Datasheet

LTC6400-26
11
640026fa
The LTC6400-26 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 LTC6400-26 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 LTC6400-26 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 capacitor
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 capacitors and a 16nH inductor create a bandpass
fi lter with 165MHz center frequency, –3dB frequencies at
138MHz and 200MHz.
Output Common Mode Adjustment
The LTC6400-26’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 disturbance. The V
OCM
pin should be tied to a DC
bias voltage with a 0.1μF bypass capacitor. When interfac-
ing with 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 LTC6400-26 has been specifi cally designed to interface
directly with high speed A/D converters. Figure 7 shows the
LTC6400-26 with a 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 LTC6400-26 is connected to V
CM
of the LTC2208
at 1.25V. Alternatively, a single-ended input signal can be
Figure 5. LTC6400-26 Internal Filter Topology Modifi ed
for Low Filter Bandwidth (Three External Capacitors)
640026 F03
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
25Ω
R
T
150Ω
0.1μF
12.5Ω
500Ω
LTC6400-26
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
640026 F04
+OUT
+OUTP
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
25Ω 12.5Ω
500Ω
LTC6400-26
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
640026 F05
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
25Ω 12.5Ω
500Ω
LTC6400-26
25Ω
500Ω
12.5Ω
50Ω
50Ω
2.7pF
13
14
15
16
7
5
6
8
8.2pF
8.2pF
12pF
FILTERED OUTPUT
(87.5MHz)