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AD603 데이터시트(PDF) 11 Page - Analog Devices

부품명 AD603
상세설명  Low Noise, 90 MHz Variable Gain Amplifier
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AD603 데이터시트(HTML) 11 Page - Analog Devices

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AD603
Rev. H | Page 11 of 24
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 it may be set to any range in between using one
external resistor between Pin 5 and Pin 7. Somewhat higher
gain can be obtained by connecting the resistor from Pin 5 to
common, but the increase in output offset voltage limits the
maximum gain to about 60 dB. For any given range, the
bandwidth is independent of the voltage-controlled gain. This
system provides an underrange 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).
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. Because 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; therefore,
the distortion can be very low. Equally important, the small-
signal gain and phase response, and thus the pulse response, are
essentially independent of gain.
Figure 29 is a simplified schematic. The input attenuator is a
7-section R-2R ladder network, using untrimmed resistors of
nominally R = 62.5 Ω, which results in a characteristic resistance 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 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 (COMM)
must be connected directly to the input ground; significant
impedance in this connection reduces the gain accuracy.
The signal applied at the input of the ladder network is attenuated
by 6.02 dB by each section; therefore, 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,
indicated by the slider in Figure 29, thus providing continuous
attenuation from 0 dB to 42.14 dB. It helps 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 Pin 1 and Pin 2. The details of the gain control
interface are in the The Gain Control Interface section.
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 5).
NOISE PERFORMANCE
An important advantage of the X-AMP is its superior noise
performance. The nominal resistance seen at inner tap points is
41.7 Ω (one third of 125 Ω), 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 is apparent that it is essential to use a
low resistance in the ladder network to achieve the very low
specified noise level. The source impedance of the signal
forms a voltage divider with the 100 Ω input resistance of the
AD603. In some applications, the resulting attenuation may
be unacceptable, requiring the use of an external buffer or
preamplifier to match a high impedance source to the low
impedance AD603.
The noise at maximum gain (that is, at the 0 dB tap) depends on
whether the input is short-circuited or open-circuited. When
short-circuited, the minimum NSD of slightly over 1 nV/√Hz is
achieved. When open-circuited, the resistance of 100 Ω looking
into the first tap generates 1.29 nV/√Hz, so the noise increases
to 1.63 nV/√Hz. (This last calculation would be important if the
AD603 were preceded by, for example, a 900 Ω 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.
Apart from the small variations just discussed, the signal-to-
noise (SNR) 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.
Therefore, for the maximum undistorted output of 1 V rms and
a 1 MHz bandwidth, the output SNR would be 86.6 dB, that is,
20 log(1 V/46.5 μV).


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