ADL5519ACPZ-R7 datasheet(30/39 Pages) AD

30 / 39 page

ADL5519

Data Sheet

Rev. C | Page 30 of 39

AUTOMATIC GAIN CONTROL

Figure 67 shows how the ADL5519 can be connected to provide

automatic gain control to an amplifier or signal chain. Additional

pins are omitted for clarity. In this configuration, both detectors

are connected in measurement mode with appropriate filtering

being used on CLPA, CLPB to provide adequate filtering of the

demodulated log output. OUTA, however, is also connected to

the VLVL pin of the on-board difference amplifier. In addition,

the OUTP output of the difference amplifier drives a variable

gain element (either VVA or VGA) and is connected back to the

FBKA input via a capacitor so that it is operating as an

integrator.

Assume that OUTA is much bigger than OUTB. Because

OUTA also drives VLVL, this voltage is also present on the

noninverting input of the op amp driving OUTP. This results in

a current flow from OUTP through the integrating capacitor into

the FBKA input. This results in the voltage on OUTP increasing. If

the gain control transfer function of the VGA/VVA is positive, this

increases the gain, which in turn increases the input signal to

INHA. The output voltage on the integrator continues to increase

until the power on the two input channels is equal, resulting in

a signal chain gain of unity.

If a gain other than 0 dB is required, an attenuator can be used

in one of the RF paths, as shown in Figure 67. Alternatively,

power splitters or directional couplers of different coupling

factors can be used. Another convenient option is to apply a

voltage on VLVL other than OUTA. Refer to Equation 18 and

the Controller Mode section for more detail.

If the VGA/VVA has a negative gain control sense, the OUTN

output of the difference amplifier can be used with the integrating

capacitor tied back to FBKB. Alternatively, the inputs could be

swapped.

The choice of the integrating capacitor affects the response time

of the AGC loop. Small values give a faster response time but

may result in instability, whereas larger values reduce the response

time. Capacitors that are too large can also cause oscillations due

to the capacitive drive capability of the op amp. In automatic gain

control, the capacitors on CLPA and CLPB, which perform the

filtering of the demodulated log output, must still be used and

also affect loop response time.

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