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參數(shù)資料
| 型號(hào)： | AD8370-EVAL |
| 廠商： | Analog Devices, Inc. |
| 元件分類： | 運(yùn)動(dòng)控制電子 |
| 英文描述： | LF to 750 MHz Digitally Controlled VGA |
| 中文描述： | LF到750兆赫數(shù)字控制VGA |
| 文件頁(yè)數(shù)： | 15/28頁(yè) |
| 文件大小： | 1043K |
| 代理商： | AD8370-EVAL |
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AD8370
APPLICATIONS
BASIC CONNECTIONS
Figure 42 shows the minimum connections required for basic
operation of the AD8370. Supply voltages between 3.0 V and
5.5 V are allowed. The supply to the VCCO and VCCI pins
should be decoupled with at least one low inductance, surface-
mount ceramic capacitor of 0.1 μF placed as close as possible to
the device.
Rev. 0 | Page 15 of 28
AD8370
I
I
V
P
V
V
O
O
O
O
V
L
C
D
I
I
6
7
8
2
3
5
1
11
10
9
15
14
16
13
12
4
SERIAL CONTROL
INTERFACE
0.1
μ
F
1nF
0.1
μ
F
+V
S
(3.0V TO 5.0V)
1nF
1nF
1nF
1nF
BALANCED
LOAD
R
L
BALANCED
SOURCE
R
S
2
R
S
2
0
Figure 42. Basic Connections
The AD8370 is designed to be used in differential signal chains.
Differential signaling allows improved even-order harmonic
cancellation and better common-mode immunity than can be
achieved using a single-ended design. To fully exploit these
benefits, it is necessary to drive and load the device in a
balanced manner. This requires some care to ensure that the
common-mode impedance values presented to each set of
inputs and outputs are balanced. Driving the device with an
unbalanced source can degrade the common-mode rejection
ratio. Loading the device with an unbalanced load can cause
degradation to even-order harmonic distortion and premature
output compression. In general, optimum designs are fully
balanced, although the AD8370 still provides impressive
performance when used in an unbalanced environment.
The AD8370 is a fine adjustment, variable gain amplifier. The
gain control transfer function is linear in voltage gain. On a
decibel scale, this results in the logarithmic transfer functions
indicated in Figure 4. At the low end of the gain transfer
function, the slope is steep, providing a rather coarse control
function. At the high end of the gain control range, the decibel
step size decreases, allowing precise gain adjustment.
GAIN CODES
The AD8370’s two gain ranges are referred to as high gain (HG)
and low gain (LG). Within each range, there are 128 possible
gain codes. Therefore, the minimum gain in the low gain range
is given by the nomenclature LG0 whereas the maximum gain
in that range is given by LG127. The same is true for the high
gain range. Both LG0 and HG0 essentially turn off the variable
transconductance stage, and thus no output is available with
these codes. See Figure 24.
The theoretical linear voltage gain can be expressed with respect
to the gain code as
A
V
= GainCode Vernier(
1
+
(
PreGain
1)
MSB)
where:
A
V
is the linear voltage gain.
GainCode
is the digital gain control word minus the MSB (the
final 7 bits).
Vernier
= 0.055744 V/V
PreGain
= 7.079458 V/V
MSB
is the most significant bit of the 8-bit gain control word.
The MSB sets the device in either high gain mode (MSB = 1 ) or
low gain mode (MSB = 0).
For example, a gain control word of HG45 (or 10101101 binary)
results in a theoretical linear voltage gain of 17.76 Volts/Volt,
calculated as
45 × 0.055744 × (1 + (7.079458 1) × 1)
Increments or decrements in gain within either gain range are
simply a matter of operating on the
GainCode
. Six –dB gain
steps, which are equivalent to doubling or halving the linear
voltage gain, are accomplished by doubling or halving the
GainCode.
When power is first applied to the AD8370, the device is
programmed to code LG0 to avoid overdriving the circuitry
following it.
POWER-UP FEATURE
The power-up feature does not affect the
GainCode
and the gain
setting is preserved when in power-down mode. Powering
down the AD8370 (bringing PWUP low while power is still
applied to the device) does not erase or change the
GainCode
from the AD8370, and the same gain code is in place when the
device is powered up, that is, when PWUP is brought high
again. Removing power from the device all together and
reapplying, however, reprograms to LG0.
CHOOSING BETWEEN GAIN RANGES
There is some overlap between the two gain ranges; users can
choose which one is most appropriate for their needs. When
deciding which preamp to use, consider resolution, noise,
linearity, and spurious-free dynamic range (SFDR). The most
important points to keep in mind are
The low gain range has better gain resolution.
The high gain range has a better noise figure.
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