Datasheet
LTC6417
19
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
gain and noise is obvious when constant noise figure circle
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.
•
•
T2
MABA-007159-000000
IN
+
IN
–
8
9
OUT
–
6417 F03
OUT
+
19
18
LTC6417
0.1µF
0Ω
50Ω
0.1µF
0Ω
•
•
T2
MABA-007159-000000
IN
+
IN
–
8
9
OUT
–
6417 F04
OUT
+
18
19
LTC6417
0.1µF
23.7Ω
50Ω
0.1µF
23.7Ω
4
5
3
1