Datasheet

LTC6430-15
11
643015f
applicaTions inForMaTion
The LTC6430-15 is a highly linear fixed-gain amplifier
which is designed for ease of use. Both the input and
output are internally matched to 100Ω differential source
and load impedance from 20MHz to 1700MHz. Biasing and
temperature compensation are also handled internally to
deliver optimized performance. The designer need only
supply input/output blocking capacitors, RF chokes and
decoupling capacitors for the 5V supply. However, because
the device is capable of such wideband operation, a single
application circuit will probably not result in optimized
performance across the full frequency band.
Differential circuits minimize the common mode noise
and 2nd harmonic distortion issues that plague many
designs. Additionally, the LTC6430’s differential topol-
ogy matches well with the differential inputs of an ADC.
However, evaluation of these differential circuits is dif-
ficult, as high resolution, high frequency, differential test
equipment is lacking.
Our test circuit is designed for evaluation with standard
single ended 50Ω test equipment. Therefore, 1:2 balun
transformers have been added to the input and output to
transform the LTC6430-15’s 100Ω differential source/load
impedance to 50Ω single-ended impedance compatible
with most test equipment.
Other than the balun, the evaluation circuit requires a
minimum of external components. Input
and output DC-
blocking capacitors are
required as this device is internally
biased for optimal operation. A frequency appropriate
choke and de-coupling capacitors provide DC bias to the
RF ±OUT nodes. Only a single 5V supply is necessary to
either of the V
CC
pins on the device. Both V
CC
pins are
connected inside the package. Two V
CC
pins are provided
for the convenience of supply routing on the PCB. An op-
tional parallel 60pF, 350Ω input network has been added
to ensure low frequency stability.
The particular element values shown in Test Circuit A are
chosen for wide bandwidth operation. Depending on the
desired frequency, performance may be improved by
custom selection of these supporting components.
Choosing the Right RF Choke
Not all choke inductors are created equal. It is always im-
portant to select an inductor with low R
LOSS
as resistance
will drop the available voltage to the device. Also look for
an inductor with high self resonant frequency (SRF) as this
will limit the upper frequency where the choke is useful.
Above the SRF, the parasitic capacitance dominates and
the choke’s impedance will drop. For these reasons, wire-
wound inductors are preferred, while multilayer ceramic
chip inductors should be
avoided for an RF choke if pos-
sible. Since the LTC6430-15 is capable of such wideband
operation, a single choke value will not result in optimized
performance across its full frequency band. Table 1 lists
common frequency bands and suggested corresponding
inductor values.
Table 1. Target Frequency and Suggested Inductor Value
FREQUENCY
BAND (MHz)
INDUCTOR
VALUE
(nH)
SRF
(MHz)
MODEL
NUMBER MANUFACTURER
20 to 100 1500nH 100 0603LS Coilcraft
www.coilcraft.com
100 to 500 560nH 525 0603LS
500 t o 1000 100nH 1150 0603LS
1000 to 2000 51nH 1400 0603LS
DC-Blocking Capacitor
The role of a DC-blocking capacitor is straightforward:
block the path of DC current and allow a low series imped-
ance path for the AC signal. Lower frequencies require a
higher value of DC-blocking capacitance. Generally, 1000pF
to 10,000pF will suffice for operation down to 20MHz.
The LTC6430-15 linearity is insensitive to the choice of
blocking capacitor.
RF Bypass Capacitor
RF bypass capacitors act to shunt the AC signals to
ground with a low impedance path. They prevent the AC
signal from getting into the DC bias supply. It is best to
place the bypass capacitor as close as possible to the DC
supply pins of the amplifier. Any extra distance translates
into additional series inductance which lowers the effec-
tiveness of the bypass capacitor network. The suggested
bypass capacitor network consists of two capacitors:
a low value 1000pF capacitor to shunt high frequencies