Linear Technology LTC2377-20 Manuale utente

dc1925af

DEMO MANUAL DC1925A

DESCRIPTION

LTC2378-20/LTC2377-20/LTC2376-20

20-Bit,1Msps/500ksps/250ksps, Low

Power, SAR ADCs with 104dB SNR

TheLT C

2378-20,LTC2377-20andLTC2376-20are20-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 perfor-

mance 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

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

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 recom-

mended 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.

dc1925af

DEMO MANUAL DC1925A

ASSEMBLY OPTIONS

DC890 QUICK START PROCEDUREDC890 QUICK START PROCEDURE

DC590 SETUP

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

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 refer-

ence 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.

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 DC1925Ashould be in the3.3V 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) sup-

plied 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.

dc1925af

DEMO MANUAL DC1925A

DC1925A SETUP

DC Power

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.

Clock Source

You must provide alowjitter 2.5VP-P (ifVCCIO isin 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

presenceof high amplitude higherfrequency 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 aspreadsheet, 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.

Reference

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.

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 single-

ended signal to drive the ADC, the circuit shown in Figure3

can be used. The circuit of Figure 3 converts a single-

ended 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 remov-

ing 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

dc1925af

DEMO MANUAL DC1925A

DC1925A SETUP

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

characteristicsof 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 Fig-

ure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 Fig-

ure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 3. Single-Ended-to-Differential Driver

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 a 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 sine generator capable of ppm frequency ac-

curacy 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 approxima-

tion 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

closetosupplypinsaspossible.Uselowimpedancereturns

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

Figure 4. Differential Level Shifter

dc1925af

DEMO MANUAL DC1925A

DC1925A SETUP

Figure 5. PScope Screen Shot

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.

dc1925af

DEMO MANUAL DC1925A

DC1925A SETUP

Figure 6. QuikEval Screen Shot

dc1925af

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 set-

ting 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.

dc1925af

DEMO MANUAL DC1925A

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

PARTS LIST

dc1925af

DEMO MANUAL DC1925A

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

DC1925A-A

1 1 GENERAL BOM DC1925A

2 1 U1 IC, HIGH SPEED, LOW NOISE SAR ADC LINEAR TECH., LTC2378CMS-20#PBF

3 1 FAB, PRINTED CIRCUIT BOARD DEMO CIRCUIT 1925A-2

DC1925A-B

1 1 GENERAL BOM DC1925A

2 1 U1 IC, HIGH SPEED, LOW NOISE SAR ADC LINEAR TECH., LTC2377CMS-20#PBF

3 1 FAB, PRINTED CIRCUIT BOARD DEMO CIRCUIT 1925A-2

DC1925A-C

1 1 GENERAL BOM DC1925A

2 1 U1 IC, HIGH SPEED, LOW NOISE SAR ADC LINEAR TECH., LTC2376CMS-20#PBF

3 1 FAB, PRINTED CIRCUIT BOARD DEMO CIRCUIT 1925A-2

PARTS LIST

dc1925af

DEMO MANUAL DC1925A

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