LTC6412CUFPBF datasheet(18/24 Pages) LINER

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LTC6412

6412f

APPLICATIONS INFORMATION

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, CO, at the frequency of interest,

ω =1/√CO(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 CO capacitor can be split into two separate

shunt capacitors to ground, so any common mode noise

is ﬁltered as well.

6412 F01

6412 F02

Figure 1. Top Silkscreen for DC1464A. Test Circuit A

Figure 2. Top Metal for DC1464A. Test Circuit A

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