REV. 0

AD8367

–12–

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, tem-

perature- and supply-stable linear-in-dB scaling will be 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-band-

width systems.

HPFL may 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 will not be able to track the variations that occur

important even when the output is ac-coupled because of the poten-

tial reduction of the upper and lower voltage range at this pin.

However, in many applications these components will be unnec-

essary, since an internal network provides a default high-pass

1 nF, the modified corner

is at ~10 kHz; it scales downward with increasing capacitance.

TPC 18 shows representative response curves for the indicated

component values.

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13

12

11

10

9

8

1

2

3

4

5

6

7

ENBL

INPT

MODE

GAIN

DETO

ICOM

ICOM

HPFL

VPSI

VPSO

VOUT

DECL

OCOM

ICOM

AD8367

VOUT

C5

10nF

C4, 0.1 F

R5

4.7

R6

4.7

C1

1 F

C2

0.1 F

C3

0.1 F

Figure 7. Basic Connections for Voltage-Controlled

Gain Mode

Modulated Gain Mode

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

level. It should be kept in mind, however, that the gain is a

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/ s. During gain slewing from close to

minimum to maximum gain (or vice versa) the internal interpo-

lation processes in an X-AMP-based VGA rapidly scan the full

range of gain values. The gain and offset ripple associated with

this process may 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 may be used as an AGC amplifier as shown in

Figure 8. 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 “set-point”

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 carry-

ing complex modulation, will invariably be somewhat greater.

However, for all waveforms having a crest factor of less than 5,

and when using a supply voltage of 4.5 V to 5.5 V, the rms value

point is determined by a band-gap reference) but the peak crest

factor capacity is reduced.

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 zero. If the input to the

amplifier increases the mean of this current also increases, and

354 mV rms causes this voltage to fall toward ground. The capacitor

voltage is the AGC bias; this may be used as an RSSI (Received

that is, 20 mV/dB.

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12

11

10

9

8

1

2

3

4

5

6

7

ENBL

INPT

MODE

GAIN

DETO

ICOM

ICOM

HPFL

VPSI

VPSO

VOUT

DECL

OCOM

ICOM

AD8367

VOUT

C5

10nF

C4, 0.1 F

R5

4.7

R6

4.7

C1

1 F

C2

0.1 F

C3

0.1 F

0.1 F

Figure 8. 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 may be

converted to an absolute power measurement for any given

source impedance. The AD8367 may 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 direc-

restore the needed balance against the set-point. Therefore, the

MODE pin must be pulled low. This accurate leveling function is

shown in Figure 9, where the rms output is held to within 0.1 dB

of the set point for >35 dB range of input levels.

Ω

loop thus operates as a single-pole system with a loop bandwidth

of 1/(2

decibels, this bandwidth is independent of absolute signal level.

Figure 10 illustrates the loop dynamics for a 30 dB change in

100 pF.