Datasheet

LTC6401-26
11
640126f
The LTC6401-26 is unconditionally stable, i.e. differential
stability factor Kf>1 and stability measure B1>0. However,
the overall differential gain is affected by both source
impedance and load impedance as shown in Figure 4:
A
V
VR
R
R
V
OUT
IN S
L
L
==
++
1000
50 25
•
The noise performance of the LTC6401-26 also depends
upon the source impedance and termination. A trade-off
between gain and noise is obvious when constant noise
fi gure 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.
Output Impedance Match and Filter
The LTC6401-26 can drive an ADC directly without external
output impedance matching. Alternatively, the differential
output impedance of 25Ω can be made larger, e.g. 50Ω,
by series resistors or LC network.
Figure 4. Calculate Differential Gain
APPLICATIONS INFORMATION
Figure 3. Input Termination for Single-Ended 50Ω Input
Impedance
The internal low pass fi lter outputs at +OUTF/–OUTF have
a –3dB bandwidth of 590MHz. External capacitors can
reduce the low pass fi lter bandwidth as shown in Figure
5. A bandpass fi lter is easily implemented with only a
few components as shown in Figure 6. Three 39pF ca-
pacitors and 16nH inductor create a bandpass fi lter with
165MHz center frequency, –3dB frequencies at 138MHz
and 200MHz.
Output Common Mode Adjustment
The LTC6401-26’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 15MHz, 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 of
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 capacitor. When
interfacing with 3V A/D converters such as the LTC22xx
families, the V
OCM
pin can be connected to the V
CM
pin
of the ADC.
Driving A/D Converters
The LTC6401-26 has been specifi cally designed to interface
directly with high speed A/D converters. Figure 7 shows the
LTC6401-26 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
Figure 5. LTC6401-26 Internal Filter Topology Modifi ed for Low
Filter Bandwidth (Three External Capacitors)
640126 F03
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
25Ω
R
T
150Ω
0.1μF
0.1μF
12.5Ω
500Ω
LTC6401-26
25Ω
R
S
50Ω
V
IN
500Ω
12.5Ω
50Ω
50Ω
2.7pF
13
14
15
16
7
5
6
8
+
–
0.1μF
37.4Ω
640126 F04
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
25Ω 12.5Ω
500Ω
LTC6401-26
25Ω
1/2 R
S
1/2 R
S
V
IN
V
OUT
500Ω
12.5Ω
50Ω
50Ω
2.7pF
13
14
15
16
7
5
6
8
+
–
1/2 R
L
1/2 R
L
640126 F05
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
25Ω 12.5Ω
500Ω
LTC6401-26
25Ω
500Ω
12.5Ω
50Ω
50Ω
2.7pF
13
14
15
16
7
5
6
8
8pF
8pF
12pF
FILTERED OUTPUT
(87.5MHz)