Linear Technology DC961B Guida rapida alla configurazione

dc961bf

DEMO MANUAL DC961B

Description

LT1994

Low Noise, Low Distortion,

Fully Differential Amplifier/Driver

Demonstration circuit 961 features an LT

1994, low noise,

low distortion, fully differential amplifier. The LT1994 is a

high precision, very low noise, low distortion, fully differ-

ential input/output amplifier (see Table 1). The LT1994’s

output common mode voltage is independent of the input

common mode voltage, and is adjustable by applying a

voltage on the VOCM pin. The DC961 board contains an

LT1994 amplifier configured as a unity gain differential

amplifier with 499Ω feedback and input resistors. Gains

greater than one require changing the input resistors to

a value lower than 499Ω (refer to Figure 2). In addition,

DC961 has surface mount pads and traces for resistors

and capacitors for building first and second order fully

differential filter circuits. The differential outputs of DC961

can be configured with a first order RC network for driv-

ing the differential inputs of an analog-to-digital converter

(ADC).

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Analog

Devices, Inc. All other trademarks are the property of their respective owners.

Connection to the differential input and output of a DC961

is through SMA connectors. Onboard jumpers configure

the DC961 for dual or single power supply operation. The

differential input of a DC961 is AC coupled with 1µF capac-

itors for ease of use as a dual or a single supply circuit.

DC coupling to the DC961 input is provided through the

shorting of the input capacitors with 0Ω surface–mount

resistor jumpers.

Table 1. LT1994 Noise and Distortion

Differential Input Referred Voltage Noise

Density

3nV/RTHz

Distortion,

2VP-P Differential Input,

VS= 3V, fIN = 1MHz, RLOAD = 800Ω

2nd Harmonic

3rd Harmonic

99dBc

96dBc

Design files for this circuit board are available at

http://www.linear.com/demo/DC961B

Figure 1. DC961B

Figure 2. Typical Application for an LT1994

VOUT–

VOUT+

V+

0.1µF

dc961b F02

499Ω

(RIN)

VIN+

VIN–499Ω

499Ω

499Ω RECOMMENDED ADCs:

LTC2202/LTC2203 AND

LTC2245/LTC2246

1µF

0.1µF AIN–

AIN+

ADC

–

LT1994VOCM

VCM

(RIN)

(VOUT+– VOUT–) 499Ω

(VIN+– VIN–)

dc961bf

DEMO MANUAL DC961B

Quick test proceDure

A. Single-Ended Input to Differential Output

1. Connect to a DC961 a dual power supply, a function

generator and an oscilloscope as shown in Figure 3

(the 50Ω termination on J2 input is used to balance

the 50Ω generator impedance on J1 input).

2. Set the function generator for a 1VP-P, 100kHz sin-

ewave and turn on the power supply.

3. The channel 2 input of the oscilloscope is in phase with

the DC961 input and the channel 1 input is 180 degrees

out of phase with the DC961 input. The single-ended

output shown on channel 1 or 2 is a 0.5VP-P sinewave

(a 1VP-P differential output).

Note 1:The LT1994 can directly drive at least a 25pF

capacitive load at each output. However the LT1994 can

drive directly a low frequency sinewave (100kHz or less)

into a capacitive load of up to 100pF. In this Quick Test

Procedure, the output signal is a sinewave and each

LT1994 output drives the capacitance of a 24 inch or

less cable plus the input capacitance of the oscilloscope

input, a capacitive load of 70pF (30pF per foot for the coax

cable and 10pF for the oscilloscope input). For testing the

transient response of the LT1994 to a squarewave or a

pulse, use a 10x low capacitance oscilloscope probe to

monitor the DC961 output at J3 or J4.

Figure 3. Single-Ended Input to Differential Output Quick Test Setup

V+

PWR_ON

JP1

SHDN

0.1µF

LT1994VOCM

0.1µF

R12, 50Ω J3

CH1 CH2

CH2

OSCILLOSCOPE

COAX CABLE

CH1

EXT

R15, 50Ω

499Ω

DC961b

0.1µF

499Ω

VOUT+SMA

BNC 24 INCHES OR

LESS COAX CABLE

DC961 JUMPERS SETTINGS:

JP1 LT1994, SHDN PIN TO GND OR OPEN

JP2 LT1994, V–PIN TO V–OR GND

JP3 LT1994, VOCM PIN TO V+/2 OR

AN EXTERNAL CONNECTION

SET THE OSCILLOSCOPE

TO EXTERNAL TRIGGER

SMA

BNC

SMA

VOUT–

JP2 SINGLE

DUAL

499Ω

C12, 1µF

0Ω 0Ω

JP3 EXT

10k

V+

V–

INT

EXT

VOCM 100Ω

499Ω

C8, 1µF

FUNCTION

GENERATOR

VIN

VIN–

50Ω

TERMINATION

BNC

SMA

VIN+

50Ω

OUTPUT

IMPEDANCE

SET FUNCTION

GENERATOR

TO A 1VP-P,

100kHz

SINEWAVE

BNC_T

DUAL POWER SUPPLY

0Ω 0Ω

5V –5V

V+V–

TO JP2

DC961

–

dc961bf

DEMO MANUAL DC961B

Quick test proceDure

Figure 4. Input and Output Common Mode Quick Test Setup

B. DC Coupled Inputs and Output Common Mode

Voltage Adjustment

1. On the DC961, install 0603 0Ω resistors at R1 and R10

to short input capacitors C8 and C12 respectively (see

DC961 schematic).

2. Connect DC961 as shown in Figure 4 (JP1 to PWR_ON,

JP2 to SINGLE, and JP3 to EXT VOCM).

3. Apply an input signal with a DC offset (VINCM) 0V to

V+–1.25V. The output common mode (VOCM) can be

set independently of the VINCM from 1.1V to V+–0.8V.

This adjustment is made by applying a DC voltage at

EXT VOCM.

V+

PWR_ON

JP1

SHDN

0.1µF

LT1994VOCM

0.1µF

R12, 50Ω J3

CH1 CH2

CH2

OSCILLOSCOPE

CH1

R15, 50Ω

499Ω

DC961b F04

0.1µF

499Ω

VOUT+

SMA

BNC

24 INCHES OR

LESS OF COAX CABLE

DC961 JUMPERS SETTINGS:

JP1 LT1994, SHDN PIN TO GND OR OPEN

JP2 LT1994, V–PIN TO V–OR GND

JP3 LT1994, VOCM PIN TO V+/2 OR

AN EXTERNAL CONNECTION

SET OSCILLOSCOPE

TRIGGER TO CH2

SMA

BNC

SMA

VOUT–

JP2 SINGLE

DUAL

499Ω

C12, 1µF

0Ω 0Ω

JP3 EXT

10k

V+

V–

INT

EXT

VOCM 100Ω

499Ω

C8, 1µF

FUNCTION

GENERATOR

VINCM

VIN–

50Ω

TERMINATION

VOCM

VOLTAGE

SOURCE

BNC

SMA

VIN+

50Ω

OUTPUT

IMPEDANCE

SET FUNCTION GENERATOR

TO A 1VP-P, 100kHz SINEWAVE

WITH VARIABLE DC OFFSET

REGISTOR

JUMPER

REGISTOR

JUMPER

0V ≤ VINCM ≤ V+–1.25V

1.1V ≤ VOCM ≤ V+–0.8V

R1, 0Ω

0Ω 0Ω

1.1V ≤ VOCM ≤ V+–0.8V

3V TO 5V

V+V–

TO JP2

DC961

–

R10, 0Ω

VOCM

dc961bf

DEMO MANUAL DC961B

D. Using a DC961 to Implement a Fully Differential,

Second Order Lowpass Filter.

Quick test proceDure

Figure 5. DC961 Output Component Values

C. Driving the Analog Inputs of an ADC

Figure 5 shows the DC961 output components when driv-

ing an ADC. Optimum values for the DC961 output com-

ponents when driving an LTC2202/LTC2203 or LTC2245/

LTC2246 ADC.

Figure 6. A DC961 Configured as a Second Order

Lowpass Filter

Figure 7. Fully Differential, 2nd Order, Lowpass Filter

Design Schematic

VOUT+

DC961

VOUT–

LT1994

OUT–

LT1994

OUT+

dc961b F05

C21

10pF

R12, 49.9Ω J3 TO DIFFERENTIAL

INPUT OF AN

LTC2202/LTC2203

LTC2245/LTC2246

ADC

R15, 49.9Ω

C22

10pF

C18

10pF VOUT–

dc961b F06

VOUT+

VIN–

VIN+

V+

C11

0.1µF

LT1994VOCM

R12, 50Ω

R15, 50Ω

0.1µF

C12, 1µF

0Ω

–

R10, 0Ω

R9R2

C8, 1µF Z1

R1, 0Ω

DC961

dc961b F07

V+

C11

0.1µF

LT1994

C22

C21

R32

R22

R12

–

R31

R11

R21

dc961bf

DEMO MANUAL DC961B

Design Procedure

The following procedure is from the LT1994 data sheet.

The design schematic resistor and capacitor designators

have the following correspondence with the DC961 resis

tor and capacitor designators:

Design Schematic DC961

R11 Z1

R21 R3

R31 R9

C11 C11

C21 C5

C22 C6

R12 Z2

R22 R7

R32 R8

Differential 2nd Order Butterworth Lowpass Filter

f3dB ≤ 2.5MHz and Gain ≤ 8.8 or Gain ≤

2.5MHz

3dB

Component Calculation:

R11 = R12, R21 = R22, R31 = R32, C21 = C22, C11 = 10

•C21, R1 = R11, R2 = R21, R3 = R31, C2 = C21 and C1

= C11

1. Calculate an absolute value for C2 (C2abs) using a

specified –3dB frequency

C2abs =4•10

3dB

C2abs IN pF AND f3dB IN kHz

( )

2. Select a standard 5% capacitor value nearest the abso-

lute value for C2(C1 = 10 •C2)

3. Calculate R3, R2 and R1 using the standard 5% C2

value, the specified f3dB and the specified passband

gain (GN):

R1, R2 and R3 equations (C2 in pF and f3dB in kHz)

R3 =

1.121– (1.131– 0.127 •GN)

( )

•108

GN+1

( )

•C2 •f3dB

R2 =1.266 •1015

R3 •C22•f

3dB

2R1=R2

Quick test proceDure

dc961bf

DEMO MANUAL DC961B

 LINEAR TECHNOLOGY CORPORATION 2017

LT 0617 • PRINTED IN USA

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

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 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

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