Linear Technology DC961B Demo Manual

Description

DEMO MANUAL DC961B

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 differ­ential 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 V

pin. The DC961 board contains an

OCM

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 driv­ing the differential inputs of an analog-to-digital converter
(ADC).

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 capac­itors 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

Distortion,

Differential Input,

2V

P-P

V

= 3V, fIN = 1MHz, R

S

LOAD

= 800Ω
2nd Harmonic
3rd Harmonic

3nV/

99dBc
96dBc

Hz

RT

Design files for this circuit board are available at

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

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.

Figure 1. DC961B

1µF

)

(R

1µF

IN

499Ω

0.1µF

499Ω

(R

IN

)

+

V

IN

–

V

IN

+

V

OCM

+––

499Ω

+

LT1994V

499Ω

0.1µF

V

OUT

V

OUT

–

+

+

–

– V

(V

OUT

(V

IN

RECOMMENDED ADCs:
LTC2202/LTC2203 AND
LTC2245/LTC2246

) 499Ω

OUT

+

– V

=

–

)

IN

(RIN)

+

A

IN

ADC

–

A

IN

V

CM

dc961b F02

Figure 2. Typical Application for an LT1994

dc961bf

1

DEMO MANUAL DC961B

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 1V

, 100kHz sin-

P-P

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 0.5V
(a 1V

SET FUNCTION
GENERATOR
TO A 1V
100kHz
SINEWAVE

differential output).

P-P

FUNCTION
GENERATOR

V

50Ω

BNC_T

V

IN

BNC

SMA

V

BNC

SMA

TO

IN

J2

TO

IN

50Ω
OUTPUT
IMPEDANCE

,

P-P

TERMINATION

J1

+

–

C12, 1µF

EXT

V

C8, 1µF

OCM

R2

0Ω 0Ω

SHDN

R6

0Ω 0Ω

100Ω

sinewave

P-P

DUAL POWER SUPPLY

5V –5V

+

V

R9

Z1

499Ω

PWR_ON

JP1

0.1µF

R8

Z2

499Ω

JP3

EXT

INT

V

8

7

2

1

+

V

10k

10k

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.

COAX CABLE

–

TO JP2

+

V

3

–

OCM

+

6

+

LT1994V

–

DUAL

R5

499Ω

0.1µF

5

4

0.1µF

R4

499Ω

JP2

SINGLE

–

V

DC961

R12, 50Ω

R15, 50Ω

J3

V

OUT

J4

V

OUT

DC961 JUMPERS SETTINGS:
JP1 LT1994, SHDN PIN TO GND OR OPEN
JP2 LT1994, V
JP3 LT1994, V
AN EXTERNAL CONNECTION

–

+

SMA
TO
BNC

SMA
TO
BNC

OSCILLOSCOPE

CH1 CH2

24 INCHES OR
LESS COAX CABLE

–

PIN TO V– OR GND

PIN TO V+/2 OR

OCM

CH2

CH1

EXT

SET THE OSCILLOSCOPE
TO EXTERNAL TRIGGER

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

2

dc961bf

Quick test proceDure

DEMO MANUAL DC961B

FUNCTION
GENERATOR

50Ω
OUTPUT
IMPEDANCE

V

CM

IN

CM ≤ V+ –1.25V

0V ≤ V

IN

TERMINATION

1.1V ≤ V

SET FUNCTION GENERATOR
TO A 1V
WITH VARIABLE DC OFFSET

50Ω

OCM

V

OCM

VOLTAGE
SOURCE

P-P,

V

BNC

SMA

V

BNC

SMA

≤ V+ –0.8V

100kHz SINEWAVE

+

IN

J1

TO

–

IN

J2

TO

R1, 0Ω

C8, 1µF

R10, 0Ω

C12, 1µF

REGISTOR
JUMPER

REGISTOR
JUMPER

EXT

V

OCM

R2

0Ω 0Ω

SHDN

JP1

R6

Z2

0Ω 0Ω

100Ω

3V TO 5V

V

Z1

499Ω

JP3

+

499Ω

PWR_ON

0.1µF

R8

EXT

INT

1.1V ≤ V

DC961

–

V

TO JP2

–

+

V

+

3

OCM

6

+

LT1994V

–

DUAL

R5

499Ω

0.1µF

5

4

0.1µF

R4

499Ω

JP2

SINGLE

–

V

R12, 50Ω

R15, 50Ω

R9

8

7

2

1

+

V

10k

10k

≤ V+ –0.8V

OCM

OSCILLOSCOPE

CH2

CH1

CH1 CH2

SMA
TO
BNC

J3

–

V

OUT

J4

+

V

OUT

SMA
TO
BNC

1.1V ≤ V

DC961 JUMPERS SETTINGS:
JP1 LT1994, SHDN PIN TO GND OR OPEN
JP2 LT1994, V
JP3 LT1994, V
AN EXTERNAL CONNECTION

24 INCHES OR
LESS OF COAX CABLE

≤ V+ –0.8V

OCM

–

PIN TO V– OR GND

OCM

SET OSCILLOSCOPE
TRIGGER TO CH2

V

OCM

PIN TO V+/2 OR

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 V

OCM

).

DC961b F04

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

+

–1.25V. The output common mode (V

V
set independently of the V

CM from 1.1V to V+ –0.8V.

IN

OCM

) can be

This adjustment is made by applying a DC voltage at

OCM

.

EXT V

dc961bf

3

DEMO MANUAL DC961B

dc961b F07

Quick test proceDure

C. Driving the Analog Inputs of an ADC

Figure 5 shows the DC961 output components when driv­ing an ADC. Optimum values for the DC961 output com­ponents when driving an LTC2202/LTC2203 or LTC2245/
LTC2246 ADC.

C21
10pF

DC961

C18
10pF

C22
10pF

J3

–

V

OUT

J4

+

V

OUT

TO DIFFERENTIAL
INPUT OF AN
LTC2202/LTC2203
OR
LTC2245/LTC2246
ADC

dc961b F05

R12, 49.9Ω

LT1994

–

OUT

R15, 49.9Ω

LT1994

+

OUT

Figure 5. DC961 Output Component Values

D. Using a DC961 to Implement a Fully Differential,
Second Order Lowpass Filter.

DC961

R1, 0Ω

+

V

IN

C8, 1µF

J1

R10, 0Ω

–

V

IN

C12, 1µF

J2

0Ω

R6

0Ω

Z1

Z2

Figure 6. A DC961 Configured as a Second Order
Lowpass Filter

C11

R3

R9R2

R8

R7

C5

+

V

0.1µF

3

8

–

+

7

LT1994V

OCM

2

1

–

+

6

C6

5

4

0.1µF

R12, 50Ω

R15, 50Ω

dc961b F06

J3

V

OUT

J4

V

OUT

–

+

R21

R11

R12

R31

C11

0.1µF

R32

R22

C21

+

V

0.1µF

3

8

5

–

+

2

LT1994

1

–

+

4

6

7

C22

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

4

dc961bf

2.5MHz

3dB

5

3dB

( )

3dB

N

Quick test proceDure

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

f

≤ 2.5MHz and Gain ≤ 8.8 or Gain ≤

3dB

f

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 (C2

) using a

abs

specified –3dB frequency

abs

=

4 • 10

f

C2

( )

IN pF AND f

abs

3dB

IN kHz

C2

2. Select a standard 5% capacitor value nearest the abso­lute value for C2(C1 = 10 • C2)

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

and the specified passband

3dB

gain (GN):

R1, R2 and R3 equations (C2 in pF and f

3dB

in kHz)

1.121– (1.131– 0.127 • G

R3 =

GN+ 1

( )

15

R2 =

1.266 • 10

R3 • C22• f

2

• C2• f

3dB

)

N

R1=

• 10

R2
G

8

dc961bf

5

DEMO MANUAL DC961B

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

Copyright © 2004, Linear Technology Corporation

6

dc961bf

LT 0617 • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 2017