AD8367 datasheet(15/24 Pages) AD

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AD8367

Rev. A | Page 15 of 24

VGA OPERATION

The AD8367 is a general-purpose VGA suitable for use in a

wide variety of applications where voltage control of gain is

needed. While having a 500 MHz bandwidth, its use is not

limited to high frequency signal processing. Its accurate,

temperature- and supply-stable linear-in-dB scaling is

valuable wherever it is important to have a more dependable

response to the control voltage than is usually offered by VGAs

of this sort. For example, there is no preclusion to its use in

speech-bandwidth systems.

Figure 33 shows the basic connections. The CHP capacitor at

Pin HPFL can be used to alter the high-pass corner frequency of

the signal path and is associated with the offset control loop that

eliminates the inherent variation in the internal dc balance of the

signal path as the gain is varied (offset ripple). This frequency

should be chosen to be about a decade below the lowest frequency

component of the signal. If made much lower than necessary, the

offset loop is not able to track the variations that occur when there

are rapid changes in VGAIN. The control of offset is important even

when the output is ac-coupled because of the potential reduction of

the upper and lower voltage range at this pin.

However, in many applications these components are

unnecessary because an internal network provides a default

high-pass corner of about 500 kHz. For CHP = 1 nF, the

modified corner is at ~10 kHz; it scales downward with

increasing capacitance. Figure 20 shows representative

response curves for the indicated component values.

AD8367

ICOM

ICOM 14

ENBL

HPFL 13

INPT

VPSI 12

MODE

VPSO 11

GAIN

VOUT 10

DETO

DECL 9

ICOM

OCOM 8

VGAIN

VOUT

VIN

10nF

μF

0.1

μF

CHP

10nF

RHP

100

4.7

0.1

μF

0.1

μF

Figure 33. Basic Connections for Voltage Controlled Gain Mode

MODULATED GAIN MODE

The AD8367 can be used as a means of modulating the signal

level. Keep in mind, however, that the gain is a nonlinear

(exponential) function of VGAIN; thus, it is not suitable for

normal amplitude-modulation functions. The small signal

bandwidth of the gain interface is ~5 MHz, and the slew rate

is of the order of ±500 dB/μs. During gain slewing from close

to minimum to maximum gain (or vice versa), the internal

interpolation processes in an X-AMP-based VGA rapidly

scan the full range of gain values. The gain and offset ripple

associated with this process can cause transient disturbances

in the output. Therefore, it is inadvisable to use high amplitude

pulse drives with rise and fall times below 200 ns.

AGC OPERATION

The AD8367 can be used as an AGC amplifier, as shown in

Figure 34. For this application, the accurate internal, square-law

detector is employed. The output of this detector is a current

that varies in polarity, depending on whether the rms value of

the output is greater or less than its internally-determined

setpoint of 354 mV rms. This is 1 V p-p for sine-wave signals,

but the peak amplitude for other signals, such as Gaussian

noise, or those carrying complex modulation, is invariably

somewhat greater. However, for all waveforms having a crest

factor of <5, and when using a supply voltage of 4.5 V to 5.5 V,

the rms value is correctly measured and delivered at VOUT.

When using lower supplies, the rms value of VOUT is unaffected

(the setpoint is determined by a band gap reference), but the

peak crest factor capacity is reduced.

The gain pin is connected to the base of a transistor internally

and thus requires only 1 μA of current drive. The output of the

detector is delivered to Pin DETO. The detector can source up

to 60 μA and can sink up to 11 μA. For a sine-wave output

signal, and under conditions where the AGC loop is settled, the

detector output also takes the form of a sine-wave, but at twice

the frequency and having a mean value of 0. If the input to the

amplifier increases, the mean of this current also increases and

charges the external loop filter capacitor, CAGC, toward more

positive voltages. Conversely, a reduction in VOUT below the

setpoint of 354 mV rms causes this voltage to fall toward

ground. The capacitor voltage is the AGC bias; this can be

used as a received signal strength indicator (RSSI) output

and is scaled exactly as VGAIN, that is, 20 mV/dB.

AD8367

ICOM

ICOM 14

ENBL

HPFL 13

INPT

VPSI 12

MODE

VPSO 11

GAIN

VOUT 10

DETO

DECL

ICOM

OCOM 8

VAGC

VOUT

VIN

10nF

CAGC

0.1

μF

0.1

μF

CHP

10nF

RHP

100

4.7

0.1

μF

0.1

μF

Figure 34. Basic Connections for AGC Operation

A valuable feature of using a square law detector is that the

RSSI voltage is a true reflection of signal power and can be

converted to an absolute power measurement for any given

source impedance. The AD8367 can thus be employed as a

true-power meter, or decibel-reading ac voltmeter, as distinct

from its basic amplifier function.

The AGC mode of operation requires that the correct gain

direction is chosen. Specifically, the gain must fall as VAGC

increases to restore the needed balance against the setpoint.

Therefore, the MODE pin must be pulled low. This accurate

leveling function is shown in Figure 35, where the rms output is

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