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LTC1250 Very Low Noise Zero-Drift Bridge Amplifier
FEATURES
s s s s s s s s s
DESCRIPTIO
Very Low Noise: 0.75VP-P Typ, 0.1Hz to 10Hz DC to 1Hz Noise Lower Than OP-07 Full Output Swing into 1k Load Offset Voltage: 10V Max Offset Voltage Drift: 50nV/C Max Common-Mode Rejection Ratio: 110dB Min Power Supply Rejection Ratio: 115dB Min No External Components Required Pin-Compatible with Standard 8-Pin Op Amps
APPLICATI
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S
The LTC(R)1250 is a high performance, very low noise zerodrift operational amplifier. The LTC1250's combination of low front-end noise and DC precision makes it ideal for use with low impedance bridge transducers. The LTC1250 features typical input noise of 0.75VP-P from 0.1Hz to 10Hz, and 0.2VP-P from 0.1Hz to 1Hz. The LTC1250 has DC to 1Hz noise of 0.35VP-P, surpassing that of low noise bipolar parts including the OP-07, OP-77, and LT1012. The LTC1250 uses the industry-standard single op amp pinout, and requires no external components or nulling signals, allowing it to be a plug-in replacement for bipolar op amps. The LTC1250 incorporates an improved output stage capable of driving 4.3V into a 1k load with a single 5V supply; it will swing 4.9V into 5k with 5V supplies. The input common mode range includes ground with single power supply voltages above 12V. Supply current is 3mA with a 5V supply, and overload recovery times from positive and negative saturation are 0.5ms and 1.5ms, respectively. The internal nulling clock is set at 5kHz for optimum low frequency noise and offset drift; no external connections are necessary. The LTC1250 is available in standard 8-pin ceramic and plastic DIPs, as well as an 8-pin SOIC package.
Electronic Scales Strain Gauge Amplifiers Thermocouple Amplifiers High Resolution Data Acquisition Low Noise Transducers Instrumentation Amplifiers
and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation.
TYPICAL APPLICATI
5V 50 GAIN TRIM
Differential Bridge Amplifier
5V 1000pF
Input Referred Noise 0.1Hz to 10Hz
2 VS = 5V AV = 10k 1
0.1F
18.2k
V
0
350 STRAIN GAUGE
2
- +
7 6 AV = 100
LTC1250 3 4
-1
1000pF -5V
18.2k -5V
1250 TA01
-2
0
U
2 6 4 TIME (s) 8 10
LT1250 TA02
UO
UO
1
LTC1250 ABSOLUTE AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW NC 1 -IN 2 +IN 3 V- 4 8 7 6 5 NC V+ OUT NC
Total Supply Voltage (V + to V -) ............................. 18V Input Voltage ........................ (V + + 0.3V) to (V - - 0.3V) Output Short Circuit Duration ......................... Indefinite Operating Temperature Range LTC1250M..................................... - 55C to 125C LTC1250C .......................................... 0C TO 70C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec.)................ 300C
ORDER PART NUMBER LTC1250MJ8 LTC1250CJ8 LTC1250CN8 LTC1250CS8 S8 PART MARKING 1250
N8 PACKAGE J8 PACKAGE 8-LEAD CERAMIC DIP 8-LEAD PLASTIC DIP S8 PACKAGE 8-LEAD PLASTIC SOIC
TJMAX = 150C, JA = 100CW (J8) TJMAX = 110C, JA = 130CW (N8) TJMAX = 110C, JA = 200CW (S8)
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER VOS Input Offset Voltage VOS Average Input Offset Drift Long Term Offset Drift en Input Noise Voltage (Note 2) in IB IOS CMRR PSRR AVOL Input Noise Current Input Bias Current Input Offset Current Common-Mode Rejection Ratio Power Supply Rejection Ratio Large-Signal Voltage Gain Maximum Output Voltage Swing Slew Rate Gain-Bandwidth Product Supply Current Internal Sampling Frequency CONDITIONS TA = 25C (Note 1) (Note 1)
VIN = 5V, TA = Operating Temperature Range, unless otherwise noted.
LTC1250M TYP MAX 5 10 0.01 0.05 50 0.75 1.0 0.2 4.0 50 150 950 100 300 500 110 130 115 130 125 170 4.0 4.3/-4.7 4.92 10 1.5 3.0 4.0 7.0 4.75 MIN MIN LTC1250C TYP MAX 5 10 0.01 0.05 50 0.75 1.0 0.2 4.0 50 200 450 100 400 500 130 130 170 4.3 /-4.7 4.95 10 1.5 3.0 4.0 5.0 4.75 UNITS V V/C nV/Mo VP-P VP-P fA/Hz pA pA pA pA dB dB dB V V V/s MHz mA mA kHz
q
TA = 25C, 0.1Hz to 10Hz TA = 25C, 0.1Hz to 1Hz f = 10Hz TA = 25C (Note 3)
q
TA = 25C (Note 3)
q
SR GBW IS fS
VCM = - 4V to 3V VS = 2.375V to 8V RL = 10k, VOUT = 4V RL = 1k RL = 100k RL = 10k, CL = 50pF No Load, TA = 25C
q q q q
110 115 125 4.0
q
TA = 25C
VIN = 5V, TA = Operating Temperature Range, unless otherwise noted.
SYMBOL VOS VOS en IB IOS PARAMETER Input Offset Voltage Average Input Offset Drift Input Noise Voltage (Note 2) Input Bias Current Input Offset Current CONDITIONS TA = 25C (Note 1) (Note 1) TA = 25C, 0.1Hz to 10Hz TA = 25C, 0.1Hz to 1Hz TA = 25C (Note 3) TA = 25C (Note 3) MIN
q
LTC1250M TYP MAX 2 5 0.01 0.05 1.0 0.3 20 100 40 200
MIN
LTC1250C TYP MAX 2 5 0.01 0.05 1.0 0.3 20 100 40 200
UNITS V V/C VP-P VP-P pA pA
2
U
W
U
U
WW
W
LTC1250
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER Maximum Output Voltage Swing IS fS Supply Current Sampling Frequency CONDITIONS RL = 1k RL = 100k TA = 25C TA = 25C
VIN = 5V, TA = Operating Temperature Range, unless otherwise noted.
MIN 4.0 LTC1250M TYP MAX 4.3 4.95 1.8 2.5 3 MIN 4.0 LTC1250C TYP MAX 4.3 4.95 1.8 2.5 3 UNITS V V mA kHz
The q denotes specifications which apply over the full operating temperature range. Note 1: These parametes are guaranteed by design. Thermocouple effects preclude measurement of these voltage levels during automated testing. Note 2: 0.1Hz to 10Hz noise is specified DC coupled in a 10s window; 0.1Hz to 1Hz noise is specified in a 100s window with an RC high-pass
filter at 0.1Hz. The LTC1250 is sample tested for noise; for 100% tested parts contact LTC Marketing Dept. Note 3: At T 0C these parameters are guaranteed by design and not tested.
TYPICAL PERFOR A CE CHARACTERISTICS
Input Noise vs Supply Voltage
1.6 TA = 25C 1.4
SAMPLING FREQUENCY (kHz)
SUPPLY CURRENT (mA)
INPUT NOISE (VP-P)
1.2 1.0 0.8 0.6 0.4 0.1Hz TO 1Hz 0.2 0 4 10 6 8 12 14 TOTAL SUPPLY VOLTAGE, V + TO V - (V) 16 0.1Hz TO 10Hz
Input Noise vs Temperature
1.2 VS = 5V 1.0 0.8 0.1Hz TO 10Hz 0.6 0.4 0.1Hz TO 1Hz 0.2 0 -50 -25
SUPPLY CURRENT (mA)
SAMPLING FREQUENCY (kHz)
INPUT NOISE (VP-P)
50 25 75 0 TEMPERATURE (C)
UW
LTC1250 G01
Supply Current vs Supply Voltage
4.0 TA = 25C 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 4
2 5 6
Sampling Frequency vs Supply Voltage
TA = 25C
4
3
10 6 8 12 14 TOTAL SUPPLY VOLTAGE, V + TO V - (V)
16
4
10 6 8 12 14 TOTAL SUPPLY VOLTAGE, V + TO V - (V)
16
LTC1250 G02
LTC1250 G03
Supply Current vs Temperature
4.5 VS = 5V
7 8
Sampling Frequency vs Temperature
VS = 5V 6 5 4 3 2 1
4.0
3.5
3.0
2.5
100
125
2.0 -50 -25
50 25 0 75 TEMPERATURE (C)
100
125
0 -50 -25
0
75 50 25 TEMPERATURE (C)
100
150
LTC1250 G04
LTC1250 G05
LTC1250 G06
3
LTC1250
TYPICAL PERFOR A CE CHARACTERISTICS
Voltage Noise vs Frequency
80 70
VOLTAGE NOISE (nV/Hz)
100
VS = 5V RS = 10
40 30 20 10 0 1 10 100 1k FREQUENCY (Hz) 10k
LTC1250 G11
GAIN (dB)
50
60 GAIN 40 20 0 -20 1k 10k VS = 5V OR SINGLE 5V TA = 25C CL = 100pF
PHASE: RL = 100k
60 40
BIAS CURRENT (|pA|)
60
Overload Recovery
8
INPUT (V)
INPUT COMMON MODE RANGE (V)
0.2
0 0
OUTPUT (V)
CMRR (dB)
-5
500s/DIV AV = 100, RL = 100k, CL = 50pF, VS = 5V
Transient Response
10 9 8
OUTPUT SWING (V)
2V/DIV
OUTPUT SWING (V)
1s/DIV AV = 1, RL = 100k, CL = 50pF, VS = 5V
4
UW
Gain/Phase vs Frequency
100 80
Bias Current (Magnitude) vs Temperature
1000 VS = 5V
80
PHASE MARGIN (DEG)
100
PHASE: RL = 1k
20 0
100k 1M FREQUENCY (Hz)
-20 10M
LTC1250 G10
10 -50
-25
50 75 0 25 TEMPERATURE (C)
100
125
LTC1250 G14
Common-Mode Input Range vs Supply Voltage
140
TA = 25C 6 4 2 0 -2 -4 -6 -8 2 3 5 4 6 SUPPLY VOLTAGE (V) 7 8
Common-Mode Rejection Ratio vs Frequency
120 100 80 60 40 20 0 1 10 100 1k FREQUENCY (Hz) 10k 100k VS = 5V VCM = 1VRMS
LTC1250 G07
LTC1250 G12
Output Swing vs Load Resistance, Dual Supplies
18
Output Voltage Swing vs Load Resistance, Single Supply
16 14 12 VS = 10V 10 8 6 4 VS = 5V V - = GND RL TO GND 0 1 2 345678 LOAD RESISTANCE (k) 9 10 VS = 16V
RL TO GND VS = 8V
7 6 5 4 3 2 1 0 0 1 2 NEGATIVE SWING POSITIVE SWING 345678 LOAD RESISTANCE (k) 9 10 VS = 2.5V VS = 5V
2 0
LTC1250 G08
LTC1250 G09
LTC1250
TYPICAL PERFOR A CE CHARACTERISTICS
Output Swing vs Output Current, 5V Supply
5 4 3
OUTPUT VOLTAGE (V)
VS = 5V
5
SHORT-CIRCUIT CURRENT (mA)
2 1 0 -1 -2 -3 -4 -5 0.01 0.1 1 OUTPUT CURRENT (mA) 10
LTC1250 G16
OUTPUT VOLTAGE (V)
TEST CIRCUITS
Offset Test Circuit
100pF
100pF
100k 5V 2 10 3
- +
7 6 OUTPUT
LTC1250 4 -5V
1250 TC01
APPLICATI
Input Noise
S I FOR ATIO
The LTC1250, like all CMOS amplifiers, exhibits two types of low frequency noise: thermal noise and 1/f noise. The LTC1250 uses several design modifications to minimize these noise sources. Thermal noise is minimized by raising the gM of the front-end transistors by running them at high bias levels and using large transistor geometries. 1/f noise is combated by optimizing the zero-drift nulling loop to run at twice the 1/f corner frequency, allowing it to reduce the inherently high CMOS 1/f noise to near thermal levels at low frequencies. The resultant noise spectrum is quite low at frequencies below the internal 5kHz clock
VOLTAGE NOISE (nV/Hz)
U
W
UW
Output Swing vs Output Current, Single 5V Supply
6 VS = SINGLE 5V
40 30 20 10 0 -10 -20 -30
Short-Circuit Current vs Temperature
VS = 15V VOUT = V -
4 3 2 1 0 0.01
VOUT = V +
0.1 1 OUTPUT CURRENT (mA)
10
LTC1250 G17
-40 -50 -25
0
50 25 75 TEMPERATURE (C)
100
125
LTC1250 G18
DC to 10Hz Noise Test Circuit (for DC to 1Hz Multiply All Capacitor Values by 10)
100k 5V 2 10 3 5V 2 6 800k 3
- +
7
-
8 1 800k
0.04F 800k
6
-
1/2 LT1057 7 OUTPUT
LTC1250 4 -5V
1/2 LT1057
+4
-5V
5
+
0.02F
0.01F
1250 TC02
U
UO
80 70 OP-27 60 50 40 30 20 10 0 0.01 0.1 FREQUENCY (Hz) 1
LTC1250 F01
OP-07
VS = 5V RS = 10
LTC1250
Figure 1. Voltage Noise vs Frequency
5
LTC1250
APPLICATI
S I FOR ATIO
frequency, approaching the best bipolar op amps at 10Hz and surpassing them below 1Hz (Figure 1). All this is accomplished in an industry-standard pinout; the LTC1250 requires no external capacitors, no nulling or clock signals, and conforms to industry-standard 8-pin DIP and 8pin SOIC packages. Input Capacitance and Compensation The large input transistors create a parasitic 55pF capacitance from each input to V +. This input capacitance will react with the external feedback resistors to form a pole which can affect amplifier stability. In low gain, high impedance configurations, the pole can land below the unity-gain frequency of the feedback network and degrade phase margin, causing ringing, oscillation, and other unpleasantness. This is true of any op amp, however, the 55pF capacitance at the LTC1250's inputs can affect stability with a feedback network impedance as low as 1.9k. This effect can be eliminated by adding a capacitor across the feedback resistor, adding a zero which cancels the input pole (Figure 2). The value of this capacitor should be: 55pF CF AV where AV = closed-loop gain. Note that CF is not dependent on the value of RF. Circuits with higher gain (AV > 50) or low loop impedance should not require CF for stability.
CF
RF RIN
-
LTC1250
CP
+
1250 F02
Figure 2. CF Cancels Phase Shift Due to Parasitic CP
Larger values of CF, commonly used in band-limited DC circuits, may actually increase low frequency noise. The nulling circuitry in the LTC1250 closes a loop that includes the external feedback network during part of its cycle. This loop must settle to its final value within 150s or it will not
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fully cancel the 1/f noise spectrum and the low frequency noise of the part will rise. If the loop is underdamped (large RF, no CF) it will ring for more than 150s and the noise and offset will suffer. The solution is to add CF as above but beware! Too large a value of CF will overdamp the loop, again preventing it from reaching a final value by the 150s deadline. This condition doesn't affect the LTC1250's offset or output stability, but 1/f noise begins to rise. As a rule of thumb, the RFCF feedback pole should be 7kHz (1/150s, the frequency at which the loop settles) for best 1/f performance; values between 100pF and 500pF work well with feedback resistors below 100k. This ensures adequate gain at 7kHz for the LTC1250 to properly null. High value feedback resistors (above 1M) may require experimentation to find the correct value because parasitics, both in the LTC1250 and on the PC board, play an increasing role. Low value resistors (below 5k) may not require a capacitor at all. Input Bias Current The inputs of the LTC1250, like all zero-drift op amps, draw only small switching spikes of AC bias current; DC leakage current is negligible except at very high temperatures. The large front-end transistors cause switching spikes 3 to 4 times greater than standard zero-drift op amps: the 50pA bias current spec is still many times better than most bipolar parts. The spikes don't match from one input pin to the other, and are sometimes (but not always) of opposite polarity. As a result, matching the impedances at the inputs (Figure 3) will not cancel the bias current, and may cause additional errors. Don't do it.
RF RIN
W
U
UO
-
LTC1250
+
1250 F03
Figure 3. Extra Resistor Will Not Cancel Bias Current Errors
LTC1250
APPLICATI
Output Drive
S I FOR ATIO
The LTC1250 includes an enhanced output stage which provides nearly symmetrical output source/sink currents. This output is capable of swinging a minimum of 4V into a 1k load with 5V supplies, and can sink or source >20mA into low impedance loads. Lightly loaded (RL 100k), the LTC1250 will swing to within millivolts of either rail. In single supply applications, it will typically swing 4.3V into a 1k load with a 5V supply. Minimizing External Errors The input noise, offset voltage, and bias current specs for the LTC1250 are all well below the levels of circuit board parasitics. Thermocouples between the copper pins of the LTC1250 and the tin/lead solder used to connect them can overwhelm the offset voltage of the LTC1250, especially if a soldering iron has been around recently. Note also that when the LTC1250's output is heavily loaded, the chip may dissipate substantial power, raising the temperature of the package and aggravating thermocouples at the inputs. Although the LTC1250 will maintain its specified accuracy under these conditions, care must be taken in the layout to prevent or compensate circuit errors. Be especially careful of air currents when measuring low frequency noise; nearby moving objects (like people) can create very large noise peaks with an unshielded circuit board. For more detailed explanations and advice on how to avoid these errors, see the LTC1051/LTC1053 data sheet. Sampling Behavior The LTC1250's zero-drift nulling loop samples the input at 5kHz, allowing it to process signals below 2kHz with no aliasing. Signals above this frequency may show aliasing behavior, although wideband internal circuitry generally keeps errors to a minimum. The output of the LTC1250 will have small spikes at the clock frequency and its harmonics; these will vary in amplitude with different feedback configurations. Low frequency or band-limited systems should not be affected, but systems with higher bandwidth (oversampling A/Ds, for example) may need to filter out these clock artifacts. Output spikes can be minimized with a large feedback capacitor, but this will adversely affect noise performance (see Input Capacitance and Compensation on
Figure 4. RC Output Pole Limits Bandwidth to 330Hz
Single Supply Operation The LTC1250 will operate with single supply voltages as low as 4.5V, and the output swings to within millivolts of either supply when lightly loaded. The input stage will common mode to within 250mV of ground with a single 5V supply, and will common mode to ground with single supplies above 11V. Most bridge transducers bias their inputs above ground when powered from single supplies, allowing them to interface directly to the LTC1250 in single supply applications. Single-ended, ground-referenced signals will need to be level shifted slightly to interface to the LTC1250's inputs. Fault Conditions The LTC1250 is designed to withstand most external fault conditions without latch-up or damage. However, unusually severe fault conditions can destroy the part. All pins are protected against faults of 25mA or 5V beyond either supply, whichever comes first. If the external circuitry can exceed these limits, series resistors or voltage clamp diodes should be included to prevent damage. The LTC1250 includes internal protection against ESD damage. All data sheet parameters are maintained to 1kV ESD on any pin; beyond 1kV, the input bias and offset currents will increase, but the remaining specs are unaffected and the part remains functional to 5kV at the input pins and 8kV at the output pin. Extreme ESD conditions should be guarded against by using standard anti-static precautions.
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.
+
-
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the previous page). Applications which require spike-free output in addition to minimum noise will need a low-pass filter after the LTC1250; a simple RC will usually do the job (Figure 4). The LTC1051/LTC1053 data sheet includes more information about zero-drift amplifier sampling behavior.
CF RF 47k 0.01 LTC1250
1250 F04
W
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7
LTC1250
TYPICAL APPLICATI
15V 7.5k 3 3 LM399 4 2 1 2
Reference Buffer
+ -
LTC1250 4
10ppm ERROR AT 15mA 1VP-P OUTPUT NOISE 2.5V/C DRIFT (DUE TO LM399)
PACKAGE DESCRIPTIO
CORNER LEADS OPTION (4 PLCS) 0.005 (0.127) MIN
J8 Package 8-Lead Ceramic DIP
0.405 (10.287) MAX 8 7 6 5
0.023 - 0.045 (0.584 - 1.143) HALF LEAD OPTION 0.045 - 0.068 (1.143 - 1.727) FULL LEAD OPTION 0.300 BSC (0.762 BSC)
0.025 (0.635) RAD TYP 1 2 3
0.008 - 0.018 (0.203 - 0.457) 0.385 0.025 (9.779 0.635)
0 - 15
0.045 - 0.068 (1.143 - 1.727) 0.014 - 0.026 (0.360 - 0.660)
NOTE: LEAD DIMENSIONS APPLY TO SOLDER DIP/PLATE OR TIN PLATE LEADS.
0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0- 8 TYP
0.016 - 0.050 0.406 - 1.270
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).
8
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 q FAX: (408) 434-0507 q TELEX: 499-3977
U
UO
7
S
Differential Thermocouple Ampliifer
R3 1M 0.1% 5V 2 C1 100pF
6
R1 10k 0.1%
- +
7 6 R6 7.5k 1% R7 500 FULL-SCALE TRIM R8 5k 1% 10mV/C VOUT 100mV/C
LTC1250 3
-
TYPE K VCM
1250 TA03
+
R2 10k 0.1% R4 1M 0.1% 5V VIN VOUT R5 3k
4 -5V C2 100pF
LT1025 GND
R9 33k -5V
FOR BEST ACCURACY, THERMOCOUPLE RESISTANCE SHOULD BE LESS THAN 100
1250 TA04
Dimensions in inches (millimeters) unless otherwise noted. N8 Package 8-Lead Plastic DIP
0.400* (10.160) MAX 8 7 6 5
0.220 - 0.310 (5.588 - 7.874)
0.255 0.015* (6.477 0.381)
4
0.200 (5.080) MAX 0.015 - 0.060 (0.381 - 1.524)
1 0.300 - 0.325 (7.620 - 8.255)
2
3
4 0.130 0.005 (3.302 0.127)
0.045 - 0.065 (1.143 - 1.651)
0.009 - 0.015 (0.229 - 0.381)
0.065 (1.651) TYP 0.125 (3.175) MIN 0.015 (0.380) MIN
0.125 3.175 0.100 0.010 MIN (2.540 0.254)
J8 0694
(
+0.025 0.325 -0.015 +0.635 8.255 -0.381
)
0.045 0.015 (1.143 0.381) 0.100 0.010 (2.540 0.254)
0.018 0.003 (0.457 0.076)
N8 0694
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTURSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm).
S8 Package 8-Lead Plastic SOIC
0.053 - 0.069 (1.346 - 1.752) 8 0.004 - 0.010 (0.101 - 0.254) 0.228 - 0.244 (5.791 - 6.197)
0.189 - 0.197* (4.801 - 5.004) 7 6 5
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) BSC
0.150 - 0.157* (3.810 - 3.988)
1
2
3
4
SO8 0294
LT/GP 0894 2K REV A * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1994

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