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參數(shù)資料
| 型號： | LT5514 |
| 廠商： | Linear Technology Corporation |
| 元件分類： | ADC |
| 英文描述： | Ultralow Distortion IF Amplifier/ADC Driver with Digitally Controlled Gain |
| 中文描述： | 超低失真IF放大器/ ADC的數(shù)字控制增益驅動 |
| 文件頁數(shù)： | 12/20頁 |
| 文件大?。?/td> | 275K |
| 代理商： | LT5514 |
12
LT5514
5514f
APPLICATIOU
Circuit Operation
The LT5514 is a high linearity amplifier with high imped-
ance output (Figure 1). It consists of the following
sections:
An input variable attenuator “gain-control” block with
100
input impedance
Two parallel, differential transconductance amplifiers,
each with independent enable inputs
An internal bias block with internal voltage regulator
A gain control logic block
The LT5514 amplifier provides amplification with very low
distortion using a linearized open-loop architecture. In
contrast with high linearity amplifiers employing negative
feedback, the LT5514 offers:
Stable operation for any practical load
A capacitive output reactance (not inductive) that pro-
vides peaking free AC response to capacitive loads
Exceptional reverse isolation of –100dB at 50MHz and
–78dB at 300MHz (package and board leakage limited)
The LT5514 is a transconductance amplifier and its opera-
tion can be understood conceptually as consisting of two
steps: First, the input signal voltage is converted to an
output current. The intermodulation distortion (in dBc) of
the LT5514 output current is determined by the input
signal level, and is almost independent of the output load
conditions. Thus, the LT5514’s input IP3 is also nearly
independent of the output load.
Next, the external output load (R
OUT
) converts the output
current to output voltage (or power). The LT5514’s volt-
age and power gain both increase with increasing R
OUT
.
Accordingly, the output power and output IP3 also im-
prove with increasing R
OUT
. The actual output linearity
performance in the application will thus be set by the
choice of output load, as well as by the output network.
W
U
U
Maximum Gain Calculation
The maximum power gain (with the 0dB attenuation step)
is:
G
PWR
(dB) = 10 log(g
m2
R
IN
R
OUT
)
where:
g
m
is the LT5514 transconductance = 0.3S in Standard
mode (0.15S in Low Power mode).
R
IN
is the LT5514 differential input impedance
108
in Standard mode (122
in Low Power mode). Input
impedance matching is assumed.
R
OUT
is the external differential output impedance as
seen by the LT5514’s differential outputs. R
OUT
should
be distinguished from the actual load impedance, R
LOAD
,
which will typically be coupled to the LT5514 output by
an impedance transformation network.
The power gain as a function of R
OUT
is plotted in Figure2.
The ideal curves are straight lines. The curved lines
indicate the roll-off due to the finite (noninfinite) output
resistance of the LT5514.
R
OUT
(
)
20
25
G
30
35
40
45
100
1000 2000
5514 F02
20
15
5
0
10
STANDARD MODE
LOW POWER MODE
STD WITH R
O
LP WITH R
O
Figure 2. Power Gain as a Function of R
OUT
The actual available output power (as well as power gain
and OIP3) will be reduced by losses in the output interface,
consisting of:
The insertion loss of the output impedance transforma-
tion network (for example the transformer insertion
loss in Figure 6)
About –3dB loss if a matching resistor (R
MATCH
in
Figure 6) is used to provide output load impedance
back-matching (for example when driving transmis-
sion lines)
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