DEMO CIRCUIT 1287
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT
GUIDE
QUICK START
1287A
LTC6416
2 GHz Low Noise Differential 16-Bit ADC Buffer
DESCRIPTION
Demonstration circuit 1287 is a 2 GHz Low Noise Differ-
ential 16-Bit ADC Buffer featuring the LTC6416. It incor-
porates a variety of passive components to support
configurations for varied applications. Both single-
ended and differential configurations are supported
at the inputs and at the outputs.
The LTC6416 is a high-speed differential buffer am-
plifier with superior distortion and noise perform-
ance. It is intended for driving high speed 14 and 16
bit ADCs. It is perfect in applications such as de-
manding communications transceivers, cellular base
stations, and other high-speed signal chain applica-
tions.
Notable features of the LTC6416 include high input
impedance, adjustable output voltage limiting and
simple output common-mode adjustment. The
LTC6416’s high input impedance allows it to be used
in 50 systems with 1:4 and 1:8 input transformers
for additional system gain. The VCLLO and VCLHI
pins set lower and upper limits on the output voltages
seen at each differential output pin. This feature helps
protect the ADC inputs from undervoltage and over-
voltage waveforms. The output common-mode volt-
age of the LTC6416 is determined by the voltage ap-
plied to the VCM pin. It is internally buffered then
drives the LTC6416’s differential inputs through 2k
series resistors. For AC-Coupled operation, no exter-
nal bias network is needed. DC-coupled inputs applied
to the LTC6416 can easily override the 2k resistance
of the internal bias network.
Design files for this cir-
cuit board are available. Call the LTC factory.
L
, LTC, LTM, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered
trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, Easy
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may be trademarks of the companies that manufacture the products.
LTC6416 2 GHZ LOW NOISE DIFFERENTIAL 16-BIT ADC BUFFER
QUICK START PROCEDURE
Table 1 shows the function of each SMA connector on
the board.
Table 1: DC1287 SMA Connector Descriptions
Single-Ended/Differential Input.
By default, this is
J1 (IN+)
configured as a single-ended input.
Use this con-
nector to supply an input to the DC1287. Drive
from a 50ohm signal source, no external termina-
tion necessary.
Differential Input.
Not connected by default.
Ca-
J2 (IN-)
pacitor C10 can be installed and C12 removed to
drive the DC1287 differentially.
Single-Ended/Differential Output.
By default, this is
J3 (OUT+)
configured as a single-ended output.
This output is
designed to drive 50 ohms, such as a network or
spectrum analyzer input.
Differential Output.
Not connected by default.
Ca-
J4 (OUT-)
pacitor C9 can be installed and C11 removed for
differential output drive.
Figure 1 shows the Top Silkscreen for the DC1287
Demo Board.
Figure 1: DC1287 Top Silkscreen
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1287A
LTC6416 2 GHZ LOW NOISE DIFFERENTIAL 16-BIT ADC BUFFER
Refer to Table 1 and Figure 1, and follow the proce-
dure below:
1.
Connect ground to both GND turrets and a 3.6V
power supply to the V+ turret. VCM, CLLO and
CLHI turrets may be left floating.
2.
Apply an input signal to J1. It should be >10MHz
because of the lower frequency limit of the 1:1 in-
put and output transformers. A good starting
point for amplitude is about 0dBm (0.63V
PP
) at the
DC1287A’s input. A low-distortion, low noise sig-
nal source with an external high-order low-pass or
band-pass filter will yield the best performance.
The DC1287’s input is impedance-matched to
50 ; no external termination is necessary.
3.
Observe the output via J3. Although the LTC6416
is intended to drive the high-impedance inputs of
an ADC, the DC1287A’s output has been designed
to drive 50 , suitable for the input of a network or
spectrum analyzer. A resistive network comprised
of R1, R3, 18 of internal resistance and the 50
load terminates the LTC6416 in a 400 differen-
tial load. T2, a 1:1 balun, transforms the output
signal for single-ended observation. Due to volt-
age division within the output network, an at-
tenuation of approximately -19dB is observed at
the output.
ADDITIONAL INFORMATION
Although the DC1287A is ready to use out of the box,
it has features that you can access by adding, remov-
ing or changing components on the board. These are
described below.
DIFFERENTIAL INPUTS
To drive the DC1287A differentially, remove capacitor
C12 and install capacitor C10. J1 and J2 can now be
driven differentially from a 50 signal source. The
input to the DC1287A will still be matched to 50
(differential) when driven in this manner.
DIFFERENTIAL OUTPUTS
To use the both outputs of the DC1287A, remove
capacitor C11 and install capacitor C9. The output
now appears differentially across J3 and J4. The
output of the DC1287A will still be matched to 50
(differentially) when used in this manner.
CHANGING THE OUTPUT COMMON-MODE
VOLTAGE
Turret E1 (labeled VCM) controls the output com-
mon-mode voltage of the DC1287A. This function
ADJUSTING THE CLLO AND CLHI VOLTAGES
The LTC6416 features high speed voltage limiting at each
of the output pins. This feature is important to help pro-
tect the inputs of the ADC. The output voltage at either of
the differential outputs will not exceed the voltage set at
the LTC6416’s CLHI pin. Similarly, it will not go below
the voltage set at the LTC6416’s CLLO pin. See the
LTC6416 datasheet for more information.
Turret E4 (labeled CLHI) drives the CLHI pin of the
LTC6416 directly. If left floating, the CLHI pin will self-
can be used to level-shift the DC output voltage for
optimum system performance. Normally, capacitors
C8 and C9 prevent the output common mode voltage
from reaching the output. However, if the common
mode voltage is desired, C8 and C9 can be shorted
or replaced by 0 resistors to allow the DC bias to
reach the output. Transformer T2 is a 1:1 transmis-
sion line type, which means DC voltages will couple
from input to output of the transformer. By default,
the LTC6416 will self-bias the VCM pin to 1.36V
(with V+=3.6V). For a different common-mode volt-
age, simply apply a voltage source to Turret E1.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1287A
LTC6416 2 GHZ LOW NOISE DIFFERENTIAL 16-BIT ADC BUFFER
bias to 2.45V (with V+=3.6V). For a different CLHI volt-
age, simply apply a voltage source to Turret E4.
Turret E5 (labeled CLLO) drives the CLLO pin of the
LTC6416 directly. If left floating, the CLLO pin will self-
bias to 0.265V (with V+=3.6V). For a different CLLO volt-
age, simply apply a voltage source to Turret E5.
OUTPUT DRIVE
Because the output of the LTC6416 is a low-
impedance voltage output, performance will degrade
when the output is directly driving low impedances
such as 50 . See the LTC6416 datasheet for more
information.
GROUND AND V+ CONNECTIONS
DC1287A’s GND connector is not only connected to
GND of the LTC6416, it also serves as the board
ground. For best performance, a low-impedance re-
turn path to the power supply from GND is crucial.
Short, low impedance wires to the VCC and GND
connectors of DC1287A will yield the best perform-
ance from the LTC6416.
CABLES AND CONNECTORS
Long BNC or SMA cables are transmission lines, and
for best circuit performance should be kept as short
as possible. BNC or SMA cables or barrels should be
characterized (or nulled out through calibration) for
good high-frequency performance. Connectors also
fall under the category of circuit elements, and must
be of good quality and well characterized to ensure
predictable results.
SIGNAL LEVELS
If we examine the schematic included with this Quick
Start Guide, we can determine approximate signal
levels seen at various points along DC1287A’s signal
chain. These signal levels assume that we start with
0dB at J1, and that there is a 50 load at the out-
put(s). If the output load is a high-impedance load,
the signal levels will vary from the schematic.
With 0dB at J1, 1:1 transformer T1 performs a sin-
gle-ended to differential conversion. There is about
0.5dB of attenuation in the transformer, so the differ-
ential signal level at the inputs of the LTC6416 is
about -0.5dB. The differential signal level at the out-
puts of the LTC6416 is about -1dB. The output resis-
tor network results in an additional 17.5dB of at-
tenuation at the input of 1:1 transformer T2 (-18.5dB
total). Transformer T2 performs a differential to sin-
gle-ended conversion and adds a final 0.5dB of at-
tenuation bringing the total attenuation to -19dB.
The external series resistors at the LTC6416’s output
have been included so that the output of the
DC1287A can plug directly into a 50 analyzer input,
but the LTC6416 sees a more benign 400 load re-
sistance.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1287A
LTC6416 2 GHZ LOW NOISE DIFFERENTIAL 16-BIT ADC BUFFER
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