ADL5519ACPZ-R7 datasheet(26/39 Pages) AD

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ADL5519

Data Sheet

Rev. C | Page 26 of 39

ALTERING THE SLOPE

As discussed in the Setpoint Interface—VSTA, VSTB section,

the slope can readily be increased by scaling the amount of

output voltage at OUTA, OUTB that is fed back to the setpoint

interface, VSTA, VSTB. When the full signal from OUTA,

OUTB is applied to VSTA, VSTB, the slope has a nominal value of

−22 mV/dB. This value can be increased by including a voltage

divider between the OUTA, OUTB and VSTA, VSTB pins, as

shown in Figure 64.

ADL5519

OUTA, OUTB

VOUT

VSTA, VSTB

Figure 64. External Network to Raise Slope

The approximate input resistance for VSTA, VSTB is 40 kΩ.

Scaling resistor values should be carefully selected to minimize

errors. Keep in mind that these resistors also load the output

pins and reduce the load-driving capabilities.

Equation 11 can be used to calculate the resistor values.

















R2'

(11)

where:

SD is the desired slope, expressed in millivolts/decibels (mV/dB).

R2' is the value of R2 in parallel with 40 kΩ.

For example, using R1 = 1.65 kΩ and R2 = 1.69 kΩ (R2' =

1.62 kΩ), the nominal slope is increased to −44 mV/dB.

When the slope is increased, the loop capacitor, CLPA, CLPB,

may need to be raised to ensure stability and to preserve a chosen

averaging time. The slope can be lowered by placing a voltage

divider after the output pin, following standard practices.

CHANNEL ISOLATION

Isolation must be considered when using both channels of the

ADL5519 at the same time. The two isolation requirements that

should be considered are the isolation from one RF channel input

to the other RF channel input and the isolation from one RF

channel input to the other channel output. When using both

channels of the ADL5519, care should be taken in the layout to

isolate the RF inputs, INHA and INHB, from each other. Coupling

on the PC board affects both types of isolation.

In most applications, the designer has the ability to adjust the

power going into the ADL5519 through the use of temperature-

stable couplers and accurate temperature-stable attenuators of

different values. When isolation is a concern, it is useful to

adjust the input power so the lowest expected detectable power

is not far from the lowest detectable power of the ADL5519 at

the frequency of operation.

The lowest detectable power point of the ADL5519 has little

variation from part to part. This equalizes the signals on both

channels at their lowest possible power level, which reduces the

overall isolation requirements and possibly adds attenuators to the

RF inputs of the device, reducing the RF channel input isolation

requirements.

Measuring the RF channel input to the other RF channel input

isolation is straightforward and is done by measuring the loss

on a network analyzer from one input to the other input. The

outcome is shown in the Specifications section of the data sheet.

Note that adding an attenuator in series with the RF signal

increases the channel input-to-input isolation by the value of

the attenuator.

The isolation between one RF channel input and the other channel

output is a little more complicated. The easiest approach

(which was used in this datasheet) to measuring this isolation is

to have one channel set to the lowest power level it is expected

to have on its input (approximately −50 dBm in this data sheet)

and then increasing the power level on the other channel input

until the output of the low power channel changes by 22 mV.

Because −50 dBm is in the linear region of the detector, 22 mV

equates to a 1 dB change in the output.

If the inputs to both RF channels are at the same frequency, the

isolation also depends on the phase shift between the RF signals

put into the ADL5519. This relationship can be demonstrated by

placing a high power signal on one RF channel input and a low

power signal slightly offset in frequency to the other RF channel.

If the output of the low power channel is observed with an

oscilloscope, it has a ripple that looks similar to a full-wave

rectified sine wave with a frequency equal to the frequency

difference between the two channels, that is, a beat tone. The

magnitude of the ripple reflects the isolation at a specific phase

offset (note that two signals of slightly different frequencies act

like two signals with a constantly changing phase), and the

frequency of that ripple is directly related to the frequency offset.

The data shown in the Specifications section assumes worst-case

amplitude and phase offset. If the RF signals on Channel A and

Channel B are at significantly different frequencies, the input-

to-output isolation increases, depending on the capacitors placed

on CLPA, CLPB and the frequency offset of the two signals, due

to the response roll-off within the ADL5519.

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