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AD600JR 데이터시트(PDF) 4 Page - Analog Devices

부품명 AD600JR
상세설명  Dual, Low Noise, Wideband Variable Gain Amplifiers
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홈페이지  http://www.analog.com
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AD600JR 데이터시트(HTML) 4 Page - Analog Devices

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AD600/AD602
REV. A
–4–
THEORY OF OPERATION
The AD600 and AD602 have the same general design and fea-
tures. They comprise two fixed gain amplifiers, each preceded
by a voltage-controlled attenuator of 0 dB to 42.14 dB with in-
dependent control interfaces, each having a scaling factor of
32 dB per volt. The gain of each amplifier in the AD600 is laser
trimmed to 41.07 dB (X113), thus providing a control range of
–1.07 dB to 41.07 dB (0 dB to 40 dB with overlap), while the
AD602 amplifiers have a gain of 31.07 dB (X35.8) and provide
an overall gain of –11.07 dB to 31.07 dB (–10 dB to 30 dB with
overlap).
The advantage of this topology is that the amplifier can use
negative feedback to increase the accuracy of its gain; also, since
the amplifier never has to handle large signals at its input, the
distortion can be very low. A further feature of this approach is
that the small-signal gain and phase response, and thus the
pulse response, are essentially independent of gain.
The following discussion describes the AD600. Figure 1 is a
simplified schematic of one channel. The input attenuator is a
seven-section R-2R ladder network, using untrimmed resistors
of nominally R = 62.5
Ω, 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 ± 2%, which ensures accurate operation (gain and HP
corner frequency) when used in conjunction with external resis-
tors or capacitors.
GAIN CONTROL
INTERFACE
V
G
RF1
20
PRECISION PASSIVE
INPUT ATTENUATOR
FIXED GAIN
AMPLIFIER
RF2
2.24k
Ω (AD600)
694
Ω (AD602)
A1OP
A1CM
C1HI
C1LO
SCALING
REFERENCE
GATING
INTERFACE
GAT1
41.07dB (AD600)
31.07dB (AD602)
500
62.5
0dB
–6.02dB –12.04dB –18.06dB
–22.08dB –30.1dB –36.12dB –42.14dB
R – 2R LADDER NETWORK
A1HI
A1LO
Figure 1. Simplified Block Diagram of Single Channel of
the AD600 and AD602
The nominal maximum signal at input A1HI 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. Each attenuator is provided with a
separate signal “LO” connection, for use in rejecting common-
mode, the voltage between input and output grounds. Circuitry
is included to provide rejection of up to
±100 mV.
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, 6.02, 12.04, 18.06, 24.08, 30.1, 36.12
and 42.14 dB. A unique circuit technique is employed to inter-
polate between these tap points, indicated by the “slider” in Fig-
ure 1, providing continuous attenuation from 0 dB to 42.14 dB.
It will help, in understanding the AD600, to think in terms of a
mechanical means for moving this slider from left to right; in
fact, it is voltage controlled. The details of the control interface
are discussed later. Note that the gain is at all times exactly de-
termined, and a linear decibel relationship is automatically guar-
anteed between the gain and the control parameter which
determines the position of the slider. In practice, the gain devi-
ates from the ideal law, by about
±0.2 dB peak (see, for ex-
ample, Figure 6).
Note that the signal inputs are not fully differential: A1LO and
A1CM (for CH1) and A2LO and A2CM (for CH2) provide
separate access to the input and output grounds. This recog-
nizes the practical fact that even when using a ground plane,
small differences will arise in the voltages at these nodes. It is
important that A1LO and A2LO be connected directly to the
input ground(s); significant impedance in these connections will
reduce the gain accuracy. A1CM and A2CM should be con-
nected to the load ground(s).
Noise Performance
An important reason for using this approach is the superior
noise performance that can be achieved. The nominal resistance
seen at the inner tap points of the attenuator is 41.7
Ω (one
third of 125
Ω), which exhibits a Johnson noise spectral density
(NSD) of 0.84 nV/
√Hz (that is, √4kTR) at 27°C, which is a
large fraction of the total input noise. The first stage of the am-
plifier contributes a further 1.12 nV/
√Hz, for a total input noise
of 1.4 nV/
√Hz.
The noise at the 0 dB tap depends on whether the input is
short-circuited or open-circuited: when shorted, the minimum
NSD of 1.12 nV/
√Hz is achieved; when open, the resistance of
100
Ω at the first tap generates 1.29 nV/√Hz, so the noise in-
creases to a total of 1.71 nV/
√Hz. (This last calculation would
be important if the AD600 were preceded, for example, by a
900
Ω resistor to allow operation from inputs up to ±10 V rms.
However, in most cases the low impedance of the source will
limit the maximum noise resistance.)
It will be apparent from the foregoing that it is essential to use a
low resistance in the design of the ladder network to achieve low
noise. In some applications this may be inconvenient, requiring
the use of an external buffer or preamplifier. However, very few
amplifiers combine the needed low noise with low distortion at
maximum input levels, and the power consumption needed to
achieve this performance is fundamentally required to be quite
high (due to the need to maintain very low resistance values
while also coping with large inputs). On the other hand, there is
little value in providing a buffer with high input impedance,
since the usual reason for this—the minimization of loading of a
high resistance source—is not compatible with low noise.
Apart from the small variations just discussed, the signal-to-
noise (S/ N) ratio at the output is essentially independent of the
attenuator setting, since the maximum undistorted output is 1 V
rms and the NSD at the output of the AD600 is fixed at 113
times 1.4 nV/
√Hz, or 158 nV/√Hz. Thus, in a 1 MHz band-
width, the output S/N ratio would be 76 dB. The input NSD of
the AD600 and AD602 are the same, but because of the 10 dB
lower gain in the AD602’s fixed amplifier, its output S/N ratio is
10 dB better, or 86 dB in a 1 MHz bandwidth.


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