DEMO CIRCUIT 1058
16-BIT HIGH PERFORMANCE ADC DRIVERS
Q UICK S TA RT G UIDE
16 -b it H ig h P e rfo rm a n c e
A DC Driv e rs
DESCRIPTION
Demonstration circuit 1058 is a ref
erence design f
eatur-
ing Linear Technology Corporation’ 16-bit High Per-
s
f
ormance ADC Drivers. DC1058 demonstrates good cir-
cuit layout techniques and recommended component
selection f optimal system perf
or
ormance. The ADC
driver input and output networks are f
lexible,allowing f
or
AC or DC coupling,single-ended or dif erential conf
f
igura-
tions,and additional f
iltering bef the ADC.
ore
DC1058 comes installed with one of Linear Technology’
s
16-bit ADC drivers—LT1994, LTC6404, LTC6406, or
LTC6403. For ease of evaluation, a 16-bit Analog-to-
Digital Converter (ADC) is included on the board.
DC1058 includes an on-board 40-pin edge connector f
or
use with the DC718 Data Acquisition demo board and
Linear Technology’ PScope data processing sof
s
tware,
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 ofLinear Technology Corporation.
Q U ICK STA RT PROCEDU RE
Validating the perf
ormance of the ADC Driver-ADC com-
bination is simple with DC1058, and requires only two
signal generators and some basic test equipment. Table
1 shows the f
unction of each I/ connector and select-
O
able j
umper on the board. Ref to Figure 1 f proper
er
or
board evaluation equipment setup and f
ollow the proce-
dure below:
1.
Connect the power supplies as shown (see Table 2 f
or
voltages). The power supply connector labeled V+
powers the ADC driver. VDD powers the ADC, and
OVP provides power to both the ADC output stage and
the two CM OS output buf ers.The entire board and all
f
components share a common ground. Check the
datasheets of the respective IC’ bef
s
ore applying
power,to avoid damage f
rom over-voltage conditions.
2.
Provide an encode clock to the ADC via SM A connec-
tor J3.For best perf
ormance,a high-quality sine wave
synthesizer with an external band-pass f will pro-
ilter
vide a stable, low-phase-noise clock source. A crystal
oscillator will also provide good perf
ormance.
DC1058A-A includes an on-board clock buf er IC to
f
provide a f
ast-edge clock to the ADC.
NOTE.
A poor quality encode clock can signif
icantly degrade the signal-
to-noise ratio (SNR) ofthe driver-ADC combination.
Table 1:DC1058 Connector and Descriptions
REFERENCE
FUNCTI
ON
Provides direct connection to DC718.CM OS
J1 (40 pin conn)
Output Buf ers provide parallel data output and
f
clock signals (see schematic).
ADC Encode Clock.For best perf
ormance,use
J3 (Encode Input)
a high-quality low-phase-noise clock source.
Analog Input (by def
ault,tied to ground via
J2 (AIN-)
resistor R3 – apply input signal at J4)
J4 (AIN+)
Analog Input (50Ω source impedance)
JP1
Invert Clock.Def is NORM position.
ault
JP2 (SHDN)
Enables or Disables U2/ the ADC driver
U3,
ADC Input Range Select.LO selects 2.
PP
25V
JP3 (PGA)
Input Range,HI selects 1.
PP
range.
5V
Output Randomizer.NORM is def
ault,RAND
JP4 (RAND)
randomizes the DC1058 digital output.
JP5 (DITH)
ADC Additive Dither.See ADC datasheet.
3.
Apply an input signal to the board. DC1058 allows
great f
lexibility in applying input signals (see the sec-
tion on Applying Input Signals).For best results,use a
low distortion, low noise signal generator with suf i-
f
cient f
iltering to avoid degrading the perf
ormance of
the amplif and ADC.
ier
4.
Observe the ADC output with demo circuit DC718, a
USB cable,a W indows computer,and Linear Technol-
ogy’ QuickEval-II (Pscope) data processing sof
s
tware.
1
16-BIT HIGH PERFORMANCE ADC DRIVERS
Amplif Power Supply
ier
NOTE.
Even a high-quality signal
synthesizer will still have noise and
harmonics that should be attenuated
with a low-pass or band-pass f
ilter.
For good-quality high order f
ilters,
see TTE,Lark Engineering,or
equivalent.
ADC Power Supply (3.
3V)
To DC718
Signal
Generator
HP 8644B
or equiv
BPF
Signal Generator
HP 8644B or equiv
BPF
Figure 1.
Proper Evaluation Equipm ent Setup
Power Supply CM OS
Output (1. 3V)
8-3.
Version
-A
-B
-C
-D
-E
-F
V+
2.
375-12.
6V
2. 25V
7-5.
2. 25V
7-5.
VDD
OVP
1.
85-3.
6V
3.
135-3.
465V
(limited by DC718)
2. 25V
7-5.
2. 5V
7-3.
2. 25V
7-5.
Table 2:Supply Voltages for various dem o board versions
2
16-BIT HIGH PERFORMANCE ADC DRIVERS
A DDITIONA L INF ORM A TION
DC1058 comes pre-installed with LT1994, LTC6404,
LTC6406, or LTC6403.
Note that the LT1994 has a dif-
ferent pinout, and thus is located on the bottom side of
the PCB (U3).
Although the DC1058 demo board is ready
to use on delivery, it has additional f
lexibility built in f
or
various types of input networks, f
iltering conf
igurations,
and single-ended or dif erential inputs. Below is some
f
inf
ormation about conf
iguring DC1058 to meet the spe-
cif needs ofyour evaluation.
ic
APPLYI G I PUT SI N ALS
N N
G
The input network consists of various components de-
signed to allow either single-ended or dif erential inputs,
f
AC-coupled or DC-coupled. Table 3 shows some possi-
ble input conf
igurations, and which components to in-
stall. Linear Technology’ 16-bit ADC driver f
s
amilies are
generally characterized and designed f excellent per-
or
f
ormance with both single-ended and dif erential input
f
drive. W hen using DC coupled inputs, the inputs to
DC1058 may need to be level-shif to avoid violating
ted
the input common-mode voltages of the amplif Check
ier.
ier
or
the amplif datasheet f details.
Table 3:DC1058 I
nput Configuration Guide
CONFI
GURATI
ON
COM PONENTS NECESSARY
Install 0Ω j
umper at R41.Install 50Ω impedance
Single-Ended Input
matching resistor at R45 ifnecessary.
AC-Coupled
Install C5,C6 f AC coupling.
or
(Def Setup)
ault
Install 0Ω j
umper at R41.Install 50Ω impedance
Single-Ended Input
matching resistor at R45 ifnecessary.
DC Coupled*
Remove C5 and C6,replace with 0Ω j
umpers.
Replace R41 and R45 with impedance matching
Dif erential Input
f
resistors ifnecessary.
AC-Coupled
Install C5 and C6 f AC coupling.
or
Replace R41 and R45 with impedance matching
Dif erential Input
f
resistors ifnecessary.
DC-Coupled*
Remove C5 and C6,replace with 0Ω j
umpers.
NOTE.
* W hen driving the ADC driver with a direct DC-coupled path,be
aware ofthe increased input currents that may occur due to the output
common-mode voltage and the amplif gain/ eedback resistors.
ier
f
PO WER SUPPLY BYPASS CAPACI
TANCE
Depending on the quality of the power supplies provided
to DC1058,it may be desirable to add larger bulk capaci-
tors at C27, C35, and C39. This is not necessary with
clean,low-impedance DC power supplies.
FI
LTER NETWO R KS
Components f an RC or LC f
or
ilter are included at the
ADC input f low-pass or bandpass f
or
ilter designs. To
achieve the low distortion necessary f driving high-
or
perf
ormance 16-bit ADCs, the ADC drivers have very
wide signal bandwidths. Reducing the bandwidth at the
ADC input will reduce the amount of wideband noise
sampled by the ADC, and theref improve the overall
ore
signal-to-noise ratio (SNR).
CH AN GI G TH E ADC DRI
N
VER’ O UTPUT CO M M O N-
S
M O DE VOLTAG E
The LT1994 and LTC6403/ 6 have internal resistive di-
4/
viders that can set the output common-mode voltage
automatically, but if the user wishes to change that volt-
age,it can be done in one ofthree ways.By changing the
resistors R59 and R54 that comprise a resistive divider
f
rom the V+ voltage, it is possible to tune the common-
mode voltage independently of all other f
actors. Alterna-
tively, R59 and R54 can be removed, and the ADC can
supply the common-mode voltage by installing R53 (this
is the def
ault). A third option is to drive E9 (VOCM ) on
the board using an external voltage source.If driving this
voltage f
rom an active buf er such as an op amp, re-
f
moval ofC42 may be necessary to avoid loading.
EN CO DE CLOCK PATHS
As shown in the schematic, there are two clock paths
that can be used on DC1058. The diferential clock path
f
with transf
ormer T2 comes installed with the LTC2207
105M sps ADC. The single-ended clock path with clock
buf er U8 comes installed with the lower speed LTC2203.
f
The common-mode voltage at the ADC drivers must re-
main within the datasheet-stated limits. W hen replacing
C5 and C6 with
0Ω
j
umpers, make sure to level-shif the
t
inputs if necessary so that the ADC driver’ input com-
s
mon-mode voltage requirements are met.
3
16-BIT HIGH PERFORMANCE ADC DRIVERS
I VERT O UTPUT CLOCK
N
Jumper JP1 inverts the clock output f
rom DC1058 to
DC718 (data acquisition board).This ensures that DC718
latches the output data on the correct edge of the clock.
If the output data/ appears unusual, try changing the
FFT
position of JP1 to latch data on the opposite clock edge.
The correct position of Jumper JP1 may change if you
vary the clock f
requency.
Q UI
CKDAACS CI CUI
R TRY
Logic gate U5, installed on the back of DC1058, enables
the CM OS output buf ers when DC1058 is plugged into
f
DC718, which pulls its input high. Device U6 is an
EEPROM device that is used by the QuickEval sof
tware,
and does not afect board operation or perf
f
ormance.
USI G PSCO PE (Q UI
N
CKEVAL I ) SOFTWARE
I
PScope,downloadable f
rom Linear Technology’ website
s
http:/
/ www.
linear.
com/
processes data f
,
rom the DC718
QuickDAACS board and displays FFT and signal analysis
inf
ormation on the computer screen. This section de-
scribes how to use the sof
tware to view the output f
rom
DC1058.
The on-board EEPROM should enable automatic board
detection and auto-conf
iguration of the sof
tware, but in
case the user wishes to change the settings, they can
easily do so.
From the Conf
igure menu in the toolbar, uncheck
“Autodetect Device”. The next step is to use the proper
settings f the DC1058 output in the Device menu, also
or
under Conf
igure. See Figure 3. Select “User Conf
igure”
f
rom the menu bar, and select the “Randomized” check-
box if the output randomizer is turned on (via JP4). The
other settings will generally be the same as shown in the
f
igure. Af conf
ter
iguration is through, the program
should be ready to collect and display data. See the Help
f f instructions on general sof
ile or
tware use.
Figure 2. Entering the correct device inform ation for your ADC. Select the correct device for your board.
4
16-BIT HIGH PERFORMANCE ADC DRIVERS
5
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