AD603
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 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. 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 independent of gain.
seven-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 will reduce the gain accuracy.
The signal applied at the input of the ladder network is
attenuated 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, indicated by the slider in Figure 29, 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 Pin 1 and Pin 2.
The details of the gain control interface are discussed later.
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 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 Ω input resistance. 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
shorted, the minimum NSD of slightly over 1 nV/√Hz is
achieved; when open, the resistance of 100 Ω 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 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 (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).
Rev. G | Page 11 of 20
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