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LTC6420CUDC-20#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P14
LTC6420-20
7
642020fb
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
640020 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
+
+
–
–
–
–
+
+
LTC6420-20
8
642020fb
APPLICATIONS INFORMATION
Figure 1. Input Termination for Differential 50 Input Impedance
Using Shunt Resistor
Figure 2. Input Termination for Differential 50 Input Impedance
Using a 1:4 Balun
Figure 3. Input Termination for Single-Ended 50 Input Impedance
642020 F03
+IN
IN
+
OUT
–
IN
–
OUT
+
–IN
100Ω
R
T
66.5Ω
0.1µF
1000Ω
1/2 LTC6420-20
100Ω
R
S
50Ω
R
S
//R
T
28.7Ω
V
IN
1000Ω
+
–
0.1µF
Circuit Operation
Each of the two channels of the LTC6420-20 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 100/1000
resistors in the feedback network. Small output resistors
of 12.5 improve the circuit stability over various load
conditions.
The LTC6420-20 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 LTC6420-20 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.
Input Impedance and Matching
The differential input impedance of the LTC6420-20 is
200. If a 200 source impedance is unavailable, then
the differential inputs may need to be terminated to a
lower value impedance, e.g. 50, in order to provide an
impedance match for the source. Several choices are avail-
able. 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 LTC6420-20 for frequency
selection and/or noise reduction.
Referring to Figure 3, LTC6420-20 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
640020 F01
+IN
IN
+
OUT
–
IN
–
OUT
+
–IN
100Ω
66.5Ω
1000Ω
1/2 LTC6420-20
100Ω
25Ω
25Ω
V
IN
1000Ω
+
–
640020 F02
+IN
IN
+
OUT
–
IN
–
OUT
+
–IN
100Ω
1000Ω
1/2 LTC6420-20
100Ω
25Ω
25Ω
V
IN
1000Ω
+
–
1:4
••
LTC6420-20
9
642020fb
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 50, it is found that the
single-ended input impedance is 202 and R
T
is 66.5
in order to match to a 50 source impedance.
The LTC6420-20 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
Output Impedance Match
The LTC6420-20 can drive an ADC directly without external
output impedance matching. Alternatively, the differential
output impedance of 25 can be matched to a higher
value impedance, e.g. 50, 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
1.1V 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
voltage with a 0.1µF bypass capacitor. When interfacing
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 LTC6420-20 has been specifi cally designed to interface
directly with high speed A/D converters. The back page of
this data sheet shows the LTC6420-20 driving an LTC2285,
which is a dual 14-bit, 125Msps ADC.
The V
OCM
pins of the LTC6420-20 are connected to the
V
CM
pins of the LTC2285, which provide a DC voltage
level of 1.5V. Both ICs are powered from the same 3V
supply voltage.
The inputs to the LTC6420-20 can be confi gured in various
ways, as described in the Input Impedance and Matching
section of this data sheet. The outputs of the LTC6420-20
may be connected directly to the analog inputs of an ADC,
or a simple lowpass or bandpass fi lter network may be
inserted to reduce out-of-band noise.
Test Circuits
Due to the fully-differential design of the LTC6420 and
its usefulness in applications with differing characteristic
specifi cations, two test circuits are used to generate the
information in this data sheet. Test Circuit A is DC1299, a
two-port demonstration circuit for the LTC6420/LTC6421
family. The schematic and silkscreen are shown in Figure 4.
This circuit includes input and output transformers (baluns)
for single-ended-to-differential conversion and impedance
transformation, allowing direct hook-up to a 2-port network
analyzer. There are also series resistors at the output to
avoid loading the amplifi er directly with a 50Ω load. Due
to the input and output transformers, the –3dB bandwidth
is reduced from 1.8GHz to approximately 1.3GHz.
Test Circuit B uses a 4-port network analyzer to measure
S-parameters and gain/phase response. This removes the
effects of the wideband baluns and associated circuitry,
for a true picture of the >1GHz S-parameters and AC
characteristics.
APPLICATIONS INFORMATION
P1-P3P4-P6P7-P9P10-P12P13-P14

LTC6420CUDC-20#TRPBF

Mfr. #:
Buy LTC6420CUDC-20#TRPBF
Manufacturer:
Analog Devices Inc.
Description:
High Speed Operational Amplifiers 2x Matched 1.8GHz Diff Amps/ADC Drvrs
Lifecycle:
New from this manufacturer.
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