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LTC6417

6417f

applicaTions inForMaTion

The noise figure of the LTC6417 application circuit also

depends upon the input termination. For example, the

input 1:4 balun in Figure 2 improves noise figure by add-

ing 6dB of voltage gain at the inputs. A trade-off between

gainandnoiseisobviouswhenconstantnoisefigurecircle

and constant gain circle are plotted within the same input

Smith Chart. This technique can be used to determine

the optimal source impedance for a given gain and noise

requirement.

Output Match and Filter

The LTC6417 provides an output resistance of 1.5Ω at

each output. In most cases, the LTC6417 can be used

to drive an ADC without back termination but for testing

purposes, Figure 3 shows the LTC6417 driving a differ-

ential 50Ω load impedance using a 1:1 balun. If output

Figure 4. Output Termination for Differential 50Ω Load

Using a 1:1 Balun

Figure 3. LTC6417 with No Back Termination Driving a

50Ω Load Using a 1:1 Balun

matching for the 1:1 balun is desired, resistors of 23.7Ω

should be inserted in series with each LTC6417 output.

This is shown in Figure 4 where the LTC6417 is driving a

differential 100Ω load impedance.

As mentioned above, the LTC6417 can drive an ADC with-

out external output impedance matching, but improved

performance can usually be obtained with the addition of

a few components. Figure 5 shows a 6th order bandpass

filter with a 148MHz center frequency, –3dB points of

85MHz and 210MHz used for driving the LTC2209 16-bit

ADC. In the passband the filter has less than 1 dB ripple.

This higher order filter has a sharp roll-off outside its

passband, therefore it rejects noise and suppresses dis-

tortion components in its stopband. To double the filter

center frequency, halve the capacitor and inductor values,

and maintain resistor values; this also doubles the filter

bandwidth.

MABA-007159-000000

IN+

IN–

OUT–

6417 F03

OUT+

LTC6417

0.1µF

MABA-007159-000000

IN+

IN–

OUT–

6417 F04

OUT+

LTC6417

0.1µF

23.7

0.1µF

23.7

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