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