LINER LT1994CDD Low noise, low distortion fully differential input/output amplifi er/driver Datasheet

LT1994
Low Noise, Low Distortion
Fully Differential Input/
Output Amplifier/Driver
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DESCRIPTIO
FEATURES
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The LT®1994 is a high precision, very low noise, low distortion, 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 differential input, single supply ADCs.
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
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.
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APPLICATIO S
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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
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.
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TYPICAL APPLICATIO
LT1994 Driving an LTC1403A-1 1MHz
Sine Wave, 8192 Point FFT Plot
499Ω
499Ω
3V
10µF
3V
VIN
2VP-P
0.1µF
– +
VOCM LT1994
0.1µF
+ –
24.9Ω
24.9Ω
VOCM = 1.5V
AIN+
47pF
VDD
SD0
CONV
LTC1403A-1
AIN–
SCK
50.4MHz
GND VREF
10µF
DIFFERENTIAL OUTPUT MAGNITUDE (dB)
A/D Preamplifier: Single-Ended Input to Differential Output with Common
Mode Level Shifting
0
FSAMPLE = 2.8Msps
–10 F = 1.001MHz
IN
–20 INPUT = 2VP-P,
SINGLE ENDED
–30
SFDR = 93dB
–40
–50
–60
–70
–80
–90
–100
–110
–120
499Ω
499Ω
1994 TA01
0
1.05
0.35
0.70
FREQUENCY (MHz)
1.40
1994 TA01b
1994fa
1
LT1994
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ABSOLUTE
AXI U RATI GS
(Note 1)
Total Supply Voltage (V+ to V–) ..............................12.6V
Input Voltage (Note 2)...............................................±VS
Input Current (Note 2)..........................................±10mA
Input Current (VOCM, ⎯S⎯H⎯D⎯N) ................................±10mA
VOCM, ⎯S⎯H⎯D⎯N .............................................................±VS
Output Short-Circuit Duration (Note 3) ............ Indefinite
Operating Temperature Range (Note 4) ... –40°C to 85°C
Specified Temperature Range (Note 5) .... –40°C to 85°C
Junction Temperature
MS8 .................................................................. 150°C
DFN8................................................................. 125°C
Storage Temperature Range
MS8 ................................................... –65°C to 150°C
DFN8.................................................. –65°C to 125°C
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PACKAGE/ORDER I FOR ATIO
TOP VIEW
IN– 1
VOCM 2
V+
3
OUT+ 4
TOP VIEW
8 IN+
IN– 1
VOCM 2
V+ 3
OUT+ 4
7 SHDN
6 V
–
5 OUT–
8
7
6
5
IN+
SHDN
V–
OUT–
MS8 PACKAGE
8-LEAD PLASTIC MSOP
TJMAX = 150°C, θJA = 140°C/W
DD PACKAGE
8-LEAD (3mm × 3mm) PLASTIC DFN
TJMAX = 125°C, θJA = 160°C/W
UNDERSIDE METAL CONNECTED TO V–
ORDER PART NUMBER
DD PART MARKING*
ORDER PART NUMBER
MS8 PART MARKING*
LT1994CDD
LT1994IDD
LBQM
LBQM
LT1994CMS8
LT1994IMS8
LTBQN
LTBQN
Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/
*The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. V+ = 3V, V– = 0V, VCM = VOCM = VICM = mid-supply, V⎯S⎯H⎯D⎯N = OPEN,
RI = RF = 499Ω, RL = 800Ω to a mid-supply voltage (See Figure 1) unless otherwise noted. VS is defined (V+ – V–). VOUTCM is defined
as (VOUT+ + VOUT–)/2. VICM is defined as (VIN+ + VIN–)/2. VOUTDIFF is defined as (VOUT+ – VOUT–). VINDIFF is defined as (VIN+ – VIN–).
SYMBOL
VOSDIFF
PARAMETER
Differential Offset Voltage
(Input Referred)
ΔVOSDIFF/ΔT
Differential Offset Voltage Drift
(Input Referred)
IB
Input Bias Current
(Note 6)
CONDITIONS
VS = 2.375V, VICM = VS/4
VS = 3V
VS = 5V
VS = ±5V
VS = 2.375V, VICM = VS/4
VS = 3V
VS = 5V
VS = ±5V
VS = 2.375V, VICM = VS/4
VS = 3V
VS = 5V
VS = ±5V
MIN
TYP
–45
–45
–45
–45
3
3
3
3
–18
–18
–18
–18
●
●
●
●
●
●
●
●
MAX
±2
±2
±2
±3
–3
–3
–3
–3
UNITS
mV
mV
mV
mV
μV/°C
μV/°C
μV/°C
μV/°C
μA
μA
μA
μA
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LT1994
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. V+ = 3V, V– = 0V, VCM = VOCM = VICM = mid-supply, V⎯S⎯H⎯D⎯N = OPEN,
RI = RF = 499Ω, RL = 800Ω to a mid-supply voltage (See Figure 1) unless otherwise noted. VS is defined (V+ – V–). VOUTCM is defined
as (VOUT+ + VOUT–)/2. VICM is defined as (VIN+ + VIN–)/2. VOUTDIFF is defined as (VOUT+ – VOUT–). VINDIFF is defined as (VIN+ – VIN–).
SYMBOL
IOS
PARAMETER
Input Offset Current
(Note 6)
RIN
Input Resistance
CIN
Input Capacitance
en
Differential Input Referred Noise Voltage f = 50kHz
Density
Input Noise Current Density
f = 50kHz
in
enVOCM
Input Referred Common Mode Output
Noise Voltage Density
Input Signal Common Mode Range
VICMR
(Note 7)
CMRRI
(Note 8)
CMRRIO
(Note 8)
PSRR
(Note 9)
PSRRCM
(Note 9)
GCM
Input Common Mode Rejection Ratio
(Input Referred) ΔVICM/ΔVOSDIFF
Output Common Mode Rejection Ratio
(Input Referred) ΔVOCM/ΔVOSDIFF
Differential Power Supply Rejection
(ΔVS/ΔVOSDIFF)
Output Common Mode Power Supply
Rejection (ΔVS/ΔVOSOCM)
Common Mode Gain (∆VOUTCM/ΔVOCM)
BAL
Common Mode Gain Error
100 • (GCM – 1)
Output Balance (ΔVOUTCM/ΔVOUTDIFF)
CONDITIONS
VS = 2.375V, VICM = VS/4
VS = 3V
VS = 5V
VS = ±5V
Common Mode
Differential Mode
Differential
MIN
●
●
●
●
f = 50kHz, VOCM Shorted to Ground
VS = 3V
VS = ±5V
VS = 3V, ΔVICM = 0.75V
●
●
TYP
±0.2
±0.2
±0.2
±0.2
700
4.5
2
MAX
±2
±2
±3
±4
UNITS
μA
μA
μA
μA
kΩ
kΩ
pF
3
nV/√Hz
2.5
pA/√Hz
15
nV/√Hz
●
0
–5
55
85
V
V
dB
VS = 5V, ΔVOCM = 2V
●
65
85
dB
VS = 3V to ±5V
●
69
105
dB
VS = 3V to ±5V
●
45
70
dB
VS = ±2.5V
●
1
V/V
VS = ±2.5V
●
–0.15
±1
%
●
●
–65
–71
±2.5
±2.5
±2.5
±2.5
5
5
5
5
–46
–50
±25
±25
±30
±40
dB
dB
mV
mV
mV
mV
μV/°C
μV/°C
μV/°C
μV/°C
V
ΔVOUTDIFF = 2V
Single-Ended Input
Differential Input
VS = 2.375V, VICM = VS/4
VS = 3V
VS = 5V
VS = ±5V
VS = 2.375V, VICM = VS/4
VS = 3V
VS = 5V
VS = ±5V
VS = 3V, ±5V
●
●
●
●
1.75
3.75
VOSCM
Common Mode Offset Voltage
(VOUTCM – VOCM)
ΔVOSCM/ΔT
Common Mode Offset Voltage Drift
VOUTCMR
(Note 7)
RINVOCM
Output Signal Common Mode Range
(Voltage Range for the VOCM Pin)
Input Resistance, VOCM Pin
●
30
40
60
VMID
Voltage at the VOCM Pin
VS = 5V
●
2.45
2.5
2.55
V
VOUT
Output Voltage, High, Either Output Pin
(Note 10)
VS = 3V, No Load
VS = 3V, RL = 800Ω
VS = 3V, RL = 100Ω
VS = ±5V, No Load
VS = ±5V, RL = 800Ω
VS = ±5V, RL = 100Ω
●
●
●
70
90
200
150
200
900
140
175
400
325
450
2400
mV
mV
mV
mV
mV
mV
●
●
●
●
V– + 1.1
V+ – 0.8
kΩ
1994fa
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LT1994
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. V+ = 3V, V– = 0V, VCM = VOCM = VICM = mid-supply, V⎯S⎯H⎯D⎯N = OPEN,
RI = RF = 499Ω, RL = 800Ω to a mid-supply voltage (See Figure 1) unless otherwise noted. VS is defined (V+ – V–). VOUTCM is defined
as (VOUT+ + VOUT–)/2. VICM is defined as (VIN+ + VIN–)/2. VOUTDIFF is defined as (VOUT+ – VOUT–). VINDIFF is defined as (VIN+ – VIN–).
SYMBOL
PARAMETER
Output Voltage, Low, Either Output Pin
(Note 10)
ISC
Output Short-Circuit Current, Either
Output Pin (Note 11)
SR
Slew Rate
GBW
Gain-Bandwidth Product
(fTEST = 1MHz)
Distortion
tS
Settling Time
AVOL
Large-Signal Voltage Gain
VS
Supply Voltage Range
IS
Supply Current
I⎯S⎯H⎯D⎯N
Supply Current in Shutdown
VIL
CONDITIONS
VS = 3V, No Load
VS = 3V, RL = 800Ω
VS = 3V, RL = 100Ω
VS = ±5V, No Load
VS = ±5V, RL = 800Ω
VS = ±5V, RL = 100Ω
VS = 2.375V, RL = 10Ω
VS = 3V, RL = 10Ω
VS = 5V, RL = 10Ω
VS = ±5V, VCM = 0V, RL = 10Ω
VS = 5V, ΔVOUT+ = –ΔVOUT– = 1V
VS = ±5V, VCM = 0V,
ΔVOUT+ = –ΔVOUT– = 1.8V
VS = 3V, TA = 25°C
VS = ±5V, VCM = 0V, TA = 25°C
VS = 3V, RL = 800Ω, fIN = 1MHz,
VOUT+ – VOUT– = 2VP-P
Differential Input
2nd Harmonic
3rd Harmonic
Single-Ended Input
2nd Harmonic
3rd Harmonic
VS = 3V, 0.01%, 2V Step
VS = 3V, 0.1%, 2V Step
VS = 3V
MIN
●
●
±25
±30
±40
±45
50
50
TYP
30
50
125
80
125
900
±35
±40
±65
±85
65
65
●
●
58
58
70
70
MHz
MHz
–99
–96
dBc
dBc
–94
–108
120
90
100
dBc
dBc
ns
ns
dB
●
●
●
●
●
●
●
●
●
●
●
2.375
MAX
70
90
250
180
250
2400
85
85
12.6
UNITS
mV
mV
mV
mV
mV
mV
mA
mA
mA
mA
V/μS
V/μS
V
●
●
●
⎯S⎯H⎯D⎯N Input Logic Low
VS = 3V
VS = 5V
VS = ±5V
VS = 3V
VS = 5V
VS = ±5V
VS = 3V to ±5V
VIH
⎯S⎯H⎯D⎯N Input Logic High
VS = 3V to ±5V
●
R⎯S⎯H⎯D⎯N
⎯S⎯H⎯D⎯N Pull-Up Resistor
VS = 2.375V to ±5V
tON
Turn-On Time
V⎯S⎯H⎯D⎯N 0.5V to 3V
1
μs
tOFF
Turn-Off Time
V⎯S⎯H⎯D⎯N 3V to 0.5V
1
μs
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The inputs are protected by a pair of back-to-back diodes. If the
differential input voltage exceeds 1V, the input current should be limited to
less than 10mA.
Note 3: A heat sink may be required to keep the junction temperature
below the absolute maximum rating when the output is shorted
indefinitely.
●
●
●
13.3
13.9
14.8
0.225
0.375
0.7
18.5
19.5
20.5
0.8
1.75
2.5
+
V – 2.1
mA
mA
mA
mA
mA
mA
V
55
75
kΩ
●
V+ – 0.6
40
V
Note 4: The LT1994C/LT1994I are guaranteed functional over the operating
temperature range –40°C to 85°C.
Note 5: The LT1994C is guaranteed to meet specified performance from
0°C to 70°C. The LT1994C is designed, characterized, and expected to
meet specified performance from –40°C to 85°C but is not tested or
QA sampled at these temperatures. The LT1994I is guaranteed to meet
specified performance from –40°C to 85°C.
Note 6: Input bias current is defined as the average of the input currents
flowing into Pin 1 and Pin 8 (IN– and IN+). Input Offset current is defined
as the difference of the input currents flowing into Pin 8 and Pin 1
(IOS = IB+ – IB–).
1994fa
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LT1994
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.
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TYPICAL PERFOR A CE CHARACTERISTICS
Common Mode Voltage Offset vs
Temperature
7.5
COMMON MODE VOLTAGE OFFSET (mV)
VS = 3V
VCM = 1.5V
VOCM = 1.5V
250 FOUR TYPICAL UNITS
0
–250
–500
–25
0
25
50
TEMPERATURE (°C)
75
100
IB, VS = ±5V
2.5
0
–15
0.5
IOS, VS = 3V
–20
IOS, VS = ±5V 0
–25
–0.5
IB, VS = 3V
–2.5
–50
–25
0
25
50
TEMPERATURE (°C)
75
1994 G01
100
–30
–50
–25
0
25
50
TEMPERATURE (°C)
75
1994 G02
72
2
–1.0
100
1994 G03
Frequency Response vs
Supply Voltage
Gain Bandwidth vs Temperature
Frequency Response vs
Load Capacitance
2
RF = RI = 499Ω
RF = RI = 499Ω
VS = 2.5V
71
VS = ±5V
VS = 3V VS = 2.5V
1
1
VS = 5V
70
GAIN (dB)
GAIN BANDWIDTH (MHz)
1.0
VS = 3V
69
GAIN (dB)
–750
–50
5.0
–10
VS = 3V
VCM = 1.5V
VOCM = 1.5V
FOUR TYPICAL UNITS
INPUT OFFSET CURRENT (µA)
DIFFERENTIAL VOS (µV)
500
Input Bias Current and Input
Offset Current vs Temperature
INPUT BIAS CURRENT (µA)
Differential Input Referred
Voltage Offset vs Temperature
0
VS = ±5V
VS = 3V
0
68
–1
–1
5pF FROM EACH
OUTPUT TO GROUND
25pF FROM EACH
OUTPUT TO GROUND
67
66
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
1994 G04
–2
0.1
1
10
FREQUENCY (MHz)
100
1994 G05
–2
0.1
1
10
FREQUENCY (MHz)
100
1994 G06
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LT1994
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TYPICAL PERFOR A CE CHARACTERISTICS
Output Impedance vs Frequency
100
Differential Power Supply
Rejection vs Frequency
Output Balance vs Frequency
–30
VS = 3V
RF = RI = 499Ω
110
∆VOUTCM
∆VOUTDIFF
1
90
DIFFERENTIAL PSRR (dB)
OUTPUT BALANCE (dB)
ZOUT OUT+, OUT– (Ω)
–40
10
–50
–60
SINGLE ENDED INPUT
–70
DIFFERENTIAL INPUT
80
–90
1
10
FREQUENCY (MHz)
50
40
30
10
VS = 3V
1k
100
0
10k
100k
1M
FREQUENCY (Hz)
10M
Input Common Mode Rejection vs
Frequency
60
∆VS
∆VOSOCM
V– SUPPLY
COMMON MODE PSRR (dB)
50
VS = ±5V
80
70
60
50
40
V+ SUPPLY
30
20
10
40
VS = 3V
0
0.1
30
1k
10k
100k
1M
FREQUENCY (Hz)
10M
100M
1
10
FREQUENCY (MHz)
100
1995 G10
10M
2ND, VICM = 1.5V
2ND, VCM = V–
3RD, VCM = V–
–100
100M
1995 G09
Input Noise vs Frequency
100
100
VS = 3V
TA = 25°C
10
10
en
in
1
10
100
1k
10k
FREQUENCY (Hz)
100k
1
1M
1995 G12
–40
VS = 3V
FIN = 1MHz
RF = RI = 499Ω
–70 R = 800Ω
L
VOCM = MID-SUPPLY
DISTORTION HD2, HD3 (dB)
DISTORTION HD2, HD3 (dB)
100k
1M
FREQUENCY (Hz)
Differential Distortion vs Input
Common Mode Level
–60
–90
10k
1995 G11
Differential Distortion vs Input
Amplitude (Single Ended Input)
–80
VS = 3V
1k
INPUT CURRENT NOISE DENSITY (pA/√Hz)
90
INPUT CMRR (dB)
Common Mode Output Power
Supply Rejection vs Frequency
∆VICM
∆VOSDIFF
VS = 3V
100M
1995 G08
1994 G07
100
V– SUPPLY
60
20
INPUT REFERRED VOLTAGE NOISE DENSITY (nV/√Hz)
0.1
V+ SUPPLY
70
–80
0.1
∆VS
∆VOSDIFF
100
VS = 3V
VIN = 2VP-P (SINGLE ENDED)
–50 F = 1MHz
IN
RF = RI = 499Ω
–60 RL = 800Ω
VOCM = MID-SUPPLY
–70
–80
–90
2ND
–100
3RD, VICM = 1.5V
–110
1
2
3
VIN (VP-P)
4
3RD
5
1994 G13
–110
0
2.5
0.5
1.5
2.0
1.0
INPUT COMMON MODE DC BIAS, IN– OR IN+ PINS (V)
1994 G14
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LT1994
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TYPICAL PERFOR A CE CHARACTERISTICS
Differential Distortion vs
Frequency
Slew Rate vs Temperature
SLEW RATE (V/µs)
66
2ND
–90
VS = 3V
64
VS = ±5V
62
–100
–110
100k
1M
FREQUENCY (Hz)
10M
60
–50
+0.1%
ERROR
VOUT
–0.1%
ERROR
VS = 3V
RF = RI = 499Ω
–25
0
25
50
TEMPERATURE (°C)
75
100
25ns/DIV
1994 G17
1994 G16
1994 G15
Small Signal Step Response
25pF LOAD
Large Signal Step Response
VS = 3V
RF = RI = 499Ω
VCM = V–
OUT+
OUT+
0.5V/DIV
OUT+
0.5V/DIV
VS = 3V
RF = RI = 499Ω
VIN = 100mVP-P,
SINGLE ENDED
Output with Large Input Overdrive
VS = 3V
RF = RI = 499Ω
0pF LOAD
20mV/DIV
DISTORTION (dB)
–80
RF = RI = 499Ω
SETTLE VOLTAGE ERROR (2mV/DIV)
VS = 3V
V = 2VP-P (SINGLE ENDED)
–50 F IN = 1MHz
IN
RF = RI = 499Ω
–60 RL = 800Ω
VOCM = MID-SUPPLY
–70 VICM = MID-SUPPLY
3RD
2V Step Response Settling
VOUT = VOUT+ – VOUT– (0.5V/DIV)
68
–40
OUT–
OUT–
OUT–
VIN = 3VP-P SINGLE ENDED
VIN = 10VP-P SINGLE ENDED
100ns/DIV
20ns/DIV
1994 G18
2µs/DIV
1994 G19
1994 G20
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LT1994
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TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs ⎯S⎯H⎯D⎯N Voltage
SHDN PIN VOLTAGE = V+
TOTAL SUPPLY CURRENT (mA)
TOTAL SUPPLY CURRENT (mA)
16
TA = 85°C
15
TA = –40°C
TA = 0°C
10
TA = 25°C
TA = 70°C
5
Supply Current vs ⎯S⎯H⎯D⎯N Voltage
16
VS = 5V
TOTAL SUPPLY CURRENT (mA)
Supply Current vs Supply Voltage
20
12
TA = –40°C
8
TA = 0°C
TA = 25°C
4
TA = 85°C
VS = 3V
12
TA = –40°C
8
TA = 85°C
TA = 0°C
TA = 70°C
TA = 25°C
4
TA = 70°C
0
0
2.5
5.0
7.5
10.0
SUPPLY VOLTAGE (V)
12.5
0
0
1
2
3
4
SHDN PIN VOLTAGE (V)
1994 G21
SHUTDOWN SUPPLY CURRENT (µA)
SHDN PIN CURRENT (µA)
TA = 0°C
TA = 25°C
TA = 70°C
TA = 85°C
–20
0.5
1.5
2.0
2.5
1.0
SHDN PIN VOLTAGE (V)
3.0
1994 G23
750
TA = 25°C
500
TA = 85°C
TA = –40°C
250
0
0
0.5
1000
TA = –40°C
–30
0
Shutdown Supply Current vs
Supply Voltage
VS = 3V
–10
0
1994 G22
⎯S⎯H⎯D⎯N Pin Current vs ⎯S⎯H⎯D⎯N
Pin Voltage
0
5
1.5
2.0
2.5
1.0
SHDN PIN VOLTAGE (V)
3.0
1994 G24
0
2.5
7.5
10.0
5.0
SUPPLY VOLTAGE (V)
12.5
1994 G25
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IN+, IN– (Pins 1, 8): Non-Inverting and Inverting Input
Pins of the Amplifier, respectively. For best performance,
it is highly recommended that stray capacitance be
kept to an absolute minimum by keeping printed circuit
connections as short as possible, and if necessary, stripping back nearby surrounding ground plane away from
these pins.
VOCM (Pin 2): Output Common Mode Reference Voltage.
The VOCM pin is the midpoint of an internal resistive voltage divider between the supplies, developing a (default)
mid-supply voltage potential to maximize output signal
swing. VOCM has a Thevenin equivalent resistance of approximately 40kΩ and can be overdriven by an external
voltage reference. The voltage on VOCM sets the output
common mode voltage level (which is defined as the average of the voltages on the OUT+ and OUT– pins). VOCM
should be bypassed with a high quality ceramic bypass
capacitor of at least 0.1μF (unless connected directly to
a low impedance, low noise ground plane) to minimize
common mode noise from being converted to differential noise by impedance mismatches both externally and
internally to the IC.
V+, V– (Pins 3, 6): Power Supply Pins. For single supply
applications (Pin 6 grounded) it is recommended that
high quality 1μF and 0.1µF ceramic bypass capacitors be
placed from the positive supply pin (Pin 3) to the negative
supply pin (Pin 6) with minimal routing. Pin 6 should be
directly tied to a low impedance ground plane. For dual
power supplies, it is recommended that high quality, 0.1μF
ceramic capacitors are used to bypass Pin 3 to ground
and Pin 6 to ground. It is also highly recommended that
high quality 1µF and 0.1µF ceramic bypass capacitors be
placed across the power supply pins (Pins 3 and 6) with
minimal routing.
OUT+, OUT– (Pins 4, 5): Output Pins. Each pin can drive
approximately 100Ω to ground with a short circuit current
limit of up to ±85mA. Each amplifier output is designed
to drive a load capacitance of 25pF. This basically means
the amplifier can drive 25pF from each output to ground
or 12.5pF differentially. Larger capacitive loads should be
decoupled with at least 25Ω resistors from each output.
SHDN (Pin 7): When Pin 7 (⎯S⎯H⎯D⎯N) is floating or when
Pin 7 is directly tied to V+, the LT1994 is in the normal
operating mode. When Pin 7 is pulled a minimum of 2.1V
below V+, the LT1994 enters into a low power shutdown
state. Refer to the ⎯S⎯H⎯D⎯N pin section under Applications
Information for description of the LT1994 output impedance in the shutdown state.
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Functional Description
The LT1994 is a small outline, wide band, low noise, and
low distortion fully-differential amplifier with accurate
output phase balancing. The LT1994 is optimized to
drive low voltage, single-supply, differential input analog-to-digital converters (ADCs). The LT1994’s output is
capable of swinging rail-to-rail on supplies as low as 2.5V,
which makes the amplifier ideal for converting ground
referenced, single-ended signals into VOCM referenced
differential signals in preparation for driving low voltage,
single-supply, differential input ADCs. Unlike traditional
op amps which have a single output, the LT1994 has two
outputs to process signals differentially. This allows for
two times the signal swing in low voltage systems when
compared to single-ended output amplifiers. The balanced
differential nature of the amplifier also provides even-order
harmonic distortion cancellation, and less susceptibility
to common mode noise (like power supply noise). The
LT1994 can be used as a single ended input to differential
output amplifier, or as a differential input to differential
output amplifier.
The LT1994’s output common mode voltage, defined as
the average of the two output voltages, is independent
of the input common mode voltage, and is adjusted
by applying a voltage on the VOCM pin. If the pin is left
open, there is an internal resistive voltage divider, which
develops a potential halfway between the V+ and V– pins.
The VOCM pin will have an equivalent Thevenin equivalent
resistance of 40kΩ, and a Thevenin equivalent voltage of
half-supply. Whenever this pin is not hard tied to a low
impedance ground plane, it is recommended that a high
quality ceramic cap is used to bypass the VOCM pin to a
low impedance ground plane (see Layout Considerations
in this document). The LT1994’s internal common mode
feedback path forces accurate output phase balancing to
reduce even order harmonics, and centers each individual
output about the potential set by the VOCM pin.
VOUT + + VOUT –
VOUTCM = VOCM =
2
The outputs (OUT+ and OUT–) of the LT1994 are capable
of swinging rail-to-rail. They can source or sink up to
approximately 85mA of current. Each output is rated to
drive approximately 25pF to ground (12.5pF differentially).
Higher load capacitances should be decoupled with at least
25Ω of series resistance from each output.
Input Pin Protection
The LT1994’s input stage is protected against differential
input voltages that exceed 1V by two pairs of back-toback diodes that protect against emitter base breakdown
of the input transistors. In addition, the input pins have
steering diodes to either power supply. If the input pair
is over-driven, the current should be limited to under
10mA to prevent damage to the IC. The LT1994 also has
steering diodes to either power supply on the VOCM, and
⎯S⎯H⎯D⎯N pins (Pins 2 and 7) and if exposed to voltages that
exceed either supply, they too should be current limited
to under 10mA.
⎯S⎯H⎯D⎯N Pin
If the ⎯S⎯H⎯D⎯N pin (Pin 7) is pulled 2.1V below the positive
supply, an internal current is generated that is used to
power down the LT1994. The pin will have the Thevenin
equivalent impedance of approximately 55kΩ to V+. If
the pin is left unconnected, an internal pull-up resistor of
120kΩ will keep the part in normal active operation. Care
should be taken to control leakage currents at this pin to
under 1μA to prevent leakage currents from inadvertently
putting the LT1994 into shutdown. In shutdown, all biasing
current sources are shut off, and the output pins OUT+
and OUT– will each appear as open collectors with a nonlinear capacitor in parallel, and steering diodes to either
supply. Because of the non-linear capacitance, the outputs
still have the ability to sink and source small amounts of
transient current if exposed to significant voltage transients. The inputs (IN+, and IN–) have anti-parallel diodes
that can conduct if voltage transients at the input exceed
1V. The inputs also have steering diodes to either supply.
The turn-on and turn-off time between the shutdown and
active states are on the order of 1μs but depends on the
circuit configuration.
General Amplifier Applications
As levels of integration have increased and, correspondingly, system supply voltages decreased, there has been
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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 interfacing 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
• (VINP – VINM )
RI
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.
βAVG is defined as the average feedback factor (or gain)
from the outputs to their respective inputs:
1 ⎛ RI2
RI1 ⎞
+
β AVG = • ⎜
⎟
2 ⎝ RI2 + RF2 RI1 + RF1 ⎠
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):
1
VICM = • (VINP + VINM )
2
and VINDIFF is defined as the difference of the input
voltages:
VINDIFF = VINP – VINM
Effects of Resistor Pair Mismatch
Figure 2 shows a circuit diagram that takes into consideration 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 – ≅
where: RF is the average of RF1 and RF2, and RI is the
average of RI1 and RI2.
RF
• VINDIFF +
RI
When the feedback ratios mismatch (Δβ), common mode
to differential conversion occurs.
Setting the differential input to zero (VINDIFF = 0), the degree of common mode to differential conversion is given
∆β
∆β
• VICM –
• VOCM,
β AVG
β AVG
RI
VIN–
VOUT+
RF
VIN– RF2
RI2
RL
VOUT+
RL
V+
0.1µF
VS
RBAL
VINM
VINM
3
3
1
VOCM
2
8
VCM
0.1µF
–
+
VOCM
1
4
0.1µF
LT1994
–
+
VOCM
VOUTCM
2
8
0.1µF
5
–
VOCM
VIN
RBAL
0.1µF
VSHDN
RF
V
–
SHDN
0.1µF
5
6
7
RI
4
LT1994
+
6
VINP
+
–
VINP
7
VSHDNB
VOUT–
+
Figure 1. Test Circuit
1994 F01
RL
RI1
RF1
VIN
VOUT–
RL
+
1994 F02
Figure 2. Real-World Application
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APPLICATIO S I FOR ATIO
by the equation:
R1 should be chosen (see Figure 3):
VOUTDIFF = VOUT + – VOUT – ≈
(VICM – VOCM ) • β∆β
AVG
VINDIFF = 0
In general, the degree of feedback pair mismatch is a source
of common mode to differential conversion of both signals
and noise. Using 1% resistors or better will provide about
28dB of common mode rejection. Using 0.1% resistors
will provide about 48dB of common mode rejection. A low
impedance ground plane should be used as a reference
for both the input signal source and the VOCM pin. A direct
short of VOCM to this ground plane or bypassing the VOCM
with a high quality 0.1µF ceramic capacitor to this ground
plane will further mitigate against common mode signals
from being converted to differential.
Input Impedance and Loading Effects
The input impedance looking into the VINP or VINM input
of Figure 1 depends on whether or not the sources VINP
and VINM are fully differential. For balanced input sources
(VINP = –VINM), the input impedance seen at either input
is simply:
R1 =
RINM • RS
RINM − RS
According to Figure 3, the input impedance looking into
the differential amp (RINM) reflects the single ended source
case, thus:
RINM =
R2 is chosen to balance R1 || RS:
R2 =
R1 • RS
R1 + RS
Input Common Mode Voltage Range
The LT1994’s input common mode voltage (VICM) is defined
as the average of the two input voltages, VIN+, and VIN–.
It extends from V– to approximately 1.25V below V+. The
input common mode range depends on the circuit configuration (gain), VOCM and VCM (refer to Figure 4). For
fully differential input applications, where VINP = –VINM,
the common mode input is approximately:
RINP = RINM = RI
For single ended inputs, because of the signal imbalance
at the input, the input impedance actually increases over
the balanced differential case. The input impedance looking
into either input is:
RINP = RINM =
RI
⎛ 1 ⎡ RF ⎤⎞
⎜ 1– 2 • ⎢ R + R ⎥⎟
⎝
⎣ I F ⎦⎠
Input signal sources with non-zero output impedances can
also cause feedback imbalance between the pair of feedback
networks. For the best performance, it is recommended
that the source’s output impedance be compensated for.
If input impedance matching is required by the source,
RI
⎛ 1 ⎡ RF ⎤⎞
⎜ 1– 2 • ⎢ R + R ⎥⎟
⎝
⎣ I F ⎦⎠
VICM =
⎛ RI ⎞
VIN+ + VIN–
≈ VOCM • ⎜
⎟+
⎝ RI + RF ⎠
2
⎛ RF ⎞
VCM • ⎜
⎟
⎝ RF + RI ⎠
RINM
RS
VS
RI
RF
R1
–
+
LT1994
R1 CHOSEN SO THAT R1 || RINM = RS
R2 CHOSEN TO BALANCE R1 || RS
+
RI
–
RF
1994 F03
R2 = RS || R1
Figure 3. Optimal Compensation for Signal Source Impedance
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RI
VIN–
VOUT+
RF
RL
VS
VINM
3
1
VOCM
2
8
VCM
–
+
VOCM
4
LT1994
–
+
SHDN
0.1µF
5
the input common mode voltage, it can be directly tied
to the VOCM pin, but must be capable of driving a 40kΩ
equivalent resistance that is tied to a mid-supply potential.
If an external reference drives the VOCM pin, it should still
be bypassed with a high quality 0.1μF capacitor to a low
impedance ground plane to filter any thermal noise and
to prevent common mode signals on this pin from being
inadvertently converted to differential signals.
6
VINP
Noise Considerations
7
VSHDNB
RI
RF
VIN+
VOUT–
RL
1994 F04
Figure 4. Circuit for Common Mode Range
With singled ended inputs, there is an input signal component to the input common mode voltage. Applying only
VINP (setting VINM to zero), the input common voltage is
approximately:
VICM
⎛ RI ⎞
VIN+ + VIN–
=
≈ VOCM • ⎜
⎟+
⎝ RI + RF ⎠
2
⎛ RF ⎞ VINP ⎛ RF ⎞
VCM • ⎜
•⎜
⎟
⎟+
⎝ RF + RI ⎠
2 ⎝ RF + RI ⎠
Output Common Mode Voltage Range
The output common mode voltage is defined as the average of the two outputs:
V + + VOUT –
VOUTCM = VOCM = OUT
2
The LT1994’s input referred voltage noise is on the order
of 3nV/√Hz. Its input referred current noise is on the
order of 2.5pA/√Hz. In addition to the noise generated
by the amplifier, the surrounding feedback resistors also
contribute noise. The output noise generated by both the
amplifier and the feedback components is given by the
equation:
2
e no =
⎛
⎡ RF ⎤⎞
2
⎜ e ni • ⎢1+ R ⎥⎟ + 2 • (In • RF ) +
⎝
I ⎦⎠
⎣
2
⎛
⎡ R ⎤⎞
2 • ⎜ e nRI • ⎢ F ⎥⎟ + 2 • e nRF2
⎝
⎣ RI ⎦⎠
A plot of this equation and a plot of the noise generated
by the feedback components are shown in Figure 6.
The LT1994’s input referred voltage noise contributes the
equivalent noise of a 560Ω resistor. When the feedback
enRI22
enRF22
RF2
in–2
The VOCM sets this average by an internal common mode
feedback loop which internally forces VOUT+ = –VOUT–. The
output common mode range extends from approximately
1.1V above V– to approximately 0.8V below V+. The VOCM
pin sits in the middle of an 80kΩ to 80kΩ voltage divider
that sets the default mid-supply open circuit potential.
In single supply applications, where the LT1994 is used
to interface to an ADC, the optimal common mode input
range to the ADC is often determined by the ADC’s reference. If the ADC makes a reference available for setting
RI2
VS/2
3
encm
2
1
2
8
in+2
–
+
VOCM
4
eno2
LT1994
–
+
5
6
7
enRI1
2
RI1
eni2
–VS/2
RF1
enRF12
1994 F05
Figure 5. Noise Analysis
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network is comprised of resistors whose values are less
than this, the LT1994’s output noise is voltage noise
dominant (See Figure 6):
⎛ R ⎞
e no ≈ e ni • ⎜ 1 + F ⎟
⎝ RI ⎠
Feedback networks consisting of resistors with values
greater than about 10kΩ will result in output noise which
is amplifier current noise dominant.
e no ≈ 2 • In • RF
Lower resistor values always result in lower noise at the
penalty of increased distortion due to increased loading of
the feedback network on the output. Higher resistor values
will result in higher output noise, but improved distortion
due to less loading on the output.
100
OUTPUT NOISE (nV/√Hz)
TOTAL (AMPLIFIER + FEEDBACK NETWORK)
OUTPUT NOISE
10
FEEDBACK NETWORK
NOISE ALONE
1
0.1
1
RF = RI (kΩ)
10
1994 F06
Figure 6. LT1994 Output Spot Noise vs Spot Noise Contributed by
Feedback Network Alone
Figure 6 shows the noise voltage that will appear differentially between the outputs. The common mode output
noise voltage does not add to this differential noise. For
optimum noise and distortion performance, use a differential output configuration.
Power Dissipation Considerations
The LT1994 is housed in either an 8-lead MSOP package
(θJA = 140°C/W or an 8-lead DD package (θJA = 160°C/
W). The LT1994 combines high speed and large output
current with a small die and small package so there is
a need to be sure the die temperature does not exceed
150°C if housed in the 8-lead MSOP package, and 125°C
if housed in the 8-lead DD package. In the 8-lead MSOP,
LT1994 has its V– lead fused to the frame so it is possible
to lower the package thermal impedance by connecting the
V– pin to a large ground plane or metal trace. Metal trace
and plated through holes can be used to spread the heat
generated by the device to the backside of the PC board.
For example, an 8-lead MSOP on a 3/32" FR-4 board with
540mm2 of 2oz. copper on both sides of the PC board tied
to the V– pin can drop the θJA from 140°C/W to 110°C/W
(see Table 1).
The underside of the DD package has exposed metal
(4mm2) from the lead frame where the die is attached.
This provides for the direct transfer of heat from the die
junction to the printed circuit board to help control the
maximum operating junction temperature. The dual-inline pin arrangement allows for extended metal beyond
the ends of the package on the topside (component side)
of a circuit board. Table 1 summarizes for both the MSOP
and DD packages, the thermal resistance from the die
junction to ambient that can be obtained using various
amounts of topside, and backside metal (2oz. copper).
On multilayer boards, further reductions can be obtained
using additional metal on inner PCB layers connected
through vias beneath the package.
In general, the die temperature can be estimated from
the ambient temperature TA, and the device power dissipation PD:
TJ = TA+ + PD • θJA
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The power dissipation in the IC is a function of the supply
voltage, the output voltage, and the load resistance. For
fully differential output amplifiers at a given supply voltage
(±VCC), and a given differential load (RLOAD), the worstcase power dissipation PD(MAX) occurs at the worst case
quiescent current (IQ(MAX) = 20.5mA) and when the load
current is given by the expression:
ILOAD =
Table 1. LT1994 MSOP and DD Package Thermal Resistivity
LT1994 8-LEAD MSOP PACKAGE
VCC
RLOAD
The worst case power dissipation in the LT1994 at
V
ILOAD = CC is:
RLOAD
(
)
PD(MAX) = 2 • VCC • ILOAD + IQ(MAX) – ILOAD2 •
RLOAD
To operate the device at higher ambient temperature,
connect more copper to the V– pin to reduce the thermal
resistance of the package as indicated in Table 1. Note that
TJMAX for the 8-lead DD package is 125°C (as opposed to
150°C for the 8-lead MSOP), and the data for the equation
above should be altered accordingly.
V 2
= CC + 2 • VCC • IQ(MAX)
RLOAD
Example: A LT1994 is mounted on a circuit board in a
MSOP-8 package (θJA = 140°C/W), and is running off of
±5V supplies driving an equivalent load (external load plus
feedback network) of 75Ω. The worst-case power that
would be dissipated in the device occurs when:
VCC2
PD(MAX ) =
+ 2 • VCC • IQ(MAX ) =
RLOAD
5V 2
+ 2 • 5V • 17.5MA = 0.54W
75Ω
The maximum ambient temperature the 8-lead MSOP is
allowed to operate under these conditions is:
LT1994 8-LEAD DD PACKAGE
Copper Area Copper Area
Thermal
Topside
Backside
Resistance
(mm2)
(mm2)
(Junction to
Ambient)
0
0
140
Copper Area
Topside
(mm2)
Thermal
Resistance
(Junction to
Ambient)
4
160
30
0
135
16
135
100
0
130
32
110
100
100
120
64
95
540
540
110
130
70
Layout Considerations
Because the LT1994 is a high speed amplifier, it is sensitive
to both stray capacitance and stray inductance. Components connected to the LT1994 should be connected with
as short and direct connections as possible. A low noise,
low impedance ground plane is critical for the highest
performance. In single supply applications, high quality
surface mount 1μF and 0.1μF ceramic bypass capacitors
with minimum PCB trace should be used directly across
the power supplies V+ to V–. In split supply applications,
high quality surface mount 1μF and 0.1μF ceramic bypass
capacitors should be placed across the power supplies
V+ to V–, and individual high quality surface mount 0.1μF
bypass caps should be used from each supply to ground
with direct (short) connections.
TA = TJMAX – PD • θJA = 150°C – (0.54W) •
(140°C/W) = 75°C
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Any stray parasitic capacitance to ground at the summing
junctions, IN+ and IN– should be kept to an absolute minimum even if it means stripping back the ground plane
away from any trace attached to this node. This becomes
especially true when the feedback resistor network uses
resistor values >500Ω in circuits with RF = RI. Excessive
peaking in the frequency response can be mitigated by
adding small amounts of feedback capacitance around RF
(2pF to 5pF). Always keep in mind the differential nature of
the LT1994, and that it is critical that the output impedances
seen by both outputs (stray or intended) should be as balanced and symmetric as possible. This will help preserve
the natural balance of the LT1994, which minimizes the
generation of even order harmonics, and preserves the
rejection of common mode signals and noise.
It is highly recommended that the VOCM pin be either hard
tied to a low impedance ground plane (in split supply
applications) or bypassed to ground with a high quality
0.1μF ceramic capacitor in single supply applications.
This will help prevent thermal noise from the internal
80kΩ-80kΩ voltage divider (25nV/√Hz) and other external sources of noise from being converted to differential
noise due to mismatches in the feedback networks. It is
also recommended that the resistive feedback networks
be comprised of 1% resistors (or better) to enhance the
output common mode rejection. This will also prevent
VOCM input referred common mode noise of the common
mode amplifier path (which cannot be filtered) from being
converted to differential noise, degrading the differential
noise performance.
W
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SI PLIFIED SCHE ATIC
V+
120k
SHUTDOWN
CIRCUIT
I1
V+
55k
I1
SHDN
CM1
V+
CM2
V+
+
BIAS
ADJUST
OUT–
Q10
V–
BIAS
+
BIAS
ADJUST
–
–
Gm2B
Gm2A
OUT+
Q12
V+
OUT+ +V
I2
R1
4k
V+
R2
4k
Q2
D2
V+
D3
D4
Q3
Q4
V+
80k
Q8
VOCM
80k
Q5 Q6
OUT–
V–
V+
I3
Q7
Q1
D1
V–
V–
V+
V+
IN–
V+
Q11
Q9
V–
V–
V+
V–
V–
CM
ADJUST
I4A
I4B
V–
IN+
1994 SS01
V–
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LT1994
U
TYPICAL APPLICATIO S
Differential 1st Order Lowpass Filter
Example: The specified –3dB frequency is 1MHz Gain = 4
Maximum –3dB frequency (f3dB) 5MHz
1. Using f3dB = 1000kHz, C11abs = 400pF
Stopband attenuation: –6dB at 2 • f3dB and 14dB at 5 • f3dB
2. Nearest standard 5% value to 400pF is 390pF and C11
= C12 = 390pF
C11
VIN–
3. Using f3dB = 1000kHz, C11 = 390pF and Gain = 4, R21
= R22 = 412Ω and R11 = R12 = 102Ω (nearest 1%
value)
R21
R11
V+
0.1µF
Differential 2nd Order Butterworth Lowpass Filter
3
1
– +
2
8
0.1µF
4
VOUT
+
LT1994
+ –
6
5
VOUT–
Maximum –3dB frequency (f3dB) 2.5MHz
Stopband attenuation: –12dB at 2 • f3dB and –28dB at 5 • f3dB
7
R21
VIN+
R12
R22
VIN–
R11
C21
R31
V+
0.1µF
C12
1994 TA03
3
1
Component Calculation:
2
C11
8
0.1µF
R11 = R12, R21 = R22
– +
5MHz
f3dB
4 • 105
C11abs =
(C11abs in pF and f3dB in kHz)
f3dB
2. Select a standard 5% capacitor value nearest the absolute
value for C11
5
VOUT–
7
VIN+
R12
1. Calculate an absolute value for C11 (C11abs) using a
specified –3dB frequency
VOUT+
LT1994
+ –
6
f3dB ≤ 5MHz and Gain ≤
4
R32
R22
C22
1994 TA04
Component Calculation:
R11 = R12, R21 = R22, R31 = R32, C21 = C22,
C11 = 10 • C21, R1 = R11, R2 = R21, R3 = R31,
C2 = C21 and C1 = C11
3. Calculate R11 and R21 using the standard 5% C11
value, f3dB and desired gain
R11 and R21 equations (C11 in pF and f3dB in kHz)
R21=
159.2 • 106
C11• f3dB
R11=
R21
Gain
1994fa
17
LT1994
U
TYPICAL APPLICATIO S
1. Calculate an absolute value for C2 (C2abs) using a
specified –3dB frequency
Example: The specified –3dB frequency is 1MHz Gain = 1
4 • 105
(C2abs in pF and f3dB in kHz) (Note 2)
f3dB
2. Nearest standard 5% value to 400pF is 390pF and C21
= C22 = 390pF and C11 = 3900pF
2. Select a standard 5% capacitor value nearest the absolute
value for C2 (C1 = 10 • C2)
3. Using f3dB = 1000kHz, C2 = 390pF and Gain = 1, R1
= 549Ω, R2 = 549Ω and R3 = 15.4Ω (nearest 1%
values). R11 = R21 = 549Ω, R21 = R22 = 549Ω and
R31 = R32 = 15.4Ω.
C2abs =
3. Calculate R3, R2 and R1 using the standard 5% C2
value, the specified f3dB and the specified passband
gain (Gn)
f3dB ≤ 2.5MHz and Gain ≤ 8.8 or Gain ≤
2.5MHz
f3dB
R1, R2 and R3 equations (C2 in pF and f3dB in kHz)
(1.121–
R3 =
R2 =
R1=
1. Using f3dB = 1000kHz, C2abs = 400pF
Note 1: The equations for R1, R2, R3 are ideal and do
not account for the finite gain bandwidth product (GBW)
of the LT1994 (70MHz). The maximum gain is set by
the C1/C2 ratio (which for convenience is set equal to
ten).
Note 2: The calculated value of a capacitor is chosen
to produce input resistors less than 600Ω. If a higher
value input resistance is required then multiply all resistor values and divide all capacitor values by the same
number.
(1.131– 0.127 • Gn) ) • 108
(Note 1)
(Gn + 1) • C2 • f3dB
1.266 • 1015
R3 • C22 • f3dB2
R2
Gn
A Single Ended to Differential Voltage Conversion with Source Impedance Matching and Level Shifting
RS = 50Ω
50Ω
374Ω
402Ω
V+
VIN
0.1µF
54.9Ω
V
3
1
V
1
2
8
0.1µF
VIN
– +
–1
VOUT–
VOCM LT1994
+ –
6
0
4
5
VOUT+
VOCM + 0.25V
VOCM
VOUT+
VOCM – 0.25V
VOUT–
0
t
7
t
1994 TA05
402Ω
402Ω
1994fa
18
LT1994
U
PACKAGE DESCRIPTIO
DD Package
8-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1698)
R = 0.115
TYP
5
0.38 ± 0.10
8
0.675 ±0.05
3.5 ±0.05
1.65 ±0.05
2.15 ±0.05 (2 SIDES)
3.00 ±0.10
(4 SIDES)
PACKAGE
OUTLINE
1.65 ± 0.10
(2 SIDES)
PIN 1
TOP MARK
(NOTE 6)
(DD8) DFN 1203
0.25 ± 0.05
4
0.25 ± 0.05
0.75 ±0.05
0.200 REF
0.50
BSC
2.38 ±0.05
(2 SIDES)
1
0.50 BSC
2.38 ±0.10
(2 SIDES)
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-1)
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON TOP AND BOTTOM OF PACKAGE
MS8 Package
8-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1660)
3.00 ± 0.102
(.118 ± .004)
(NOTE 3)
0.889 ± 0.127
(.035 ± .005)
5.23
(.206)
MIN
3.20 – 3.45
(.126 – .136)
0.254
(.010)
8
7 6 5
3.00 ± 0.102
(.118 ± .004)
(NOTE 4)
4.90 ± 0.152
(.193 ± .006)
DETAIL “A”
0.52
(.0205)
REF
0° – 6° TYP
GAUGE PLANE
0.42 ± 0.038
(.0165 ± .0015)
TYP
0.65
(.0256)
BSC
1
0.53 ± 0.152
(.021 ± .006)
RECOMMENDED SOLDER PAD LAYOUT
DETAIL “A”
1.10
(.043)
MAX
2 3
4
0.86
(.034)
REF
0.18
(.007)
SEATING
PLANE
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.22 – 0.38
(.009 – .015)
TYP
0.65
(.0256)
BSC
0.127 ± 0.076
(.005 ± .003)
MSOP (MS8) 0204
1994fa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However,
no responsibility is assumed for its use. Linear Technology Corporation makes no representation that
the interconnection of its circuits as described herein will not infringe on existing patent rights.
19
LT1994
U
TYPICAL APPLICATIO
RFID Receiver Front-End, 20kHz < –3dB BW < 5MHz
(Baseband Gain = 2)
0.056µF
82pF
140Ω
402Ω
5V
0.1µF
3
1
5V
VCC
IOUT
0°
RF–
270pF
LT5516
LT1994
2
BPF
RF+
4
7
5V
+
8
0.1µF
0.056µF
IOUT
5
6
140Ω
402Ω
IOUT–
82pF
270pF
5V
LO INPUT
82pF
270pF
LO+
QOUT
0.056µF
+
140Ω
402Ω
0°/90°
5V
90°
LO–
0.1µF
QOUT–
1
270pF
ENABLE
3
4
7
EN
5V
2
8
0.1µF
0.056µF
LT1994
QOUT
5
6
140Ω
402Ω
82pF
1994 TA02
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
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Precision, Instrumentation Amp
Single Gain Set Resistor: G = 1 to 10,000
LT1806/LT1807
Single/Dual Low Distortion Rail-to-Rail Amp
325MHz, 140V/µs Slew Rate, 3.5nV/√Hz Noise
LT1809/LT1810
Single/Dual Low Distortion Rail-to-Rail Amp
180MHz, 350V/µs Slew Rate, Shutdown
LT1990
High Voltage Gain Selectable Differential Amp
±250V Common Mode, Micropower, Gain = 1, 10
LT1991
Precision Gain Selectable Differential Amp
Micropower, Pin Selectable Gain = –13 to 14
LTC1992/LTC1992-x
Fully Differential Input/Output Amplifiers
Programmable Gain or Fixed Gain (G = 1, 2, 5, 10)
LT1993-2/-4/-10
Low Distortion and Noise, Differential In/Out
Fixed Gain (G = 2, 4, 10)
LT1995
High Speed Gain Selectable Differential Amp
30MHz, 1000V/µs, Pin Selectable Gain = –7 to 8
LT1996
Precision, 100µA, Gain Selectable Differential Amp
Pin Selectable Gain = 9 to 117
LT6600-2.5/-5/-10/-15/-20
Differential Amp and Lowpass, Chebyshev Filter
Filter Cutoff = 2.5MHz, 5MHz, 10MHz, 15MHz or 20MHz
1994fa
20 Linear Technology Corporation
LT 0306 REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2005
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