Linear Technology DC961B Quick setup guide

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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)
(RIN)
VIN+
VIN–499Ω
499Ω
499Ω RECOMMENDED ADCs:
LTC2202/LTC2203 AND
LTC2245/LTC2246
1µF
1µF
0.1µF AIN–
AIN+
ADC
+
–
–
+
LT1994VOCM
VCM
(RIN)
=
(VOUT+– VOUT–) 499Ω
(VIN+– VIN–)

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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+
3
6
1
2
7
PWR_ON
JP1
SHDN
85
4
0.1µF
LT1994VOCM
0.1µF
R12, 50Ω J3
J4
CH1 CH2
CH2
OSCILLOSCOPE
COAX CABLE
CH1
EXT
R15, 50Ω
R4
499Ω
DC961b
0.1µF
499Ω
VOUT+SMA
TO
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
TO
BNC
J2
BNC
TO
SMA
VOUT–
JP2 SINGLE
DUAL
R8
499Ω
R6
C12, 1µF
0Ω 0Ω
R5
JP3 EXT
10k
10k
V+
V–
INT
EXT
VOCM 100Ω
Z2
R9
499Ω
R2
C8, 1µF
FUNCTION
GENERATOR
VIN
VIN–
50Ω
TERMINATION
J1
BNC
TO
SMA
VIN+
50Ω
OUTPUT
IMPEDANCE
SET FUNCTION
GENERATOR
TO A 1VP-P,
100kHz
SINEWAVE
BNC_T
DUAL POWER SUPPLY
0Ω 0Ω
Z1
5V –5V
V+V–
TO JP2
DC961
–
+
–
+

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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+
3
6
1
2
7
PWR_ON
JP1
SHDN
85
4
0.1µF
LT1994VOCM
0.1µF
R12, 50Ω J3
J4
CH1 CH2
CH2
OSCILLOSCOPE
CH1
R15, 50Ω
R4
499Ω
DC961b F04
0.1µF
499Ω
VOUT+
SMA
TO
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
TO
BNC
J2
BNC
TO
SMA
VOUT–
JP2 SINGLE
DUAL
R8
499Ω
R6
C12, 1µF
0Ω 0Ω
R5
JP3 EXT
10k
10k
V+
V–
INT
EXT
VOCM 100Ω
Z2
R9
499Ω
R2
C8, 1µF
FUNCTION
GENERATOR
VINCM
VIN–
50Ω
TERMINATION
VOCM
VOLTAGE
SOURCE
J1
BNC
TO
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
Z1
1.1V ≤ VOCM ≤ V+–0.8V
1.1V ≤ VOCM ≤ V+–0.8V
3V TO 5V
V+V–
TO JP2
DC961
–
+
–
+
R10, 0Ω
VOCM

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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
OR
LTC2245/LTC2246
ADC
J4
R15, 49.9Ω
C22
10pF
C18
10pF VOUT–
dc961b F06
J3
VOUT+
J4
VIN–
J2
VIN+
J1
V+
3
6
1
2
7
C11
85
4
0.1µF
LT1994VOCM
R12, 50Ω
R15, 50Ω
0.1µF
C6
C5
R8
R7
R6
C12, 1µF
0Ω
0Ω
Z2
–
+
–
+
R10, 0Ω
R9R2
C8, 1µF Z1
R1, 0Ω
R3
DC961
dc961b F07
V+
3
6
7
1
2
C11
0.1µF
85
4
0.1µF
LT1994
C22
C21
R32
R22
R12
–
+
–
+
R31
R11
R21

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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
f
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
5
f
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
G
N
Quick test proceDure

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DEMO MANUAL DC961B
 LINEAR TECHNOLOGY CORPORATION 2017
LT 0617 • PRINTED IN USA
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