA,

depending on the voltage applied to the VOCM pin and the

voltages present on VRMSF and VRMSR. VOCM can connect

directly to VREF. However, the connection must include a 1 kΩ

resistor to ground, as shown in Figure 38.

VRMSF

VRMSR

2kΩ

1kΩ

2kΩ

1kΩ

2kΩ

1kΩ

VDIFF–

VDIFF+

VOCM

Figure 40. Differential Output Stage

TEMPERATURE DRIFT COMPENSATION

The TADJI and TADJS pins provide the option to optimize the

temperature drift of the output voltages of ADL5920. The voltage

on TADJI provides compensation of intercept temperature drift

and the voltage on TADJS compensates for temperature drift of

the slope.

Table 5 shows the recommended voltages for VTADJI and VTADJS to

minimize temperature drift over the intended temperature range

(−40°C < TA < +85°C).

Table 5. Recommended VTADJI and VTADJS Values for Selected

Frequencies

Frequency (GHz)

VTADJI (V)

VTADJS (V)

0.01

0.1

0.2

0.8

The TADI and TADJS pins have a high input impedance and

can be conveniently driven from an external source or from an

attenuated value of VREF using a resistor divider.

SETTING VTGT

The voltage on the VTGT pin determines the settling point of

internal automatic level control (ALC) loops that are part of the

rms computation core. The recommended value for VTGT is

1 V, which 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 in Figure 38. Like the resistors chosen to set

the voltage on TADJI and TADJS, 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 voltages

on TADJI, TADJS and VTGT (which cannot exceed 4 mA).

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