LTC6412 Datasheet, PDF (18/24 Pages) Linear Technology – 800MHz, 31dB Range Analog-Controlled VGA

LTC6412
APPLICATIONS INFORMATION
6412 F01
6412 F02
Figure 2. Top Metal for DC1464A. Test Circuit A
Figure 1. Top Silkscreen for DC1464A. Test Circuit A
On the output side, the differential port admittance is very
close to 300Ω||1.5pF across all gain settings and applica-
tion frequencies. This output port circuit must provide a
path for DC output supply current as well as any balun,
matching, or filtering functions required by the application.
Thus, the design options for the output circuitry are more
varied. A brief list of the more common output circuits
is shown in Figure 9 along with a few design guidelines
to estimate component values. Final design simulations
should use the small-signal equivalent circuit model in
Figure 8 to properly account for loading effects of the
output terminals.
Figure 9a shows the simplest differential output configura-
tion employing two suitable inductors, L1 = L2, to pass
the DC supply current without loading the output nodes
at the application frequency. The PCB trace widths from
the output pins should be narrow in keeping with the high
impedance of these terminals; 8 to 10mil trace width on
1oz copper is a good choice. The 0.1μF capacitors serve to
DC block and decouple as needed. These capacitor values
are adequate down to a few MHz and can be scaled down
for higher application frequencies.
If bandpass filtering is needed at the VGA output of
Figure 9a, then L1 and L2 can be designed to resonate
with a shunt capacitor, C_O, at the frequency of interest,
ω =1/√C_O(L1 + L2). Alternately, L1 = L2 can be designed
to resonate with two separate capacitors, C1 = C2, so any
common mode noise is filtered as well.
Figure 9b shows a further variation of the tuned differential
output where the DC blocking capacitors are brought inside
the tank resonator to participate in the bandpass filter and
transform the VGA output impedance to a lower value.
Here too, the C_O capacitor can be split into two separate
shunt capacitors to ground, so any common mode noise
is filtered as well.
6412f
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