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AD603AQ 参数 Datasheet PDF下载

AD603AQ图片预览
型号: AD603AQ
PDF下载: 下载PDF文件 查看货源
内容描述: 低噪声, 90 MHz可变增益放大器 [Low Noise, 90 MHz Variable-Gain Amplifier]
分类和应用: 放大器
文件页数/大小: 14 页 / 222 K
品牌: ADI [ ADI ]
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AD603  
indicated by the “slider” in Figure 1, thus providing continuous  
attenuation from 0 dB to 42.14 dB. It will help, in understanding  
the AD603, to think in terms of a mechanical means for moving  
this slider from left to right; in fact, its “position” is controlled  
by the voltage between Pins 1 and 2. The details of the gain-  
control interface are discussed later.  
THEORY OF OPERATION  
The AD603 comprises a fixed-gain amplifier, preceded by a  
broadband passive attenuator of 0 dB to 42.14 dB, having a  
gain-control scaling factor of 40 dB per volt. The fixed gain is  
laser-trimmed in two ranges, to either 31.07 dB (×35.8) or  
50 dB (×358), or may be set to any range in between using one  
external resistor between Pins 5 and 7. Somewhat higher gain  
can be obtained by connecting the resistor from Pin 5 to com-  
mon, but the increase in output offset voltage limits the  
maximum gain to about 60 dB. For any given range, the band-  
width is independent of the voltage-controlled gain. This system  
provides an under- and overrange of 1.07 dB in all cases;  
for example, the overall gain is –11.07 dB to 31.07 dB in the  
maximum-bandwidth mode (Pin 5 and Pin 7 strapped).  
The gain is at all times very exactly determined, and a linear-in-  
dB relationship is automatically guaranteed by the exponential  
nature of the attenuation in the ladder network (the X-AMP  
principle). In practice, the gain deviates slightly from the ideal  
law, by about ±0.2 dB peak (see, for example, Figure 16).  
Noise Performance  
An important advantage of the X-AMP is its superior noise per-  
formance. The nominal resistance seen at inner tap points is  
41.7 (one third of 125 ), which exhibits a Johnson noise-  
spectral density (NSD) of 0.83 nV/Hz (that is, 4kTR) at 27°C,  
which is a large fraction of the total input noise. The first stage  
of the amplifier contributes a further 1 nV/Hz, for a total input  
noise of 1.3 nV/Hz. It will be apparent that it is essential to use  
a low resistance in the ladder network to achieve the very low  
specified noise level. The signal’s source impedance forms a  
voltage divider with the AD603’s 100 input resistance. In  
some applications, the resulting attenuation may be unaccept-  
able, requiring the use of an external buffer or preamplifier to  
match a high impedance source to the low impedance AD603.  
This X-AMP structure has many advantages over former methods  
of gain-control based on nonlinear elements. Most importantly,  
the fixed-gain amplifier can use negative feedback to increase its  
accuracy. Since large inputs are first attenuated, the amplifier  
input is always small. For example, to deliver a ±1 V output in  
the –1 dB/+41 dB mode (that is, using a fixed amplifier gain of  
41.07 dB) its input is only 8.84 mV; thus the distortion can be  
very low. Equally important, the small-signal gain and phase  
response, and thus the pulse response, are essentially indepen-  
dent of gain.  
Figure 1 is a simplified schematic. The input attenuator is a  
seven-section R-2R ladder network, using untrimmed resistors  
of nominally R = 62.5 , which results in a characteristic resis-  
tance of 125 Ω ± 20%. A shunt resistor is included at the input  
and laser trimmed to establish a more exact input resistance of  
100 Ω ± 3%, which ensures accurate operation (gain and HP  
corner frequency) when used in conjunction with external resistors  
or capacitors.  
The noise at maximum gain (that is, at the 0 dB tap) depends  
on whether the input is short-circuited or open-circuited: when  
shorted, the minimum NSD of slightly over 1 nV/Hz is achieved;  
when open, the resistance of 100 looking into the first tap  
generates 1.29 nV/Hz, so the noise increases to a total of  
1.63 nV/Hz. (This last calculation would be important if the  
AD603 were preceded by, for example, a 900 resistor to allow  
operation from inputs up to 10 V rms.) As the selected tap  
moves away from the input, the dependence of the noise on  
source impedance quickly diminishes.  
The nominal maximum signal at input VINP is 1 V rms (±1.4 V  
peak) when using the recommended ±5 V supplies, although  
operation to ±2 V peak is permissible with some increase in HF  
distortion and feedthrough. Pin 4 (SIGNAL COMMON) must  
be connected directly to the input ground; significant impedance in  
this connection will reduce the gain accuracy.  
Apart from the small variations just discussed, the signal-to-  
noise (S/N) ratio at the output is essentially independent of the  
attenuator setting. For example, on the –11 dB/+31 dB range  
the fixed gain of ×35.8 raises the output NSD to 46.5 nV/Hz.  
Thus, for the maximum undistorted output of 1 V rms and a  
1 MHz bandwidth, the output S/N ratio would be 86.6 dB, that  
is, 20 log (1 V/46.5 µV).  
The signal applied at the input of the ladder network is attenu-  
ated by 6.02 dB by each section; thus, the attenuation to each of  
the taps is progressively 0 dB, 6.02 dB, 12.04 dB, 18.06 dB,  
24.08 dB, 30.1 dB, 36.12 dB and 42.14 dB. A unique circuit  
technique is employed to interpolate between these tap-points,  
VPOS  
VNEG  
SCALING  
REFERENCE  
PRECISION PASSIVE  
INPUT ATTENUATOR  
FIXED GAIN  
AMPLIFIER  
GPOS  
GNEG  
V
OUT  
V
G
6.44k*  
GAIN  
CONTROL  
INTERFACE  
AD603  
FDBK  
694*  
20*  
0dB  
–6.02dB –12.04dB18.06dB –24.08dB –30.1dB –36.12dB –42.14dB  
VINP  
R
R
R
R
R
R
R
2R  
2R  
2R  
2R  
2R  
2R  
R
COMM  
R = 2R LADDER NETWORK  
*NORMAL VALUES  
Figure 1. Simplified Block Diagram of the AD603  
–4–  
REV. C