Datasheet

LTC6421-20
9
642120fb
APPLICATIONS INFORMATION
Referring to Figure 3, LTC6421-20 can be easily confi gured
for single-ended input and differential output without a
balun. The signal is fed to one of the inputs through a
matching network while the other input is connected to the
same matching network and a source resistor. Because the
return ratios of the two feedback paths are equal, the two
outputs have the same gain and thus symmetrical swing. In
general, the single-ended input impedance and termination
resistor R
T
are determined by the combination of R
S
, R
G
and R
F
. For example, when R
S
is 50Ω, it is found that the
single-ended input impedance is 202Ω and R
T
is 66.5Ω
in order to match to a 50Ω source impedance.
The LTC6421-20 is unconditionally stable. However,
the overall differential gain is affected by both source
impedance and load impedance as follows:
A
V
=
V
OUT
V
IN
=
2000
R
S
+ 200
•
R
L
25 + R
L
Figure 3. Input Termination for Single-Ended 50Ω Input Impedance
642120 F03
+IN
IN
+
OUT
–
IN
–
OUT
+
–IN
100Ω
R
T
66.5Ω
0.1μF
1000Ω
LTC6421-20
100Ω
R
S
50Ω
R
S
//R
T
28.7Ω
V
IN
1000Ω
+
–
0.1μF
Output Impedance Match
The LTC6421-20 can drive an ADC directly without external
output impedance matching. Alternatively, the differential
output impedance of 25Ω can be matched to a higher
value impedance, e.g. 50Ω, by series resistors or an LC
network.
Output Common Mode Adjustment
The 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. The 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 common mode
noise generation at the outputs. The internal common
mode feedback loop has a –3dB bandwidth of 300MHz,
allowing fast rejection of any common mode output voltage
disturbance. The V
OCM
pin should be tied to a DC bias