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LMP8276 High Common Mode, Gain of 20, Bidirectional Precision Voltage Difference Amplifier
August 2005
LMP8276 High Common Mode, Gain of 20, Bidirectional Precision Voltage Difference Amplifier
General Description
The LMP8276 is a fixed gain differential amplifier with a -2V to 27V input common mode voltage range and a supply voltage range of 4.75V to 5.5V. The LMP8276 is part of the LMPTM precision amplifier family which will detect, amplify and filter small differential signals in the presence of high common mode voltages. The gain is fixed at 20 and is adequate to drive an ADC to full scale in most cases. This fixed gain is achieved in two separate stages, a pre-amplifier with gain of +10 and a second stage amplifier with a gain of +2. The internal signal path is brought out on two pins that provide a connection for a filter network. The LMP8276 will operate with reduced specifications over the extended common mode input voltage range of -2V to 36V. This feature makes the device suitable for applications with load dump in automotive systems. The mid-rail offset adjustment pin enables the user to utilize this device for bidirectional current sensing. This is achieved by adjusting an externally set voltage reference. Accurate bidirectional load current measurements are achieved when monitoring the output with respect to this reference voltage.
Features
Typical Values, TA = 25C n Bidirectional current sense capability 2 mV max n Input offset Voltage 30 V/C max n TCVos n CMRR 80 dB Min n Extended CMVR -2V to 36V n Output voltage swing Rail-to-Rail n Bandwidth 80 kHz n Operating temperature range (ambient) -40C to 125C n Supply voltage 4.75V to 5.5V n Supply current 1 mA
Applications
n Fuel injection control n High and low side driver configuration current sensing n Power management systems
Typical Application
Typical Application: Low Side Current Sensing
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LMPTM is a trademark of National Semiconductor Corporation.
(c) 2005 National Semiconductor Corporation
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LMP8276
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. ESD Tolerance (Note 2) Human Body Model For input pins only For All other pins Machine Model Supply Voltage (VS - GND)
Storage Temperature Range Junction Temperature (Note 3) Soldering Information Infrared or Convection (20 sec) Wave Soldering Lead Temp. (10 sec)
-65C to +150C +150C max 235C 260C
4000V 2000V
200V 5.75V
Operating Ratings (Note 1)
Temperature Range Packaged Devices (Note 3) Supply Voltage (VS -GND) Package Thermal Resistance (JA (Note 3)) 8-Pin SOIC -40C to +125C 4.75V to 5.5V 190C/W
Voltage on +IN and -IN Transient (400 ms)
-5V to 42V -7V to 45V (Note 4)
5V Electrical Characteristics
Symbol VOS TC VOS A2 IB IS RCM RDM CMVR ECMVR DC CMRR Parameter Input Offset Voltage Input Offset Voltage Drift Input Bias Current of A2 Supply Current Input impedance Common Mode Input impedance Differential Mode Input Common-Mode Voltage Range Extended Common-Mode Voltage Range
Unless otherwise specified, all limits guaranteed for TA = 25C, VS = 5V, GND= 0, -2V VCM 27V, RL= OPEN. Boldface limits apply at the temperature extremes. Conditions VCM = VS/2 VCM = VS/2 (Note 6) 1.0 160 320 -2 -2 -2V VCM 27V -2V VCM 36V -2V VCM 27V -2V VCM 36V f = 1 kHz f = 10 kHz 70 97 80 60 77 60 80 78 80 100 20 1 19.8 9.9 1.98 VOL VOH A2 VOUT A2 Output Voltage Swing (Notes 8, 9) RL= 100 k on Output RL= 10 k on Output VOL VOH VOL VOH
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Min
Typ (Note 5)
Max
Units mV V/C nA mA k k V V
25C TA 125C -40C TA 25C
0.25 20 20
2.0 30 35 20
1.2 1.4 240 480 +27 36
200 400
DC Common Mode Rejection Ratio 0C TA 125C -40C TA 0C
103 dB
AC CMRR PSRR RF-INT TCRF-INT
AC Common Mode Rejection Ratio -2V VCM 27V (Note 7) Power Supply Rejection Ratio Filter Resistor Filter Resistor Drift Midscale Offset Scaling Accuracy (Pin 7) 4.75V VS 5.5V
dB dB 103 k ppm/C % V/V ppm/C V/V V/V V V V
AV AV1 AV2 A1 VOUT
Total Gain Gain Drift A1 Gain A2 Gain A1 Output Voltage Swing
20 10 2 0.004 4.95 0.007
20.2
25
10.1 2.02 0.001 0.02
4.80 4.80
4.99 0.03 4.95
LMP8276
5V Electrical Characteristics
Symbol SR BW Noise Parameter Slew Rate (Note 10) Bandwidth 0.1 Hz to 10 Hz Spectral Density
(Note 4)
(Continued)
Unless otherwise specified, all limits guaranteed for TA = 25C, VS = 5V, GND= 0, -2V VCM 27V, RL= OPEN. Boldface limits apply at the temperature extremes. Conditions Min Typ (Note 5) 0.7 80 5.7 f = 1kHz 452 Max Units V/s kHz Vpp nV/
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see the Electrical Characteristics Tables. Note 2: Human Body Model, 1.5 k in series with 100 pF. Machine Model: 0 in series with 200 pF. Note 3: The maximum power dissipation is a function of TJ(MAX), JA, and TA. The maximum allowable power dissipation at any ambient temperature is PD = (TJ(MAX) - TA)/ JA . All numbers apply for packages soldered directly onto a PC board. Note 4: Electrical table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very limited self-heating of the device. Note 5: Typical values represent the parametric norm at the time of characterization. Note 6: Positive current corresponds to current flowing into the device. Note 7: AC Common Mode Signal is a 24 VPP sine-wave (0V to 24V) at the given frequency. Note 8: For VOL, RL is connected to VS and for VOH, RL is connected to GND. Note 9: For this test input is driven from A1 stage. Note 10: Slew rate is the average of the rising and falling slew rates.
Connection Diagram
8-Pin SOIC
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Top View
Ordering Information
Package 8-Pin SOIC Part Number LMP8276MA LMP8276MAX Package Marking LMP8276MA Transport Media 95 Units/Rail 2.5k Units Tape and Reel NSC Drawing M08A
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LMP8276
Typical Performance Characteristics
VOS vs. VCM Over Temperature
Unless otherwise specified: TA = 25C, VS = 5V, VCM = VS/2 Typical VOS vs. Temperature
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Input Bias Current Over Temperature (A1 Inputs)
Input Bias Current Over Temperature (A2 Inputs)
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Input Bias Current Over Temperature (A2 Inputs)
Input Referred Voltage Noise vs. Frequency
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LMP8276
Typical Performance Characteristics Unless otherwise specified: TA = 25C, VS = 5V, VCM = VS/2
(Continued) PSRR vs. Frequency VOS vs. Supply Voltage
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Gain vs. Frequency Over Temperature
Gain vs. Frequency Over Temperature
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Filter Resistor
CMRR vs. Frequency
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LMP8276
Typical Performance Characteristics Unless otherwise specified: TA = 25C, VS = 5V, VCM = VS/2
(Continued) Settling Time (Rising Edge) Settling Time (Falling Edge)
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Output Voltage vs. RL to VS
Output Voltage vs. RL to GND
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Step Response
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LMP8276
Application Note
LMP8276 The LMP8276 is a single supply amplifier with a fixed gain of 20 and an extended common mode voltage range of -2V to
36V. The fixed gain is achieved in two separate stages, a preamplifier with gain of +10 and a second stage amplifier with gain of +2. A block diagram of the LMP8276 is shown in Figure 1.
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FIGURE 1. The overall offset of the LMP8276 is minimized by trimming amplifier A1. This is done so that the output referred offset of A1 cancels the input referred offset of A2 or 10VOS1 = -VOS2 Because of this offset voltage relationship, the offset of each individual amplifier stage may be more than the limit specified for the overall system in the datasheet tables. Care must be given when pin 3 and 4, A1 and A2, are connected to each other. If the signal going from A1 to A2 is amplified or attenuated (by use of amplifiers and resistors), the overall LMP8276 offset will be affected as a result. Filtering the signal between A1 and A2 or simply connecting the two pins will not change the offset of the LMP8276. Referencing the input referred offset voltages of each stage, the following relationship holds: that offset. Namely, if the offset is connected to VX, then the output increases by 1/2VX. This relationship is shown in Figure 2:
If the signal on pin 3 is scaled, attenuated or amplified, by a factor X then the offset of the overall system will become:
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FIGURE 2. Note that the offset pin, pin 7, needs to be connected at all times. If the pin is left floating, the LMP8276 will be operating in an undefined mode. Also pin 7 should be driven from a low impedance source. POWER SUPPLY DECOUPLING In order to decouple the LMP8276 from AC noise on the power supply, it is recommended to use a 0.1 F on the supply pin. It is best to use a 0.1 F capacitor in parallel with a 10 F capacitor. This will generate an AC path to ground for most frequency ranges and will greatly reduce the noise introduced by the power supply.
MID-RAIL OFFSET ADJUSTMENT PIN The external mid-rail offset adjustment pin enables the user to utilize the LMP8276 for bidirectional current sensing. If the offset pin, pin 7, is connected to ground then the LMP8276 is capable of sensing positive signals. When the offset pin is connected to VS then 1/2VS is added to the output and the amplifier can sense both positive and negative signals. If this offset pin is connected to any voltage between ground and VS, then the output is increased by a voltage equal to half of
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LMP8276
Application Note
CURRENT LOOP RECEIVER
(Continued)
central control room. The LMP8276 can be used as a current loop receiver as shown in Figure 3.
Many types of process control instrumentation use 4 to 20 mA transmitters to transmit the sensor's analog value to a
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FIGURE 3. HIGH SIDE CURRENT SENSING High side current measurement requires a differential amplifier with gain. Here the DC voltage source represent a common mode voltage with the +IN input at the supply voltage and the -IN input very close to the supply voltage. The LMP8276 can be used with a common mode voltage, VDC in this case, of up to 36 volts The LMP8276 can be used for high side current sensing. The large common mode voltage range of this device allows it to sense signals outside of its supply voltage range. Also, the LMP8276 has very high CMRR, which enables it to sense very small signals in the presence of larger common mode signals. The system in Figure 4 couples these two characteristics of the LMP8276 in an automotive application. The signal through RS1 is detected and amplified by the LMP8276 in the presence of a common mode signal of up to 27 Volts with the highest accuracy.
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LMP8276
Application Note
(Continued)
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FIGURE 4. LOW SIDE CURRENT SENSING Low side current measurements can cause a problem for operational amplifiers by exceeding the negative common mode voltage limit of the device. In Figure 5, the load current is returning to the power source through a common connection that has a parasitic resistance. The voltage drop across the parasitic resistances can cause the ground connection of the measurement circuits to be at a positive voltage with respect to the common side of the sense resistor. This will result in one or both of the inputs being negative with respect to the circuit's ground. The LMP8276 has a wide input common mode voltage range of -2 to 36 volts and will function in this condition.
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LMP8276
Application Note
(Continued)
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FIGURE 5. SECOND ORDER LOW PASS FILTER The LMP8276 can be used to build a second order SallenKey low pass filter. The general filter is shown in Figure 6 and
K represents the sum of DC closed loop gain and the nonideal behavior of the operational amplifier. Assuming ideal behavior, the equation for K reduces simply to DC gain, which is +2 for the LMP8276. The LMP8276 can be used to realize this configuration as shown in Figure 7:
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FIGURE 6. With the general transfer function
(1) Where:
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LMP8276
Application Note
(Continued)
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FIGURE 7. Using Equation (1), the filter parameters can be calculated as follows: For the LMP8276, R1 = 100K. Setting R1 = R2 and C1 = C2results in a low pass filter with Q = 1. Since the values of resistors are predetermined, the corner frequency of this implementation of the filter depends on the capacitor values. GAINS OTHER THAN 20 The LMP8276 has an internal gain of +20; however, this gain can be modified. The signal path between the two amplifiers is available as external pins. GAINS LESS THAN 20 Figure 8 shows the configuration used to reduce the LMP8276 gain.
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LMP8276
Application Note
(Continued)
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FIGURE 8. Where: GAINS GREATER THAN 20 A higher gain can be achieved by using positive feedback on the second stage amplifier, A2, of LMP8276. Figure 9 shows the configuration:
and
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FIGURE 9. The total gain is given by:
which can be rearranged to calculate RG:
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The inverting gain of the second amplifier is set at 2, giving the total system gain of 20. The non-inverting gain which is achieved through positive feedback can be less than or
LMP8276
Application Note
(Continued)
equal to this gain without any issues. This implies a total
system gain of 40 or less is easily achievable. Once the positive gain surpasses the negative gain, the system might oscillate.
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LMP8276 High Common Mode, Gain of 20, Bidirectional Precision Voltage Difference Amplifier
Physical Dimensions
inches (millimeters) unless otherwise noted
8-Pin SOIC NS Package Number M08A
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. For the most current product information visit us at www.national.com. LIFE SUPPORT POLICY NATIONAL'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. BANNED SUBSTANCE COMPLIANCE National Semiconductor manufactures products and uses packing materials that meet the provisions of the Customer Products Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ``Banned Substances'' as defined in CSP-9-111S2. Leadfree products are RoHS compliant.
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