lt5554 Linear Technology Corporation, lt5554 Datasheet - Page 19

• Manufacturer Part Number: lt5554
• Description: Broadband Ultra Low Distortion 7-bit Digitally Controlled Vga
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
The differential outputs can also be converted to single-
ended 50Ω load using a center-tap transformer interface
shown in Figure 6 and Figure 16.
The internal 400Ω differential resistor (R
impedance and the maximum voltage gain (G
when outputs OUT
Figure 7 shows the Voltage and Power Gains as a func-
tion of R
collector amplifi er output including the internal resistor
R
The gain vs R
equations:
Where R
For wide band applications, the amplifi er bandwidth can
be extended by inductive peaking technique. The inductor
in series with the LT5554 outputs (OUT
a value up to some tens of nH depending on R
and board capacitance.
The current limiting will occur with R
case the instantaneous signal current at the output exceeds
I
ODC
O
G
G
= 400Ω.
V
P
= 45mA.
Figure 7. Maximum Voltage and Power Gain vs R
= 20log(G
= 10log(R
OUT
IN
= 50Ω and G
, which is the total output loading at the open
36
30
24
18
12
6
0
10
OUT
M
IN
• R
• G
+
relationship is given by the following
, OUT
OUT
M2
50
M
VOLTAGE GAIN
) in dB
• R
–
R
= 0.15 siemens at G
OUT
are open.
100
OUT
POWER GAIN
(Ω)
) in dB
OUT
400
+
O
<140Ω, in which
5554 F07
) sets the output
OUT
1000
MAX
–
) can have
MAX
OUT
) to 36dB
OUT
value
Voltage clipping will occur with R
the instantaneous voltage at each OUT
is either <2V or >8V.
The output OP1dB = 20dBm can be achieved when R
130Ω. In this case, the LT5554 outputs reach both current
and voltage limiting for maximum output power.
Gain Control Interface
The MODE pin selects the interface to the LT5554 gain
control pins.
The PGx and STROBE control inputs can be confi gured
to be either DC-coupled (for TTL interface) or AC-coupled
(for ECL or low-voltage CMOS interfaces).
In addition, the STROBE input can be driven such that the
LT5554 gain state is updated asynchronously (PGx latch
control in transparent-mode) or controlled by positive
STROBE transition (PGx latch control in strobed-mode).
There are several options available for coupling type and
latch control which are given in the following tables:
Table1. MODE Input Options
Table2. MODE Input Levels
Alternatively, the MODE pin can be left open (2V
internal).
(State)
MODE
OPEN
OPEN
HIGH
HIGH
LOW
(State)
MODE
OPEN
HIGH
LOW
AC Positive
0.6 to 2.2V
0.6 to 2.2V
Transition
DC >2.2V
DC >2.2V
STROBE
COUPLING TYPE
(Min Level)
V
CC
MODE
1.5V
– 0.4V
0
OUT
PGx
DC
DC
AC
AC
AC
>140Ω, in which case
+
and OUT
PGx (Latch Control)
(Max Level)
LT5554
MODE
Transparent
Transparent
0.6V
2.5V
V
Strobe
Strobe
Strobe
CC
–
outputs
19
OUT
5554f
=