LT5554IUH#TRPBF Linear Technology, LT5554IUH#TRPBF Datasheet - Page 22

• Manufacturer Part Number: LT5554IUH#TRPBF
• Description: IC VGA DIG CONTROL 32-QFN
• Manufacturer: Linear Technology

Specifications of LT5554IUH#TRPBF

• Applications: Variable Gain (VGA)
• Number Of Circuits: 1
• -3db Bandwidth: 1GHz
• Current - Supply: 200mA
• Voltage - Supply, Single/dual (±): 4.75 V ~ 5.25 V
• Mounting Type: Surface Mount
• Package / Case: 32-WFQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

LT5554
APPLICATIONS INFORMATION
The LT5554 Noise and Linearity performance throughout
the 16dB gain range has an obvious discontinuity at every
4dB gain step. The noise fi gure is fairly constant from 0dB
(Maximum Gain) to –3.875dB attenuation when the gain is
decreased by lowering the amplifi er transconductance. And
then, the NF increases by 4dB when the input attenuator
is switched to –4dB attenuation while the amplifi er gain
is switched back to maximum transconductance. This
pattern repeats for each 4dB gain step change.
SECOND ORDER HARMONIC DISTORTION
Balanced differential inputs and outputs are important
for achieving excellent second order harmonic distortion
(HD2) of the LT5554. When confi gured in single-ended
input and output interfaces, therefore, the single-ended
to differential conversion at the input and differential to
single-ended conversion at the output will have signifi cant
impact on the HD2 performance.
Figure 14, for example, shows the desirable singe-ended
input and output confi guration using external transformers
for the single-ended to differential conversion and differ-
ential to single-ended conversion. To assure a good HD2
performance, R5 and R6 should also be matched to better
22
Figure 14. Recommended Single-Ended Input and Output
Confi guration, HD2 = –80dBc at 10dBm, 140MHz
ETC1-1-13
•
1:1
T1
0.1μF
•
C4
50Ω
DEC
C5
1μF
IN
IN
+
–
400Ω
R
O
V
CC
OUT
OUT
= 5V
–
+
LT5554
68.1Ω
68.1Ω
R5
R6
0.1μF
C3
V
CCO
= 5V
TC2-1T
•
•
T2
•
5554 F14
than 1% or use these two resistors with 1% component
tolerance. In this case, the HD2 can be as good as -80dBc
when the output power is 10dBm at 140MHz.
When the single-ended input is not converted into well
balanced inputs to LT5554, the HD2 performance will
be degraded. For instance, when the T1 transformer is
improperly rotated by 90 degrees as shown in Figure 15,
the imbalance of the differential input signals will result in
14dB degradation in HD2. It is also important to split the
differential R7 resistor into two single-ended R5 and R6
resistors at the outputs to reduce the imbalance of the T2
transformer. If not, 3dB degradation in HD2 performance
can also be observed.
The HD2 performance can be further improved by mounting
a capacitor from IN
tor from OUT
these capacitors cancels to some degree the T1 and T2
imbalance as shown in Figure 15.
For optimum HD2 performance, fully differential input and
output interfaces to the LT5554 part are recommended.
Figure 15. Not Recommended Single-Ended Input and Output
Confi guration, HD2 = –63dBc at 10dBm, 140MHz
ETC1-1-13
1:1
T1
47nF
47nF
0.1μF
C1
C2
–
C4
to ground. For narrow band applications,
+
50Ω
C5
1μF
DEC
IN
IN
to ground (a few pF) and a capaci-
+
–
400Ω
R
O
V
CC
OUT
OUT
= 5V
–
+
LT5554
R7
134Ω
0.1μF
C3
V
CCO
= 5V
TC2-1T
•
•
T2
•
5554f
5554 F15