LTC6401-8
11
64018f
FREQUENCY (MHz)
0
IMD3 (dBc)
–40
–90
–80
–70
–60
–50
–100
–110
20 40 60 80 100 120 140 160 180
64018 F08
200
SINGLE-ENDED INPUT
F
S
= 122.8Msps
DRIVER V
OUT
= 2V
P-P
COMPOSITE
Figure 6. LTC6401-8 Modifi ed 165MHz for Bandpass Filtering
(Three External Capacitors, One External Inductor)
only a few components as shown in Figure 6. Three
39pF capacitors and a 16nH inductor create a bandpass
fi lter with 165MHz center frequency, –3dB frequencies at
138MHz and 200MHz.
Figure 7. Single-Ended Input to LTC6401-8 and LTC2208
Figure 8. IMD3 for the Combination of LTC6401-8 and LTC2208
Output Common Mode Adjustment
The LTC6401-8’s output common mode voltage is set
by the V
OCM
pin, which is a high impedance input. The
output common mode voltage is capable of tracking V
OCM
in a range from 1V to 1.6V. Bandwidth of V
OCM
control is
typically 14MHz, which is dominated by a low pass fi lter
connected to the V
OCM
pin and is aimed to reduce com-
mon mode noise generation at the outputs. The internal
common mode feedback loop has a –3dB bandwidth
around 400MHz, allowing fast rejection of any common
mode output voltage disturbance. The V
OCM
pin should
be tied to a DC bias voltage with a 0.1μF bypass capaci-
tor. When interfacing with 3V A/D converters such as the
LT22xx families, the V
OCM
pin can be connected to the
V
CM
pin of the ADC.
Driving A/D Converters
The LTC6401-8 has been specifi cally designed to interface
directly with high speed A/D converters. Figure 7 shows the
LTC6401-8 with single-ended input driving the LTC2208,
which is a 16-bit, 130Msps ADC. Two external 5Ω resistors
help eliminate potential resonance associated with bond
wires of either the ADC input or the driver output. V
OCM
of the LTC6401-8 is connected to V
CM
of the LTC2208 at
1.25V. Alternatively, an input single-ended signal can be
converted to differential signal via a balun and fed to the
input of the LTC6401-8.
Figure 8 summarizes the IMD3 performance of the whole
system as shown in Figure 7.
APPLICATIONS INFORMATION
64018 F06
IN+ OUT–
IN– OUT+
200Ω 12.5Ω
500Ω
LTC6401-8
LTC2208
200Ω
500Ω
12.5Ω
10Ω
10Ω
4.99Ω
4.99Ω
50Ω
50Ω
13
14
15
16
7
5
6
8
39pF
16nH
39pF
+OUT
+OUTF
–OUTF
–OUT
+IN
+IN
–IN
–IN
1.7pF
39pF
27.4Ω
59.0Ω
0.1μF
0.1μF
64018 F07
LTC6401-8
V
OCM
ENABLE
LTC2208
4.99Ω
0.1μF
LTC2208 130Msps
16-Bit ADC
1.25V
4.99Ω
8dB GAIN
+OUT
+OUTF
–OUTF
–OUT
AIN
–
AIN
+
V
CM
–IN
+IN
IF IN
