參數(shù)資料
型號(hào)： AD8330
廠商： Analog Devices, Inc.
英文描述： Low Cost DC-150 MHz Variable Gain Amplifier
中文描述： 低成本DC - 150 MHz的可變?cè)鲆娣糯笃?/td>
文件頁(yè)數(shù)： 16/28頁(yè)
文件大?。?/td> 681K
代理商： AD8330
REV. A
–16–
AD8330
Amplitude/Phase Response
The ac response of the AD8330 is remarkably consistent not only
over the full 50 dB of its basic gain range, but also with changes
of gain due to alteration of V
MAG
, as demonstrated in Figure 8.
This is an overlay of two sets of results: first with a very low V
MAG
of 16 mV, which reduces the overall gain by 30 dB [20
(500 mV/16 mV)]; second, with V
MAG
= 5 V, which increases the
gain by 20 dB = 20
log10 (5 V/0.5 V).
log10
FREQUENCY – Hz
90
70
100k
0
G
30
10
–10
–30
–50
–350
50
1M
10M
100M
300M
G
–50
–100
–150
–200
–250
–300
G = +70dB
G = –20dB
100k
1M
10M
100M 300M
Figure 8. AC Performance over a 100 dB Gain
Range Obtained by Using Two Values of V
MAG
This 50 dB step change in gain produces the two sets of gain
curves, having a total gain span of 100 dB. It is apparent that
the amplitude and phase response are essentially independent of
the gain over this wide range, an aspect of the AD8330
’
s perfor-
mance potential unprecedented in any prior VGA.
It is unusual for an application to require such a wide range of
gains, of course; and as practical matter, the peak output voltage
for V
MAG
= 16 mV is reduced by the factor 16/500, compared to
its nominal value of
±
2 V, to only
±
64 mV. As already noted,
most applications of VGAs require that they operate in a mode
that is predominantly of either an IVGA or OVGA style, rather
than mixed modes.
With this limitation in mind, and simply in order to illustrate
the unusual possibilities afforded by the AD8330, it is noted
that with appropriate drive to V
DBS
and V
MAG
in tandem, the
gain span is a remarkable 120 dB, extending from
–
50 dB to
+70 dB, as shown in Figure 9 for operation at 1 MHz and
100 MHz. In this case, V
DBS
and V
MAG
are driven from a
common control voltage, V
GAIN
, which is varied from 1.2 mV
to 5 V, with 30% (1.5/5) of V
GAIN
applied to V
DBS
, and 100%
applied to V
MAG
.
The gain varies in a linear-in-dB manner with V
DBS
, while the
response from V
MAG
is linear-in-magnitude. Consequently, the
overall numerical gain is the product of these two functions:
GAIN
=
In rare cases where such a wide gain range might be of value, the
calibration will still be accurate and temperature stable.
×
V
V
GAIN
V
V
GAIN
.
/0 5
.
0 3 10
0 6
(7)
V
GAIN
– V
80
60
.001
1
N
H
G
–40
–60
100k
–20
0
20
40
10
100
1k
10k
.01
.1
1
10
Figure 9. 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 4) shows that the input voltage
is converted to current-mode form by the two 500
resistors at
pins INHI and 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
μ
V 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 log
10
(4.5 mV/2.24
μ
V),
or 66 dB. The value of V
MAG
has essentially no effect on the
input-referred noise, but we assume it to be 0.5 V.
Below midgain (25 dB, V
DBS
= 0.75 V), noise in the output
section dominates, and the total input noise is 11 nV/
√
Hz
, or
4.9
μ
V rms in a 200 kHz bandwidth, while the peak input is
78 mV rms. Thus, the dynamic range has increased 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 channels bandwidths,
f, subtract 10 log
10
(
f /200 kHz) from these illustrative values.
A system operating with a 2 MHz bandwidth, for example, will
exhibit dynamic range values that are uniformly 10 dB lower;
used in an audio application with a 20 kHz bandwidth, they will
be 10 dB higher.
Noise figure is a misleading metric for amplifiers that are not
impedance matched at their input, which is the special condition
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 R
S
is terminated using a resistor of R
S
(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 k
internal termina-
tion resistance would result in a minimum noise figure of 3 dB for
an R
S
of 1 k
if the amplifier were noise-free. However, this is
not the case and the minimum noise figure will occur at a slightly
different value of R
S
(see Figure 10 and Using the AD8330).
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