Datasheet

LTC6412
18
6412fa
APPLICATIONS INFORMATION
6412 F02
Figure 2. Top Metal for DC1464A. Test Circuit A
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 fi lter 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 fi ltered as well.
Figure 9c shows a fl ux transformer used to achieve a
50Ω single-ended output. The fl ux transformer does
not provide the large bandwidth typical of the output
transmission-line transformer shown in Figure 3, but it
usually performs well over smaller bandwidths, especially
when tuned with shunt capacitors (not shown). The fl ux
transformer design eliminates DC blocking capacitors and
is attractive in rugged applications where the amplifi er
output is subjected to ESD events and other forms of
transient electrical overstress that do not pass through a
typical RF fl ux transformer such as the MABAES0061.
Figure 9d shows a discrete LC balun suitable for bandwidths
of approximately 15% to 30%. Larger bandwidths are
diffi cult to achieve with the number of components shown,
and smaller bandwidths are often limited by component
tolerance effects. Despite these limitations, the discrete
LC balun can be a cost effective output circuit solution.
At resonance, the tuned circuit produces an impedance
transformation along with the differential-to-single-ended
conversion.
DC-Coupled Operation
The LTC6412 is intended for AC-coupled operation. The
translation between the fi xed input DC common mode
voltage and higher open-collector output DC bias point
makes it impractical to use in DC-coupled applications.
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 fi ltering 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 fi ltered as well.