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PDF LT1994 Data sheet ( Hoja de datos )
| Número de pieza | LT1994 | |
| Descripción | Low Noise - Low Distortion Fully Differential Input/Output Amplifier/Driver | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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FEATURES
■ Fully Differential Input and Output
■ Wide Supply Range: 2.375V to 12.6V
■ Rail-to-Rail Output Swing
■ Low Noise: 3nV/√Hz
■ Low Distortion, 2VP-P, 1MHz: –94dBc
■ Adjustable Output Common Mode Voltage
■ Unity Gain Stable
■ Gain-Bandwidth: 70MHz
■ Slew Rate: 65V/μs
■ Large Output Current: 85mA
■ DC Voltage Offset <2mV MAX
■ Open-Loop Gain: 100V/mV
■ Low Power Shutdown
■ 8-Pin MSOP or 3mm × 3mm DFN Package
U
APPLICATIO S
■ Differential Input A/D Converter Driver
■ Single-Ended to Differential Conversion
■ Differential Amplification with Common Mode
Translation
■ Rail-to-Rail Differential Line Driver/Receiver
■ Low Voltage, Low Noise, Differential Signal
Processing
LT1994www.DataSheet4U.com
Low Noise, Low Distortion
Fully Differential Input/
Output Amplifier/Driver
DESCRIPTIO
The LT®1994 is a high precision, very low noise, low distor-
tion, fully differential input/output amplifier optimized for
3V, single supply operation. 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. A separate internal common mode feedback
path provides accurate output phase balancing and reduced
even-order harmonics. This makes the LT1994 ideal for
level shifting ground referenced signals for driving dif-
ferential input, single supply ADCs.
The LT1994 output can swing rail-to-rail and is capable
of sourcing and sinking up to 85mA. In addition to the
low distortion characteristics, the LT1994 has a low input
referred voltage noise of 3nV/√Hz. This part maintains
its performance for supply voltages as low as 2.375V. It
draws only 13.3mA of supply current and has a hardware
shutdown feature that reduces current consumption to
225µA.
The LT1994 is available in an 8-pin MSOP or 8-pin DFN
package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATIO
A/D Preamplifier: Single-Ended Input to Differential Output with Common
Mode Level Shifting
VIN
2VP-P
499Ω
0.1µF
499Ω
3V
0.1µF
–+
VOCM LT1994
+–
24.9Ω
24.9Ω
47pF
VOCM = 1.5V
3V
10µF
AIN+
VDD SD0
CONV
LTC1403A-1
SCK
AIN–
GND VREF
50.4MHz
10µF
499Ω
499Ω
1994 TA01
LT1994 Driving an LTC1403A-1 1MHz
Sine Wave, 8192 Point FFT Plot
0
–10
FSAMPLE = 2.8Msps
FIN = 1.001MHz
–20 INPUT = 2VP-P,
–30
SINGLE ENDED
SFDR = 93dB
–40
–50
–60
–70
–80
–90
–100
–110
–120
0
0.35 0.70 1.05
FREQUENCY (MHz)
1.40
1994 TA01b
1994fa
1
1 page  
LT1994www.DataSheet4U.com
ELECTRICAL CHARACTERISTICS
Note 7: Input Common Mode Range is tested using the Test Circuit of
Figure 1 (RF = RI) by applying a single ended 2VP-P, 1kHz signal to VINP
(VINM = 0), and measuring the output distortion (THD) at the common
mode Voltage Range limits listed in the Electrical Characteristics table,
and confirming the output THD is better than –40dB. The voltage range for
the output common mode range (Pin 2) is tested using the Test Circuit of
Figure 1 (RF = RI) by applying a 0.5V peak, 1kHz signal to the VOCM
Pin 2 (with VINP = VINM = 0) and measuring the output distortion (THD)
at VOUTCM with VOCM biased 0.5V from the VOCM pin range limits listed
in the Electrical Characteristics Table, and confirming the THD is better
than –40dB.
Note 8: Input CMRR is defined as the ratio of the change in the input
common mode voltage at the pins IN+ or IN– to the change in differential
input referred voltage offset. Output CMRR is defined as the ratio of the
change in the voltage at the VOCM pin to the change in differential input
referred voltage offset.
Note 9: Differential Power Supply Rejection (PSRR) is defined as the ratio
of the change in supply voltage to the change in differential input referred
voltage offset. Common Mode Power Supply Rejection (PSRRCM) is
defined as the ratio of the change in supply voltage to the change in the
common mode offset, VOUTCM – VOCM.
Note 10: Output swings are measured as differences between the output
and the respective power supply rail.
Note 11: Extended operation with the output shorted may cause junction
temperatures to exceed the 150°C limit for the MSOP package (or 125°C
for the DD package) and is not recommended.
TYPICAL PERFOR A CE CHARACTERISTICS
Differential Input Referred
Voltage Offset vs Temperature
500
VS = 3V
VCM = 1.5V
VOCM = 1.5V
250 FOUR TYPICAL UNITS
Common Mode Voltage Offset vs
Temperature
7.5
VS = 3V
VCM = 1.5V
VOCM = 1.5V
5.0 FOUR TYPICAL UNITS
Input Bias Current and Input
Offset Current vs Temperature
–10
IB, VS = ±5V
–15
1.0
0.5
0
–250
–500
2.5
0
–750
–50 –25
0
25 50
TEMPERATURE (°C)
75 100
1994 G01
–2.5
–50 –25
0
25 50
TEMPERATURE (°C)
75 100
1994 G02
IOS, VS = 3V
–20
IOS, VS = ±5V 0
–25
IB, VS = 3V
–30
–50 –25 0
25 50
TEMPERATURE (°C)
–0.5
–1.0
75 100
1994 G03
Gain Bandwidth vs Temperature
72
71
VS = ±5V
70
VS = 3V
69
68
67
66
–50 –25 0
25 50
TEMPERATURE (°C)
75 100
1994 G04
Frequency Response vs
Supply Voltage
2
RF = RI = 499Ω
VS = 3V VS = 2.5V
1
VS = 5V
0
VS = ±5V
–1
–2
0.1
1 10
FREQUENCY (MHz)
100
1994 G05
Frequency Response vs
Load Capacitance
2
RF = RI = 499Ω
VS = 2.5V
1
VS = 3V
0
–1
–2
0.1
5pF FROM EACH
OUTPUT TO GROUND
25pF FROM EACH
OUTPUT TO GROUND
1 10
FREQUENCY (MHz)
100
1994 G06
1994fa
5
5 Page 
LT1994www.DataSheet4U.com
APPLICATIO S I FOR ATIO
a need for ADCs to process signals differentially in order
to maintain good signal to noise ratios. These ADCs are
typically supplied from a single supply voltage that can be
as low as 2.5V and will have an optimal common mode
input range near mid-supply. The LT1994 makes interfac-
ing to these ADCs trivial, by providing both single ended
to differential conversion as well as common mode level
shifting. Figure 1 shows a general single supply application
with perfectly matched feedback networks from OUT+ and
OUT–. The gain to VOUTDIFF from VINM and VINP is:
( )VOUTDIFF
= VOUT+
– VOUT –
≈
RF
RI
•
VINP
– VINM
Note from the above equation that the differential output
voltage (VOUT+ – VOUT–) is completely independent of input
and output common mode voltages, or the voltage at the
common mode pin. This makes the LT1994 ideally suited
pre-amplification, level shifting, and conversion of single
ended signals to differential output signals in preparation
for driving differential input ADCs.
Effects of Resistor Pair Mismatch
Figure 2 shows a circuit diagram that takes into consid-
eration that real world resistors will not perfectly match.
Assuming infinite open loop gain, the differential output
relationship is given by the equation:
VOUTDIFF
=
VOUT +
–
VOUT –
≅
RF
RI
• VINDIFF
+
∆β
βAVG
• VICM
–
∆β
βAVG
• VOCM,
where: RF is the average of RF1 and RF2, and RI is the
average of RI1 and RI2.
βAVG is defined as the average feedback factor (or gain)
from the outputs to their respective inputs:
βAVG
=
1
2
•
⎛
⎝⎜
RI2
RI2 + RF2
+
RI1R+I1RF1⎞⎠⎟
Δβ is defined as the difference in feedback factors:
∆β = RI2 – RI1
RI2 + RF2 RI1 + RF1
VICM is defined as the average of the two input voltages,
VINP and VINM (also called the input common mode
voltage):
( )VICM
=
1•
2
VINP
+ VINM
and VINDIFF is defined as the difference of the input
voltages:
VINDIFF = VINP – VINM
When the feedback ratios mismatch (Δβ), common mode
to differential conversion occurs.
Setting the differential input to zero (VINDIFF = 0), the de-
gree of common mode to differential conversion is given
VINM
VCM
VINP
RI VIN– RF
V+
0.1µF
3
VOCM
0.1µF
1–
2
8
VOCM
+
+4
LT1994
–
5
6
7
VSHDN
0.1µF
RI RF V–
VIN+
VOUT+
RL
RBAL
0.1µF
VOUTCM
RBAL
VOUT–
RL
1994 F01
Figure 1. Test Circuit
RI2 VIN– RF2
VOUT+ RL
VINM
VINP
VS
3
VOCM
0.1µF
1–
2
8
VOCM
+
SHDN
+4
LT1994
–
5
6
7
VSHDNB
RI1 RF1
VIN+
0.1µF
VOUT– RL
1994 F02
Figure 2. Real-World Application
1994fa
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LT1994.PDF ] | |
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