AD8330ACP-R2 datasheet(18/36 Pages) AD

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AD8330

Rev. D | Page 18 of 36

VGAIN (V)

0.001

–40

–60

–20

100

10k

100k

0.01

0.1

Figure 52. Gain Control Function and Input Referred Noise Spectral Density

over a 120 dB Range

Noise, Input Capacity, and Dynamic Range

The design of variable gain amplifiers invariably incurs some

compromises in noise performance. However, the structure of

the AD8330 is such that this penalty is minimal. Examination

of the simplified schematic (Figure 47) shows that the input

voltage is converted to current-mode form by the two 500 Ω

resistors at Pin INHI and Pin INLO, whose combined Johnson

noise contributes 4.08 nV/√Hz. The total input noise at full

gain, when driven from a low impedance source, is typically

5 nV/√Hz after accounting for the voltage and current noise

contributions of the loop amplifier. For a 200 kHz channel

bandwidth, this amounts to 2.24 μV rms. The peak input at full

gain is ±6.4 mV, or +4.5 mV rms for a sine wave signal. The

signal-to-noise ratio at full input, that is, the dynamic range, for

these conditions is, thus, 20 log10(4.5 mV/2.24 μV), or 66 dB.

The value of VMAG has essentially no effect on the input referred

noise, but it is assumed to be 0.5 V.

Below midgain (25 dB, VDBS = 0.75 V), noise in the output

section dominates, and the total input noise is 11 nV/√Hz, or

4.9 μV rms in a 200 kHz bandwidth, and the peak input is

78 mV rms. Thus, the dynamic range increases to 84 dB.

At minimum gain, the input noise is up to 120 nV/√Hz, or

53.7 mV rms in the assumed 200 kHz bandwidth, while the

input capacity is ±2 V, or +1.414 V rms (sine), a dynamic range

of 88.4 dB. In calculating the dynamic range for other channel

bandwidths, Δf, subtract 10 log10(Δf/200 kHz) from these

illustrative values. A system operating with a 2 MHz bandwidth,

for example, exhibits dynamic range values that are uniformly

10 dB lower; used in an audio application with a 20 kHz band-

width, they are 10 dB higher.

Noise figure is a misleading metric for amplifiers that are not

impedance matched at their input, which is the special condi-

tion resulting only when both the voltage and current components

of a signal, that is, the signal power, are used at the input port.

When a source of impedance (RS) is terminated using a resistor

of RS (a condition that is not to be confused with matching),

only one of these components is used, either the current (as in

the AD8330) or the voltage. Then, even if the amplifier is

perfect, the noise figure cannot be better than 3 dB. The 1 kΩ

internal termination resistance would result in a minimum

noise figure of 3 dB for an RS of 1 kΩ if the amplifier were

noise-free. However, this is not the case, and the minimum

noise figure occurs at a slightly different value of RS (for an

example, see Figure 53 and the Using the AD8330 section).

RS (Ω)

10k

100

Figure 53. Noise Figure for Source Resistance of 50 Ω to 5 kΩ, at f = 10 MHz

(Lower) and 100 MHz (Simulation)

VDBS (V)

00.1 0.20.30.40.5 0.60.70.80.91.01.11.21.31.41.5

144

132

128

124

120

140

136

116

CONSTANT 1V rms

OUTPUT, BOTH CASES

X-AMP WITH 40dB

OF GAIN AND AN

INPUT NSD

OF nV/√Hz

Figure 54. Dynamic Range in dB/√Hz vs. VDBS (VMAG = 0.5 V, 1 V rms Output)

Compared with a Representative X-AMP (Simulation)

Dynamic Range

The ratio of peak output swing, expressed in rms terms, to the

output-referred noise spectral density provides a measure of

dynamic range, in dB/√Hz. For a certain class of variable gain

amplifiers, exemplified by the Analog Devices X-AMP® family,

the dynamic range is essentially independent of the gain setting

because the peak output swing and noise are both constant. The

AD8330 provides a different dynamic range profile because

there is no longer a constant relationship between these two

parameters. Figure 54 compares the dynamic range of the

AD8330 to a representative X-AMP.

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