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AD8367ARUZ 参数 Datasheet PDF下载

AD8367ARUZ图片预览
型号: AD8367ARUZ
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品牌: ADI [ ADI ]
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AD8367  
VGA Operation  
AGC 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.  
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  
will be correctly measured and delivered at VOUT. When using  
lower supplies, the rms value of VOUT is unaffected (the set-  
point is determined by a band-gap reference) but the peak crest  
factor capacity is reduced.  
Figure 7 shows the basic connections. The capacitor CHP at Pin  
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  
when there are rapid changes in VGAIN. The control of offset is  
important even when the output is ac-coupled because of the poten-  
tial reduction of the upper and lower voltage range at this pin.  
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  
charges the external loop filter capacitor CAGC toward more positive  
voltages. Conversely, a reduction in VOUT below the set-point of  
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  
However, in many applications these components will be unnec-  
essary, since 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.  
TPC 18 shows representative response curves for the indicated  
component values.  
Signal Strength Indicator) output, and is scaled exactly as VGAIN  
,
that is, 20 mV/dB.  
V
P
V
P
C1  
R6  
R5  
C1  
1F  
R6  
R5  
1F  
4.74.7⍀  
C3  
4.74.7⍀  
C3  
C2  
0.1F  
1
2
3
4
5
6
7
ICOM  
ENBL  
INPT  
14  
13  
12  
11  
10  
9
ICOM  
HPFL  
VPSI  
C2  
0.1F  
1
2
3
4
5
6
7
ICOM  
ENBL  
INPT  
14  
13  
12  
11  
10  
9
0.1F  
ICOM  
HPFL  
VPSI  
C
, 10nF  
0.1F  
HP  
C
, 0.1F  
HP  
V
IN  
AD8367  
V
IN  
AD8367  
MODE  
GAIN  
DETO  
ICOM  
VPSO  
VOUT  
DECL  
MODE  
GAIN  
DETO  
ICOM  
VPSO  
VOUT  
DECL  
C4, 0.1F  
VOUT  
C4, 0.1F  
VOUT  
V
AGC  
V
GAIN  
C
AGC  
0.1F  
C5  
C5  
10nF  
10nF  
8
OCOM  
8
OCOM  
Figure 8. Basic Connections for AGC Operation  
Figure 7. Basic Connections for Voltage-Controlled  
Gain Mode  
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.  
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  
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/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.  
The AGC mode of operation requires that the correct gain direc-  
tion is chosen. Specifically, the gain must fall as VAGC increases to  
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.  
The dynamics of this loop are controlled by CAGC acting in  
conjunction with an on-chip equivalent resistance RAGC of 10 kΩ  
which form an effective time-constant TAGC RAGC CAGC. The  
loop thus operates as a single-pole system with a loop bandwidth  
of 1/(2TAGC). Because the gain control function is linear in  
decibels, this bandwidth is independent of absolute signal level.  
Figure 10 illustrates the loop dynamics for a 30 dB change in  
input signal level with CAGC 100 pF.  
–12–  
REV. 0