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LTC6421CUDC-20#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LTC6421-20
7
642120fb
PIN FUNCTIONS
+INA, –INA, –INB, +INB (Pins 1, 2, 5, 6): Differential
Inputs of A and B channel respectively.
V
–
(Pins 3, 4, 13, 14, 21): Negative Power Supply. All
four pins, as well as the exposed back, must be connected
to same voltage/ground.
ENABLEA, ENABLEB (Pins 9, 18): Logic inputs. If low,
the amplifi er is enabled. If high, the amplifi er is disabled
and placed in a low-power shutdown mode, making
the amplifi er outputs high impedance. These pins are
internally separate. These pins should not be left
fl oating.
V
+
A , V
+
B (Pins 15, 20, 7, 12 ): Positive Power Supply
(Normally tied to 3V or 3.3V). Supply pins of A and B channels
are internally separate. Bypass each pin with 1000pF and
0.1μF capacitors as close to the pins as possible.
–OUTA, +OUTA, –OUTB, +OUTB (Pins 16, 17, 11, 10):
Differential Outputs of channels A and B respectively.
V
OCMA
, V
OCMB
(Pins 19, 8): These pins set the output
common mode voltage for the respective channel. They
are internally separate. A 0.1μF external bypass capacitor
is recommended.
Exposed Pad (Pin 21): V
–
. The Exposed Pad must be
connected to same voltage/ground as pins 3, 4, 13, 14.
BLOCK DIAGRAM
642120 BD
7
V
+
B
V
+
B
V
+
A
–OUTA
–OUTB
8
V
OCMB
V
OCMA
+INA
–INA
V
–
V
–
R
OUT
12.5Ω
R
OUT
12.5Ω
R
F
1000Ω
R
G
100Ω
R
F
1000Ω
R
F
1000Ω
R
F
1000Ω
10
V
–
V
–
9
20
V
+
A
+OUTB
+OUTA
19 1718
ENABLEA
ENABLEB
R
OUT
12.5Ω
R
OUT
12.5Ω
R
G
100Ω
R
G
100Ω
R
G
100Ω
1
2
3
4
5
6
16
15
14
13
12
11
–INB
+INB
–
–+
+
+
–
–
+
LTC6421-20
8
642120fb
Circuit Operation
Each of the two channels of the LTC6421-20 is composed
of a fully differential amplifi er with on chip feedback and
output common mode voltage control circuitry. Differential
gain and input impedance are set by 100Ω/1000Ω
resistors in the feedback network. Small output resistors
of 12.5Ω improve the circuit stability over various load
conditions.
The LTC6421-20 is very fl exible in terms of I/O coupling.
It can be AC- or DC-coupled at the inputs, the outputs or
both. If the inputs are AC-coupled, the input common mode
voltage is automatically biased close to V
OCM
and thus
no external circuitry is needed for bias. The LTC6421-20
provides an output common mode voltage set by V
OCM
,
which allows driving an ADC directly without external
components such as a transformer or AC coupling capacitors.
The input signal can be either single-ended or differential
with only minor differences in distortion performance.
Figure 1. Input Termination for Differential 50Ω Input Impedance
Using Shunt Resistor
Figure 2. Input Termination for Differential 50Ω Input Impedance
Using a 1:4 Balun
APPLICATIONS INFORMATION
642120 F01
+IN
IN
+
OUT
–
IN
–
OUT
+
–IN
100Ω
66.5Ω
1000Ω
1/2 LTC6421-20
100Ω
25Ω
25Ω
V
IN
1000Ω
+
–
642120 F02
+IN
IN
+
OUT
–
IN
–
OUT
+
–IN
100Ω
1000Ω
1/2 LTC6421-20
100Ω
25Ω
TCM4-19
25Ω
V
IN
1000Ω
+
–
1:4
••
Input Impedance and Matching
The differential input impedance of the LTC6421-20 is
200Ω. If a 200Ω source impedance is unavailable, then
the differential inputs may need to be terminated to a
lower value impedance, e.g. 50Ω, in order to provide
an impedance match for the source. Several choices
are available. One approach is to use a differential shunt
resistor (Figure 1). Another approach is to employ a wide
band transformer (Figure 2). Both methods provide a
wide band impedance match. The termination resistor or
the transformer must be placed close to the input pins in
order to minimize the refl ection due to input mismatch.
Alternatively, one could apply a narrowband impedance
match at the inputs of the LTC6421-20 for frequency
selection and/or noise reduction.
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
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LTC6421CUDC-20#TRPBF

Mfr. #:
Buy LTC6421CUDC-20#TRPBF
Manufacturer:
Analog Devices Inc.
Description:
High Speed Operational Amplifiers 2x Matched 1.3GHz Diff Amps/ADC Drvrs
Lifecycle:
New from this manufacturer.
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