Linear Technology LT6411 User manual

LT6411 AND LTC2249 A
DC
1
DESCRIPTION
Demonstration circuit 1057 is a reference design
featuring Linear Technology Corporation’s LT6411
High Speed Amplifier/ADC Driver with an on-board
LTC2249 14-bit, 80MSPS ADC. DC1057 demon-
strates good circuit layout techniques and recom-
mended component selection for optimal system
performance. The ADC driver input and output
networks are flexible, allowing for AC or DC cou-
pling, gains of 0dB or 6dB, single-ended or differ-
ential configurations, and signal low-pass or band-
pass filtering before the ADC.
DC1057 also allows the flexibility of selecting the
CMOS output supply voltage. DC1057 includes an
on-board 40-pin edge connector for use with the
DC718 Data Acquisition demo board and Linear
Technology’s QuickEval-II data processing soft-
ware, available on our website at
http://www.linear.com.
Design files for this circuit board are available.
Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
QUICK START PROCEDURE
Validating the performance of the LT6411 ADC
Driver-ADC combination is simple with DC1057,
and requires only two signal generators and some
basic lab equipment. Table 1 shows the function of
each I/O connector and selectable jumper on the
board. Refer to Figure 1 for proper board evalua-
tion equipment setup and follow the procedure be-
low:
1. Connect the power supplies as shown. The
power supply connector labeled VCC powers
the ADC driver. VDD powers the ADC, and
OVDD provides power to both the ADC output
stage and the two CMOS output buffers. The
entire board and all components share common
ground planes. Check the supply voltages be-
fore applying power, to avoid damage from over-
voltage conditions.
2. Provide an encode clock to the ADC via SMA
connector J4. For best performance, a high-
quality sine wave synthesizer with an external
band-pass filter will provide a stable, low-phase-
noise clock source. A crystal oscillator will also
provide good performance. DC1057 includes an
on-board clock amplifier/buffer IC to provide a
large-amplitude clock source to the ADC.
NOTE. A poor quality encode clock can significantly degrade the
signal-to-noise ratio (SNR) of the driver-ADC combination.
Table 1: DC1057 Connectors and Descriptions
REFERENCE FUNCTION
J1 (AIN+) Analog Input (50
Ω termination imped-
ance)
J2 (AIN-) Analog Input (by default, tied to ground
via resistor R16 for single-ended-to-
differential conversion)
J3 (40-pin conn) Provides direct connection to DC718.
CMOS Output Buffers provide parallel
data output and clock signals (see sche-
matic)
J4 (Encode
Clock)
ADC Encode Clock. For best perform-
ance, use a high-quality low-jitter clock
source.
JP1 (ENABLE) Enable/Disable LT6411. Default is EN.
3. Apply an input signal to the board. DC1057 al-
lows great flexibility in applying input signals
(see the section on Applying Input Signals). For
best results, use a low distortion, low noise sig-
nal generator with high order low-pass or band-
DEMO CIRCUIT 1057
QUICK START GUIDE
LT6411 High-
Speed
ADC Driver Combo
Board

LT6411 AND LTC2249 A
DC
2
pass filtering to avoid degrading the perform-
ance of the ADC driver and ADC.
4. Observe the ADC output with demo circuit
DC718, a USB cable, a Windows computer, and
Linear Technology’s QuickEval-II data process-
ing software. See Figure 2 for the general board
evaluation setup diagram.
NOTE. See the DC718 Quick Start Guide for instructions on using
the DC718 QuickDAACS data acquisition demo board.
Figure 1. Proper Evaluation Equipment Setup
Figure 2. Evaluation Setup with DC718, Computer, and QuickEval-II Software
Signal
Generator
HP 8644B
or equiv
Signal Generator
HP 8644B or equiv
Amplifier Power Supply ADC Power Supply
Power Supply
CMOS Output
To DC718
BPF
DC718
QuickDAACS
Data Acquisition
Board
Input Signal Generator
with LPF, BPF
Windows PC
w/ QuickEval-II
Processing
Software
USB
NOTE. Even a high-quality signal
synthesizer will still have noise
and harmonics that could be
attenuated with a low-pass or
band-pass filter. For good-quality
high order filters, see TTE, Lark
Engineering, or equivalent.
BPF

LT6411 HIGH-SPEED ADC DRIVER COMBO BOARD
3
Figure 3. DC1057 Input Network, see Table 2 and text for configuration options
ADDITIONAL INFORMATION
Although the DC1057 demo board is ready to use,
it has additional flexibility built in for various types
of input networks, filtering configurations, and sin-
gle-ended or differential inputs. Below is some in-
formation about configuring DC1057 to meet the
specific needs of any application.
APPLYING INPUT SIGNALS
The input network consists of various components
designed to allow either single-ended or differential
inputs, transformer-coupled or DC-coupled. Table
2 and Figure 4 show some possible input configu-
rations, and which components to install. Linear
Technology’s ADC driver families are generally
characterized and designed for both single-ended
and differential input drive, but for optimal per-
formance differential input drive (or transformer-
coupled drive) is recommended. By default, trans-
former drive is used on DC1057 so that only a sin-
gle-ended input is needed. See the LT6411 data-
sheet for more comprehensive information on pos-
sible configurations.
Table 2: DC1057 Input Configuration Guide
CONFIGURATION COMPONENTS NECESSARY
Single-Ended In-
put
Transformer Drive
Gain = +2
(default, not
shown)
Install 0 Ω jumper at R16 or R6.
Install transformer T1. Install 25 Ω at R7
and R13 for impedance matching.
Establish DC bias by using R37 and R38
or R2/R3 and R17/R18.
Tie feedback resistors to AC ground
through 0.1µF at C5 and C9 (or replace
with 0 Ω resistors if using dual sup-
plies).
Single-Ended In-
put
Single-Ended
Drive
Gain = +2
(Figure 4a)
Install 0 Ω jumper at R16. Install R6 for
impedance matching.
Remove T1, C5, C9. Install 0Ω at R5, R10,
R39, R42.
Install 0.1µF at C33, C34. Establish DC
bias with R17, R18 resistors.
Install 0 Ω at R5, R10.

LT6411 HIGH-SPEED ADC DRIVER COMBO BOARD
4
Differential Input
AC-Coupled
Gain = +2
(Figure 4b)
Remove R16 and R6. Install 0 Ω at R39,
R42.
Remove T1. Install 0.1µF at C33, C34.
Install R7 and R13 for impedance match-
ing.
Establish DC bias by using R37 and R38
or R2/R3 and R17/R18.
Tie feedback resistors to AC ground
through 0.1µF at C5 and C9 (or replace
with 0 Ω resistors if using dual sup-
plies).
Differential Input
AC-Coupled
Gain = +1
(Figure 4c)
Remove R16 and R6. Install 0 Ω at R39,
R42.
Remove T1. Install 0.1µF at C33, C34.
Install R7 and R13 for impedance match-
ing.
Establish DC bias by using R37 and R38
or R2/R3 and R17/R18.
Install 0 Ω at R5, R14 to establish gain of
+1.
Differential Input
AC-Coupled
Gain = -1
(Figure 4d)
Remove R16 and R6.
Remove T1. Install 0.1µF at C33, C34.
Install R7 or R13 for impedance matching.
Establish DC bias by using R2/R3 and
R17/R18.
Install 0 Ω at R40, R41 to establish gain of -
1.
Differential Input
AC-Coupled
Gain = +3
(Figure 4e)
Remove R16 and R6
Remove T1. Install 0.1µF at C33, C34.
Install R7 or R13 for impedance matching.
Establish DC bias by using R37 and R38
or R2/R3 and R17/R18.
Install 0 Ω jumpers in the positions shown
in Figure 4e, to establish gain of +3.
Differential Input
DC-Coupled*
(Figure 4f)
Remove R16 and R6.
Remove T1. Install 0 Ω at R39, R42.
Install R7 or R13 for impedance matching.
Remove CIN+ and CIN-, replace with 0 Ω .
Install 0 Ω jumpers at C5, C9.
NOTE. * When driving the ADC driver with a direct DC-coupled
path, be aware of the increased input currents that may occur due to
the output common-mode voltage and internal resistor networks.
The DC common-mode voltage requirements of
Linear Technology’s ADC drivers do not always
extend to ground (the negative supply voltage).
When replacing CIN+ and CIN- with 0Ω jumpers,
make sure to level-shift the inputs so that the ADC
driver’s input common-mode voltage requirement
is met.
Recommended 1:1 transformers for populating T1
are M-A/Com’s ETC1-1-13 and ETC1-1T.
POWER SUPPLY BYPASS CAPACITANCE
Depending on the quality of the power supplies
provided to DC1057, it may be desirable to add
larger bulk capacitors at C1, C29, C30, and C31.
This will not be necessary with clean, low-
impedance power supplies.
FILTER NETWORKS
For narrowband input signals, L1 and C6 are in-
cluded at the output of the ADC driver for easy
band-pass filter design. In addition, there are extra
pads for resistors, capacitors, and other filter net-
work components at the output of the LT6411.
CHANGING THE ADC DRIVER’S OUTPUT
COMMON-MODE VOLTAGE
The output common-mode voltage of the ADC
driver must be set either at the input of the LT6411
(see Table 2), or by AC coupling the output of the
LT6411. R24 and R25 are optional resistors con-
nected to the VCM output of the ADC that can be
installed to apply the proper common-mode volt-
age to the ADC inputs.
QUICKDAACS CIRCUITRY
Logic gate U6, installed on the back of DC1057,
enables the CMOS output buffers when DC1057 is
plugged into DC718, which pulls its input high. De-
vice U5 is an EEPROM device that is used by the
QuickEval software, and does not affect board op-
eration or performance.
USING QUICKEVAL-II SOFTWARE
QuickEval-II, downloadable from Linear Technol-
ogy’s website http://www.linear.com/, processes
data from the DC718 QuickDAACS board and dis-
plays FFT and signal analysis information on the
computer screen. This section describes how to
use the software to view the output from DC1057.
The first step is to select the correct input board.
See Figure 5 From the Configure menu in the

LT6411 HIGH-SPEED ADC DRIVER COMBO BOARD
5
toolbar, select DC718 (QuickDAACs). The next
step is to use the proper settings for the DC1057
output. See Figure 6 Select the correct device from
the menu bar, and select the “Randomized” option
if the output randomizer is turned on (via JP4). Af-
ter configuration is through, the program should be
ready to collect and display data. See the Help file
for instructions on general software use.

LT6411 HIGH-SPEED ADC DRIVER COMBO BOARD
6
Figure 4. Pictures of DC1057 input network configurations. Blocks show where components should be mounted, ac-
cording to Table 2.
A B
C D
E F

LT6411 HIGH-SPEED ADC DRIVER COMBO BOARD
7
Figure 5. Selecting the correct input to QuickEval-II
Figure 6. Entering the correct device information for your ADC. Select the correct device for your board.

LT6411 HIGH-SPEED ADC DRIVER COMBO BOARD
8

LT6411 HIGH-SPEED ADC DRIVER COMBO BOARD
9

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DC1057A