Linear Technology DC2581A Guide de configuration rapide

dc2581af

DEMO MANUAL DC2581A

DESCRIPTION

LTC2341

16-/18-Bit, Dual 666ksps, SAR ADC

Demonstration circuit 2581A shows the proper way to drive

the LT C

2341 ADC. The LTC2341 is a low noise, high speed,

simultaneoussampling16-/18-bitsuccessiveapproximation

™

interface that allows conversion-by-conversion control of

the input voltage span on a per-channel basis. An internal

2.048V reference and 2×buffer simplify basic operation

while an external reference can be used to increase the input

range and the SNR of the ADC.

The DC2581A demonstrates the DC and AC performance of

the LTC2341 in conjunction with the DC590/DC2026 and

DC890 data collection boards. Use the DC590/DC2026 to L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and PScope,

QuikEval and SoftSpan are trademarks of Linear Technology Corporation. All other trademarks

are the property of their respective owners.

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 DC2581A is intended to

demonstrate recommended grounding, component place-

ment and selection, routing and bypassing for this ADC. A

suggesteddrivercircuitfortheanaloginputsisalsopresented.

Designfilesfor this circuitboardincluding the schematic,

layout and BOM are available at

http://www.linear.com/demo/DC2581A

Figure 1. DC2581A Connection Diagram

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DEMO MANUAL DC2581A

DC890 QUICK START PROCEDURE

Check to make sure that all jumpers are set to their default

settings as described in the DC2581A Jumpers section of

this manual. The default connections configure the ADC to

use the onboard reference and regulators to generate all

the required bias voltages. The analog inputs by default are

DC coupled. Connect the DC2581A 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 signal source to J5 and J6. Use J7 to route

the signal sources of J5 and J6 to the desired AIN0-AIN1

inputs. Observe the recommended input voltage range

for each analog input. Connect a low jitter 2.5VP-P sine

wave or square wave to connector J1. See the Assembly

Options table for the appropriate clock frequency. Note

that J1 has a 50Ω termination resistor to ground.

Run the PScope

™

software (Pscope.exe version K91 or

later) which can be downloaded from www.linear.com/

designtools/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 DC2581A and

configure itself automatically.

Click the Collect button (See Figure 3) to begin acquiring

data. The Collect button then changes to Pause, which

can be clicked to stop data acquisition.

DC590/DC2026 QUICK START PROCEDURE

IMPORTANT! To avoid damage to the DC2581A, make

sure that VCCIO (JP6 of the DC590, JP3 of the DC2026)

of the DC590/DC2026 is set to 3.3V before connecting

the DC590/DC2026 to the DC2581A.

To usetheDC590/DC2026with the DC2581A,itisnecessary

to apply ±9V and ground to the ±9V and GND terminals of

the DC2581A. Connect the DC590/DC2026 to a host PC

with a standard USB A/B cable. Connect the DC2581A to

a DC590/DC2026 USB serial controller using the supplied

14-conductor ribbon cable. Apply a signal source to J5

and J6. Use J7 to route the signal sources of J5 and J6

to the desired AIN0-AIN1 inputs. No Clock is required on

J1 when using the DC590/DC2026. The clock signal is

provided by the DC590/DC2026.

Run the QuikEval software (quikeval.exe version K110 or

later)which isavailable from www.linear.com/designtools/

software. The correct control panel will be loaded auto-

matically. Click the COLLECT button (Figure 6) to begin

reading the ADC.

ASSEMBLY OPTIONS

Assembly

Version U1 Part Number

Max Conversion

Rate

Number of

Channels

Number of

Bits

Max CLK IN

Frequency

CLK IN/fs

Ratio

DC2581A-A LTC2341-18 666ksps 2 18 60MHz 90

DC2581A-B LTC2341-16 666ksps 2 16 60MHz 90

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DEMO MANUAL DC2581A

DC2581A SETUP

–

AIN0 AIN0+

AIN0–

VCM

R107

49.9Ω

U25B

LT6237IDD

U25A

LT6237IDD

C96

0.1µF

C94

1000pF

C98

OPT

C116

OPT

R129

R108

24.9Ω

R123

24.9Ω

C104

0.1µF

VCC

VEE

R124

49.9Ω

C100

39pF

C112

39pF

C102

1000pF

DC2581a F02

R113

0Ω

R119

DC Power

TheDC2581A requires±9VDC anddraws +132mA/–52mA.

Most of the supply current is consumed by the FPGA, 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

FPGA and op amps.

Clock Source

You must provide a low jitter 2.5VP-P sine or square wave

to the clock input, J1. The clock input is AC coupled so the

DC level of the clock signal is not important. A generator

such as the Rohde & Schwarz SMB100A high speed clock

source is recommended to drive the clock input. Even a

good 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 sample rate. The ratio of clock frequency to

conversion rate is shown in the Assembly Options table. If

the clock input is to be driven with logic, it is recommended

that the 49.9Ω termination resistor (R4) be removed.

Driving R4 with discrete logic may result in slow rising

edges. These 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 intoa 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 should be latched using the

negative edge of this signal. The data output signal levels

at P1 can also be increased to 0V-3.3V if the application

circuit requires a higher voltage. This is accomplished by

moving JP2 to the 3.3V position.

Figure 2. 0V–4.096V Single-Ended to Fully Differential DC Coupled Driver

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DEMO MANUAL DC2581A

Reference

The default reference is the LTC2341 internal 4.096V

reference. Alternatively, if a higher reference voltage is

desired, the LTC6655-5 reference (U7) can be used by

setting the REF jumper (JP1) to the EXT position and

installing a 0Ω resistor in the R7 position. This should

result in better SNR performance but may slightly degrade

the THD performance of the LTC2341.

Analog Inputs

Both inputs have the same driver circuitry. The circuit of

Figure 2 shows the driver for AIN0. It provides aDC coupled

single-ended to fully differential output to the analog inputs

of the LTC2341 with a maximum 0V-4.096V input signal.

DC890 Data Collection

For SINAD, THD or SNR testing a low noise, low distortion

generatorsuch asthe B&KType 1051 or Stanford Research

SR1 should be used. A low jitter RF oscillator such as the

Rohde & Schwarz SMB100A is used to drive the clock

input. This demo board is tested in house by attempting

to duplicate the FFT plot shown in Typical Performance

Characteristics section of the LTC2341 data sheet. This

involves using a 60MHz clock source, along with a sinu-

soidal generator at a frequency of approximately 2kHz. The

input signal level is approximately –1dBFS. A typical FFT

obtained with DC2581A is shown in Figure 3. Note that

to calculate the real SNR, the signal level (F1 amplitude =

–1.133dB) has to be added back to the SNR that PScope

displays. With the example shown in Figure 3 this means

that the actual SNR would be 94.63dB instead of the

93.50dB that PScope displays. Taking the RMS sum of the

recalculated SNR and the THD yields a SINAD of 94.58dB

which is fairly close to the typical number for this ADC.

DC2581A SETUP

To change the default settings for the LTC2341 in PScope,

click on the Set Demo Bd Options button in the PScope

tool bar shown in Figure 4. This will open the Configure

Channels menu of Figure 5. In this menu it is possible to

set the input signal range setting for each channel. There

is also a button to return PScope to the default DC2581A

settings which are optimized for the default hardware

settings of the DC2581A.

There are a number of scenarios that can produce mislead-

ing results when evaluating an ADC. One that is common

is feeding the converter with an input frequency that is

a sub-multiple 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 signal 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 an

amplifier or clock source with poor phase noise is used,

the windowing will not improve the SNR.

DC590/DC2026 Data Collection

Due to the relatively low and somewhat unpredictable

sample rate of the DC590/DC2026 its usefulness is lim-

ited to noise measurement and data collection of slowly

moving signals. A typical data capture and histogram are

shown in Figure 6. To change the default settings for the

LTC2341 in QuikEval, click on the Channel Config but-

ton. This will open the Config Dialog menu of Figure 7.

In this menu it is possible to set the input signal range for

each sequence. There is also a button to return QuikEval

to the default DC2581A settings which are optimized for

the default hardware settings of the DC2581A.

dc2581af

DEMO MANUAL DC2581A

DC2581A SETUP

Layout

As with any high performance ADC, this part is sensitive

to layout. The area immediately surrounding the ADC on

the DC2581A 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 LTC2341. A ground plane is necessary

to obtain maximum performance. Keep bypass capacitors

as close to supply pins as possible. Use individual low

impedance returns for all bypass capacitors. Use of a

symmetrical layout around the analog inputs will minimize

the effectsof parasitic elements.Shield analog 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

LTC2341, 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 heat-

ing. Because of their low voltage coefficients, to further

reduce distortion NPO or silver mica capacitors should be

used. Any buffer used to drive the LTC2341 should have

low distortion, low noise and a fast settling time such as

the LT6237.

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DEMO MANUAL DC2581A

DC2581A SETUP

Figure 3. PScope Screen Shot

Figure 4. PScope Tool Bar

dc2581af

DEMO MANUAL DC2581A

DC2581A SETUP

Figure 5. PScope Configuration Menu

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DEMO MANUAL DC2581A

DC2581A SETUP

Figure 6. QuikEval Screen Shot

Figure 7. QuikEval Configuration Menu

dc2581af

DEMO MANUAL DC2581A

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 representa-

tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

DC2581A CONNECTORS

DC2581A JUMPERS

Definitions

JP1: REF selects INT or EXT reference for the ADC. The

default setting is INT.

JP2: 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 DC2026.

JP3: I/O selects LVDS or CMOS logic levels. The default

setting is CMOS. Only CMOS is currently supported.

Definitions

P1: DC890 interface is used to communicate with the

DC890 controller.

J1: CLK provides the master clock for the DC2581A when

interfaced to the DC890.

J2: FPGA PROGRAM is used to program the FPGA. This

is for factory use only.

JP4: EEPROM is for factory use only. The default posi-

tion is WP.

JP5, JP8: AIN0-AIN1 can be used to short individual AIN

inputs to ground or can be used drive the individual AIN

inputs. The default is to leave these open.

JP13-JP14: Coupling selects AC or DC coupling for VIN1

and VIN2. Default setting is DC.

JP15: Sets VCM for the single-end to differential drivers

of AIN0-AIN1

J3: JTAG is for factory use only.

J4: DC590/DC2026 interface is used to communicate with

the DC2026 Linduino controller or DC590.

J5 and J6: Provide analog input voltages to AIN0-AIN1

of the ADC.

J7: Routes the signals of J5 and J6 to AIN0-AIN1.

dc2581af

DEMO MANUAL DC2581A

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ●FAX: (408) 434-0507 ●www.linear.com

 LINEAR TECHNOLOGY CORPORATION 2016

LT 1016 • PRINTED IN USA

DEMONSTRATION BOARD IMPORTANT NOTICE

Linear Technology Corporation (LT C ) 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 LT C 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

FOR ANY PARTICULAR PURPOSE. EXCEPT TO THE EXTENT OF THIS INDEMNITY, NEITHER PARTY SHALL BE LIABLE TO THE OTHER FOR

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 LT C 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. LT C assumes no liability for applications assistance,

customer product design, software performance, or infringement of patents or any other intellectual property rights of any kind.

LT C 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 LT C applica-

tion engineer.

Mailing Address:

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1630 McCarthy Blvd.

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