DC1925A - Demo Manual

DEMO MANUAL DC1925A
LTC2378-20/LTC2377-20/LTC2376-20
20-Bit,1Msps/500ksps/250ksps, Low
Power, SAR ADCs with 104dB SNR
DESCRIPTION
The LTC®2378-20, LTC2377-20 and LTC2376-20 are 20‑bit,
low power, low noise SAR ADCs with serial outputs that
operate from a single 2.5V supply. The following text
refers to the LTC2378-20 but applies to all parts in the
family, the only difference being the maximum sample
rate. The LTC2378-20 supports a ±5V fully differential
input range with a 104dB SNR, consumes only 21mW
and achieves ±2ppm INL max with no missing codes at 20
bits. The DC1925A demonstrates the DC and AC performance of the LTC2378-20 in conjunction with the DC590
QuikEval™ and DC890 PScope™ data collection boards.
Use the DC590 to demonstrate DC performance such as
peak-to-peak noise and DC linearity. Use the DC890 if
precise sampling rates are required or to demonstrate AC
performance such as SNR, THD, SINAD and SFDR. The
demonstration circuit 1925A is intended to show recommended grounding, component placement and selection,
routing and bypassing for this ADC.
Design files for this circuit board are available at
http://www.linear.com/demo or scan the QR code on
the back of the board.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
QuikEval and PScope are trademarks of Linear Technology Corporation. All other trademarks are
the property of their respective owners.
BOARD PHOTO
Figure 1. DC1925A Connection Diagram
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DEMO MANUAL DC1925A
ASSEMBLY OPTIONS
Table 1. DC1925A Assembly Options
ASSEMBLY VERSION
U1 PART NUMBER
MAX CONVERSION RATE
NUMBER OF BITS
MAX CLK IN FREQUENCY
DC1925A-A
LTC2378CMS-20
1Msps
20
80MHz
DC1925A-B
LTC2377CMS-20
500ksps
20
40MHz
DC1925A-C
LTC2376CMS-20
250ksps
20
20MHz
DC890 QUICK START PROCEDURE
Check to make sure that all switches and jumpers are
set as shown in the connection diagram of Figure 1.
In particular make sure that VCCIO (JP3) is set to the
2.5V position. Operating the DC1925A with the DC890
while JP3 is in the 3.3V position will cause noticeable
performance degradation in SNR and THD. The default
connections configure the ADC to use the onboard reference and regulators to generate the required common
mode voltages. The analog input is DC coupled. Connect
the DC1925A to a DC890 USB high speed data collection
board using connector P1. Then, connect the DC890 to a
host PC with a standard USB A/B cable. Apply ±9V to the
indicated terminals. Then apply a low jitter differential sine
source to J2 and J4. Connect a low jitter 80MHz 2.5VP-P
sine wave or square wave to connector J1. Note that J1
has a 50Ω termination resistor to ground.
Run the PScope software (PScope.exe version K73 or
later) supplied with the DC890 or download it from www.
linear.com/software.
Complete software documentation is available from the
Help menu. Updates can be downloaded from the Tools
menu. Check for updates periodically as new features
may be added.
The PScope software should recognize the DC1925A and
configure itself automatically.
Click the Collect button (See Figure 5) to begin acquiring
data. The Collect button then changes to Pause, which
can be clicked to stop data acquisition.
DC590 SETUP
IMPORTANT! To avoid damage to the DC1925A, make
sure that VCCIO (JP6) of the DC590 is set to 3.3V before
connecting the DC590 to the DC1925A.
VCCIO (JP3) of the DC1925A should be in the 3.3V position
for DC590 operation. To use the DC590 with the DC1925A,
it is necessary to apply –9V and ground to the –9V and
GND terminals. Connect the DC590 to a host PC with a
standard USB A/B cable. Connect the DC1925A to a DC590
USB serial controller using the supplied 14-conductor
ribbon cable. Apply a signal source to J4 and J2 or J4
depending on how the DC1925A is configured.
Run the QuikEval software (version K92-01 or later) supplied with the DC590 or download it from www.linear.
com/software. The correct control panel will be loaded
automatically. Click the Collect button (See Figure 6) to
begin reading the ADC.
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DEMO MANUAL DC1925A
DC1925A SETUP
DC Power
Reference
The DC1925A requires ±9VDC and draws approximately
100mA. Most of the supply current is consumed by the
CPLD, op amps, regulators and discrete logic on the board.
The +9VDC input voltage powers the ADC through LT1763
regulators which provide protection against accidental
reverse bias. Additional regulators provide power for the
CPLD and op amps. See Figure 1 for connection details.
The default reference is a LTC6655 5V reference. If an
external reference is used it must settle quickly in the
presence of glitches on the REF pin. To use an external
reference, unsolder R37 and apply the reference voltage
to the VREF terminal.
Clock Source
You must provide a low jitter 2.5VP-P (if VCCIO is in the 3.3V
position, the clock amplitude should be 3.3VP-P) sine or
square wave to J1. The clock input is AC coupled so the DC
level of the clock signal is not important. A clock generator
like the Rohde & Schwarz SMB100A or the DC1216A-C is
recommended. Even a good clock generator can start to
produce noticeable jitter at low frequencies. Therefore it
is recommended for lower sample rates to divide down a
higher frequency clock to the desired input frequency. The
ratio of clock frequency to conversion rate is 80:1. If the
clock input is to be driven with logic, it is recommended
that the 50Ω terminator (R5) be removed. Slow rising
edges may compromise the SNR of the converter in the
presence of high amplitude higher frequency input signals.
Data Output
Parallel data output from this board (0V to 2.5V default),
if not connected to the DC890, can be acquired by a logic
analyzer, and subsequently imported into a spreadsheet, or
mathematical package depending on what form of digital
signal processing is desired. Alternatively, the data can be
fed directly into an application circuit. Use Pin 50 of P1 to
latch the data. The data can be latched using either edge
of this signal. The data output signal levels at P1 can also
be changed to 0V to 3.3V if the application circuit requires
a higher voltage. This is accomplished by moving VCCIO
(JP3) to the 3.3V position.
Analog Input
The default driver for the analog inputs of the LTC2378-20
on the DC1925A is shown in Figure 2. This circuit buffers
a fully differential 0V to 5V input signal applied at AIN+
and AIN–. In addition, this circuit bandlimits the input
frequencies to approximately 1.2MHz.
Alternatively, if your application circuit requires a singleended signal to drive the ADC, the circuit shown in Figure 3
can be used. The circuit of Figure 3 converts a singleended signal to the fully-differential signal required by the
ADC. Additionally, this circuit further bandlimits the input
frequencies to 100kHz which is the bandwidth limit of the
ADC for low distortion performance.
The single-ended-to-differential circuit results in reduced
THD performance, (approximately –112dB) due to the
slight phase shift of the inverting op amp. The circuit in
Figure 3 can be implemented on the DC1925A by removing R44 and R52 and adding R57 and R58. At this point
it will only be necessary to drive AIN+ (J4).
Figure 2. Fully-Differential Driver
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DEMO MANUAL DC1925A
DC1925A SETUP
Figure 3. Single-Ended-to-Differential Driver
Data Collection
For SINAD, THD or SNR testing a low noise, low distortion
differential output sine generator such as the Stanford
Research DS360 should be used. A low jitter RF oscillator
such as the Rohde & Schwarz SMB100A or DC1216A-C
is used as the clock source.
This demo board is tested in house by attempting to
duplicate the FFT plot shown in the typical performance
characteristics of the LTC2378-20 data sheet. This involves
using an 80MHz clock source, along with a differential
output sinusoidal generator at a frequency of 2.0kHz.
The input signal level is approximately –1dBfs. The input
is level shifted and filtered with the circuit shown in Figure 4. A typical FFT obtained with DC1925A is shown in
Figure 5. Note that to calculate the real SNR, the signal level
(F1 amplitude = –0.998dB) has to be added back to the
SNR that PScope displays. With the example shown in Figure 5 this means that the actual SNR would be 103.668dB
instead of the 102.67dB that PScope displays. Taking the
RMS sum of the recalculated SNR and the THD yields a
SINAD of 103.62dB which is fairly close to the typical
number for this ADC.
Figure 4. Differential Level Shifter
There are a number of scenarios that can produce misleading results when evaluating an ADC. One that is common
is feeding the converter with a frequency, that is a submultiple of the sample rate, and which will only exercise
a small subset of the possible output codes. The proper
method is to pick an M/N frequency for the input sine wave
frequency. N is the number of samples in the FFT. M is
a prime number between one and N/2. Multiply M/N by
the sample rate to obtain the input sine wave frequency.
Another scenario that can yield poor results is if you do
not have a sine generator capable of ppm frequency accuracy or if it cannot be locked to the clock frequency. You
can use an FFT with windowing to reduce the “leakage” or
spreading of the fundamental, to get a close approximation of the ADC performance. If windowing is required,
the Blackman-Harris 92dB window is recommended. If an
amplifier or clock source with poor phase noise is used,
windowing will not improve the SNR.
Layout
As with any high performance ADC, this part is sensitive
to layout. The area immediately surrounding the ADC on
the DC1925A should be used as a guideline for placement,
and routing of the various components associated with the
ADC. Here are some things to remember when laying out a
board for the LTC2378-20. A ground plane is necessary to
obtain maximum performance. Keep bypass capacitors as
close to supply pins as possible. Use low impedance returns
directly to the ground plane for each bypass capacitor.
Use of a symmetrical layout around the analog inputs will
minimize the effects of parasitic elements. Shield analog
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DEMO MANUAL DC1925A
DC1925A SETUP
input traces with ground to minimize coupling from other
traces. Keep traces as short as possible.
Component Selection
When driving a low noise, low distortion ADC such as
the LTC2378-20, component selection is important so
as to not degrade performance. Resistors should have
low values to minimize noise and distortion. Metal film
resistors are recommended to reduce distortion caused
by self heating. Because of their low voltage coefficients,
to further reduce distortion NPO or silver mica capacitors
should be used. Any buffer used to drive the LTC2378-20
should have low distortion, low noise and a fast settling
time such as the LT6203.
Figure 5. PScope Screen Shot
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DEMO MANUAL DC1925A
DC1925A SETUP
Figure 6. QuikEval Screen Shot
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DEMO MANUAL DC1925A
DC1925A JUMPERS
Definitions
JP1 – EEPROM is for factory use only. Leave this in the
default WP position.
JP2 – V+ Selects 8V or 5V for V+. The default is position
is 8V. Setting V+ to 5V is useful for evaluating single 5V
supply operation of the buffer when operating the ADC
with Digital Gain Compression turned on.
JP3 – VCCIO sets the output levels at P1 to either 3.3V
or 2.5V. Use 2.5V to interface to the DC890 which is the
default setting. Use 3.3V to interface to the DC590.
JP4 – VCM sets the DC bias for AIN+ and AIN– if the single
ended to differential mode is enabled, and the inputs are
AC coupled. VREF/2 is the default setting.
JP5 – V– Selects –3.6V or ground for V–. The default setting is –3.6V. Setting V– to ground is useful for evaluating
single supply operation of the buffer when operating the
ADC with Digital Gain Compression turned on.
JP6 – FS selects whether the Digital Gain Compression is
on or off. In the VREF position, Digital Gain Compression
is off and the analog input range at AIN+ and AIN– is 0V to
VREF. In the 0.8VREF position, Digital Gain Compression is
turned on and the analog input range at AIN+ and AIN– is
0.1VREF to 0.9VREF. The default setting is off.
JP7 – Coupling selects AC or DC coupling of AIN+. The
default setting is DC.
JP8 – Coupling selects AC or DC coupling of AIN–. The
default setting is DC.
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DEMO MANUAL DC1925A
PARTS LIST
ITEM
QTY
REFERENCE
PART DESCRIPTION
MANUFACTURER/PART NUMBER
DC1925A General BOM
1
13
C1-C5, C7, C10, C11, C13-C16, C56
CAP., X7R, 0.1µF, 16V 10% 0603
AVX, 0603YC104KAT2A
2
10
C6, C9, C18, C24, C26, C29, C59, C60, C62, C69 CAP., X5R, 10µF, 6.3V 20% 0603
AVX, 06036D106MAT2A
3
1
C8
CAP., X7R, 1µF, 16V 10% 0603
AVX, 0603YC105KAT2A
4
9
C12, C19, C42, C43, C45, C47, C48, C57, C77
CAP., X7R, 0.1µF, 25V 20% 0603
AVX, 06033C104MAT2A
5
12
C17, C22, C25, C28, C40, C44, C49, C51, C54,
C55, C58, C74
CAP., X5R, 1µF, 25V 10% 0603 X5R OK
AVX, 06033D105KAT2A
6
1
C20
CAP., X7R, 47µF, 10V 10% 1210
MURATA, GRM32ER71A476KE15L
7
1
C21
CAP., X5R, 22µF, 25V 20% 1210
AVX, 12103D226MAT2A
8
4
C23, C27, C30, C50
CAP., X7R, 0.01µF, 25V 10% 0603
AVX, 06033C103KAT2A
9
8
C31, C32, C33, C34, C35, C36, C37, C38
CAP., X7R, 0.1µF, 16V 10% 0402
AVX, 0402YC104KAT2A
10
0
C39, C61, C63-C67, C70, C75, C76
CAP., OPT, 0603
OPTION
11
1
C46
CAP., X5R, 2.2µF, 10V 10% 0603
AVX, 0603ZD225KAT2A
12
2
C52 ,C53
CAP., X5R, 10µF, 25V 10% 0805
AVX, 08053D106KAT2A
13
1
C68
CAP., COG, 15pF, 50V 10% 0603
AVX, 06035A150KAT2A
14
2
C71, C73
CAP., NPO, 6800pF, 50V 5% 1206
MURATA, GRM3195C1H682JA01D
15
1
C72
CAP., NPO, 3300pF, 50V 10% 1206
AVX, 12065A332KAT2A
16
1
C78
CAP., X5R, 4.7µF, 6.3V 20% 0603
AVX, 06036D475MAT2A
17
7
E1, E2, E3, E4, E5, E9, E10
TEST POINT, TURRET, 0.061
MILL MAX, 2308-2-00-80-00-00-07-0
18
3
E6, E7, E8
TESTPOINT, TURRET, 0.094, PBF
MILL MAX, 2501-2-00-80-00-00-07-0
19
8
JP1-JP8
HEADER, 3-PIN SINGLE ROW 0.100
SAMTEC, TSW-103-07-L-S
20
3
J1, J2, J4
CONNECTOR, BNC
CONNEX, 112404
21
1
J3
CONN HEADER 14-POS 2MM VERT GOLD MOLEX, 87831-1420
22
1
J5
HEADER, 2X5, 0.100"
SAMTEC, TSW-105-07-L-D
23
4
MH1, MH2, MH3, MH4
STANDOFF, NYLON 0.25"
KEYSTONE, 8831 (SNAP ON)
24
4
R1, R3, R8, R15
RES., CHIP, 33Ω, 1/10W, 5% 0603
YAGEO, RC0603JR-0733RL
25
8
R2, R6, R7, R13, R19, R24, R29, R43
RES., CHIP, 1k, 1/10W, 1% 0603
YAGEO, RC0603JR-071KL
26
2
R4, R9
RES., CHIP, 0Ω, 1/16W, 0402
YAGEO, RC0402JR-070RL
27
1
R5
RES., CHIP, 49.9Ω, 1/4W, 1% 1206
YAGEO, RC1206FR-0749R9L
28
4
R10, R11, R12, R81
RES., CHIP, 4.99k, 1/10W, 1% 0603
YAGEO, RC0603FR-74K99L
29
24
R14, R61-R79, R82, R83, R84, R85
RES., CHIP, 33Ω, 1/16W, 5% 0402
YAGEO, RC0402JR-0733RL
30
1
R16
RES., CHIP, 300Ω, 1/16W, 5% 0402
YAGEO, RC0402JR-07300RL
31
1
R17
RES., CHIP, 2k, 1/10W, 5% 0603
YAGEO, RC0603JR-072KL
32
2
R18, R38
RES., CHIP, 249Ω, 1/10W, 1% 0603
YAGEO, RC0603FR-07249RL
33
3
R20, R22, R59
RES., CHIP, 1k, 1/16W, 5% 0402
YAGEO, RC0402JR-071KL
34
1
R21
RES., CHIP, 10k, 1/10W, 5% 0603
YAGEO, RC0603JR-0710KL
35
1
R23
RES., CHIP, 6.49k, 1/10W, 1% 0603
YAGEO, RC0603FR-076K49L
36
1
R25
RES., CHIP, 1.69k, 1/10W, 1% 0603
YAGEO, RC0603FR-071K69L
37
1
R26
RES., CHIP, 1.54k, 1/10W, 1% 0603
YAGEO, RC0603FR-071K54L
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DEMO MANUAL DC1925A
PARTS LIST
ITEM
QTY
REFERENCE
PART DESCRIPTION
MANUFACTURER/PART NUMBER
38
1
R27
RES., CHIP, 2.8k, 1/10W, 1% 0603
YAGEO, RC0603FR-072K8L
39
2
R28, R80
RES., CHIP, 2k, 1/10W, 1% 0603
YAGEO, RC0603FR-072KL
40
10
R30, R37, R39, R41, R44, R46, R50, R52, R53,
R55
RES., CHIP, 0Ω, 1/10W, 0603
YAGEO, RC0603JR-070RL
41
4
R33, R34, R86, R87
RES., CHIP, 499Ω, 1/10W, 1% 0603
VISHAY, CRCW0603499RFKEA
42
10
R35, R36, R40, R45, R47, R49, R54, R56, R57,
R58
RES., CHIP, OPT, 0603
OPTION
43
1
R42
RES., CHIP, 5.62k, 1/10W, 1% 0603
YAGEO, RC0603FR-075K62L
44
2
R48,R51
RES., CHIP, 10Ω, 1/10W, 1% 0603
YAGEO, RC0603FR-0710RL
45
1
R60
RES., CHIP, 10k, 1/16W, 5% 0402
YAGEO, RC0402JR-0710KL
46
1
R88
RES., CHIP, 1Ω, 1/10W, 5% 0603
YAGEO, RC0603JR-071RL
47
2
U2, U4
IC, UNBUFFERED INVERTER, SC70-5
FAIRCHILD, NC7SVU04P5X
48
1
U3
IC, D FLIP-FLOP, US8
ON SEMI., NL17SZ74USG
49
1
U5
IC, OP AMP, LOW NOISE, TSOT-23
LINEAR TECH., LT6202CS5#PBF
50
1
U6
IC, SINGLE SPST BUS SWITCH, SC70-5
FAIRCHILD, NC7SZ66P5X
51
1
U7
IC, SERIAL EEPROM, TSSOP
MICROCHIP, 24LC024-I/ST
52
2
U8, U9
IC, UHS INVERTER, SC70-5
FAIRCHILD, NC7SZ04P5X
53
2
U10, U18
IC, DUAL OP AMP, MS8
LINEAR TECH., LT6203CMS8#PBF
54
1
U11
IC, MAX II CPLD, TQFP100
ALTERA, EPM240GT100C5N
55
1
U12
IC, MICROPOWER REGULATOR, SO-8
LINEAR TECH., LT1763CS8-1.8#PBF
56
2
U13, U16
IC, MICROPOWER REGULATOR, SO-8
LINEAR TECH., LT1763CS8#PBF
57
1
U14
IC, MICROPOWER REGULATOR, SO-8
LINEAR TECH., LT1763CS8-2.5#PBF
58
1
U15
IC, VOLTAGE REFERENCE, MSOP
LINEAR TECH., LTC6655BHMS8-5#PBF
59
1
U17
IC, MICROPOWER NEG. REGULATOR
LINEAR TECH., LT1964ES5-SD#PBF
60
8
XJP1-XJP8
SHUNT, 0.100 CENTERS
SAMTEC, SNT-100-BK-G
61
2
STENCIL, (TOP & BOTTOM)
STENCIL DC1925A
1
1
GENERAL BOM
DC1925A
2
1
IC, HIGH SPEED, LOW NOISE SAR ADC
LINEAR TECH., LTC2378CMS-20#PBF
3
1
FAB, PRINTED CIRCUIT BOARD
DEMO CIRCUIT 1925A-2
1
1
GENERAL BOM
DC1925A
2
1
IC, HIGH SPEED, LOW NOISE SAR ADC
LINEAR TECH., LTC2377CMS-20#PBF
3
1
FAB, PRINTED CIRCUIT BOARD
DEMO CIRCUIT 1925A-2
1
1
GENERAL BOM
DC1925A
2
1
IC, HIGH SPEED, LOW NOISE SAR ADC
LINEAR TECH., LTC2376CMS-20#PBF
3
1
FAB, PRINTED CIRCUIT BOARD
DEMO CIRCUIT 1925A-2
DC1925A-A
U1
DC1925A-B
U1
DC1925A-C
U1
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DEMO MANUAL DC1925A
SCHEMATIC DIAGRAM
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DEMO MANUAL DC1925A
SCHEMATIC DIAGRAM
dc1925af
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
11
DEMO MANUAL DC1925A
DEMONSTRATION BOARD IMPORTANT NOTICE
Linear Technology Corporation (LTC) provides the enclosed product(s) under the following AS IS conditions:
This demonstration board (DEMO BOARD) kit being sold or provided by Linear Technology is intended for use for ENGINEERING DEVELOPMENT
OR EVALUATION PURPOSES ONLY and is not provided by LTC for commercial use. As such, the DEMO BOARD herein may not be complete
in terms of required design-, marketing-, and/or manufacturing-related protective considerations, including but not limited to product safety
measures typically found in finished commercial goods. As a prototype, this product does not fall within the scope of the European Union
directive on electromagnetic compatibility and therefore may or may not meet the technical requirements of the directive, or other regulations.
If this evaluation kit does not meet the specifications recited in the DEMO BOARD manual the kit may be returned within 30 days from the date
of delivery for a full refund. THE FOREGOING WARRANTY IS THE EXCLUSIVE WARRANTY MADE BY THE SELLER TO BUYER AND IS IN LIEU
OF ALL OTHER WARRANTIES, EXPRESSED, IMPLIED, OR STATUTORY, INCLUDING ANY WARRANTY OF MERCHANTABILITY OR FITNESS
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ANY INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES.
The user assumes all responsibility and liability for proper and safe handling of the goods. Further, the user releases LTC from all claims
arising from the handling or use of the goods. Due to the open construction of the product, it is the user’s responsibility to take any and all
appropriate precautions with regard to electrostatic discharge. Also be aware that the products herein may not be regulatory compliant or
agency certified (FCC, UL, CE, etc.).
No License is granted under any patent right or other intellectual property whatsoever. LTC assumes no liability for applications assistance,
customer product design, software performance, or infringement of patents or any other intellectual property rights of any kind.
LTC currently services a variety of customers for products around the world, and therefore this transaction is not exclusive.
Please read the DEMO BOARD manual prior to handling the product. Persons handling this product must have electronics training and
observe good laboratory practice standards. Common sense is encouraged.
This notice contains important safety information about temperatures and voltages. For further safety concerns, please contact a LTC application engineer.
Mailing Address:
Linear Technology
1630 McCarthy Blvd.
Milpitas, CA 95035
Copyright © 2004, Linear Technology Corporation
dc1925af
12 Linear Technology Corporation
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