LT1817 datasheet(12/16 Pages) LINER

12 / 16 page

LTC6400-14

640014fb

In general, the single-ended input impedance and

termination resistor RT are determined by the combination

of RS, RG and RF. For example, when RS is 50Ω, it is found

that the single-ended input impedance is 202

Ω and RT is

68.5

Ω in order to match to a 50Ω source impedance.

The LTC6400-14 is unconditionally stable under normal

bias conditions. However, the overall differential gain is

affected by both source impedance and load impedance

as shown in Figure 4:

OUT

1000

200 25

•

The noise performance of the LTC6400-14 also depends

upon the source impedance and termination. For example,

an input 1:4 balun transformer in Figure 2 improves SNR

by adding 6dB of voltage gain at the inputs. A trade-off

between gain and noise is obvious when constant noise

ﬁgure circle and constant gain circle are plotted within

the same input Smith Chart, based on which users can

choose the optimal source impedance for a given gain

and noise requirement.

Figure 4. Calculate Differential Gain

Output Match and Filter

The LTC6400-14 can drive an ADC directly without

external output impedance matching. Alternatively, the

differential output impedance of 25

Ω can be matched to

higher value impedance, e.g. 50

Ω, by series resistors or

an LC network.

The internal low pass ﬁlter outputs at +OUTF/–OUTF have

a –3dB bandwidth of 590MHz. External capacitors can

reduce the low pass ﬁlter bandwidth as shown in Figure 5.

A bandpass ﬁlter is easily implemented with only a few

componentsasshowninFigure6.Three39pFcapacitorsand

a 16nH inductor create a bandpass ﬁlter with 165MHz center

frequency, –3dB frequencies at 138MHz and 200MHz.

Output Common Mode Adjustment

The output common mode voltage is set by the VOCM pin,

which is a high impedance input. The output common mode

voltage is capable of tracking VOCM in a range from 1V to

1.6V. The bandwidth of VOCM control is typically 16MHz,

APPLICATIONS INFORMATION

Figure 3. Input Termination for Single-Ended 50

Ω Input

Impedance

Figure 5. LTC6400-14 Internal Filter Topology Modiﬁed for Low

Filter Bandwidth (Three External Capacitors)

640014 F03

+OUT

+OUTF

–OUTF

–OUT

+IN

IN+

OUT–

IN–

OUT+

+IN

–IN

100Ω

68.5Ω

0.1μF

12.5Ω

1000Ω

LTC6400-14

100Ω

50Ω

VIN

1000Ω

12.5Ω

50Ω

2.7pF

–

0.1μF

29Ω

640014 F04

+OUT

+OUTF

–OUTF

–OUT

+IN

IN+

OUT–

IN–

OUT+

+IN

–IN

100Ω

12.5Ω

500Ω

LTC6400-14

100Ω

1/2 RS

VIN

VOUT

500Ω

12.5Ω

50Ω

2.7pF

–

1/2 RL

640014 F05

IN+

OUT–

IN–

OUT+

100Ω

12.5Ω

500Ω

LTC6400-14

100Ω

500Ω

12.5Ω

50Ω

8.2pF

12pF

FILTERED OUTPUT

(87.5MHz)

+OUT

+OUTF

–OUTF

–OUT

+IN

–IN

2.7pF

Figure 6. LTC6400-14 Internal Filter Topology Modiﬁed

for Bandpass Filtering (Three External Capacitors, One

External Inductor)

640014 F06

IN+

OUT–

IN–

OUT+

100Ω

12.5Ω

500Ω

LTC6400-14

LTC2208

100Ω

500Ω

12.5Ω

10Ω

4.99Ω

50Ω

39pF

16nH

39pF

+OUT

+OUTF

–OUTF

–OUT

+IN

–IN

2.7pF

39pF

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