LMP7731, Low Noise, Precision, RRIO Amplifier
June 16, 2008
LMP7731 2.9 nV/sqrt(Hz) Low Noise, Precision, RRIO Amplifier
General Description
The LMP7731 is a single, low noise, low offset voltage, railto-rail input and output, low voltage precision amplifier. The LMP7731 is part of the LMP® precision amplifier family and is ideal for precision and low noise applications with low voltage requirements. This operational amplifier offers low voltage noise of 2.9 nV/ with a 1/f corner of only 3 Hz and low DC offset with a maximum value of ±40 µV, targeting high accuracy, low frequency applications. The LMP7731 has bipolar input stages with a bias current of only 1.5 nA. This low input bias current, complemented by the very low AC and DC levels of voltage noise, makes the LMP7731 an excellent choice for photometry applications. The LMP7731 provides a wide GBW of 22 MHz while consuming only 2 mA of current. This high gain bandwidth along with the high open loop gain of 130 dB enables accurate signal conditioning in applications with high closed loop gain requirements. The LMP7731 has a supply voltage range of 1.8V to 5.5V, making it an ideal choice for battery operated portable applications. The LMP7731 is offered in the space saving 5-Pin SOT-23 and 8-Pin SOIC packages.
Features
(Typical values, TA = 25°C, VS = 5V) ■ Input voltage noise — f = 3 Hz — f = 1 kHz ■ Offset voltage (max) ■ Offset voltage drift (max) ■ CMRR ■ Open loop gain ■ GBW ■ Slew rate ■ THD @ f = 10 kHz, AV = +1, RL = 2 kΩ ■ Supply current per channel ■ Supply voltage range ■ Operating temperature range ■ Input bias current ■ RRIO 3.3 nV/√Hz 2.9 nV/√Hz ±40 µV ±1.0 µV/°C 130 dB 130 dB 22 MHz 2.4 V/µs 0.001% 2.2 mA 1.8V to 5.5V −40°C to 125°C ±1.5 nA
Applications
■ ■ ■ ■
Thermopile amplifier Gas analysis instruments Photometric instrumentation Medical instrumentation
Typical Application
Thermopile Signal Amplifier
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LMP® is a registered trademark of National Semiconductor Corporation.
© 2008 National Semiconductor Corporation
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LMP7731
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 Inputs pins only All other pins Machine Model Charge Device Model VIN Differential Supply Voltage (VS = V+ – V−)
Storage Temperature Range Junction Temperature (Note 3) Soldering Information Infrared or Convection (20 sec) Wave Soldering Lead Temp. (10 sec)
−65°C to 150°C +150°C max 235°C 260°C
2000V 2000V 200V 1000V ±2V 6.0V (Note 4)
Operating Ratings
Temperature Range Supply Voltage (VS = V+ – V–)
(Note 1) −40°C to 125°C 1.8V to 5.5V 265°C/W 190°C/W
Package Thermal Resistance (θJA) 5-Pin SOT-23 8-Pin SOIC
2.5V Electrical Characteristics
Symbol VOS Parameter Input Offset Voltage (Note 7)
Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = 2.5V, V− = 0V, VCM = V+/2, RL >10 kΩ to V+/2. Boldface limits apply at the temperature extremes. Conditions VCM = 2.0V VCM = 0.5V TCVOS Input Offset Voltage Temperature Drift VCM = 2.0V VCM = 0.5V Input Offset Voltage Time Drift IB Input Bias Current VCM = 0.5V and VCM = 2.0V VCM = 2.0V VCM = 0.5V IOS Input Offset Current VCM = 2.0V VCM = 0.5V TCIOS CMRR Input Offset Current Drift Common Mode Rejection Ratio VCM = 0.5V and VCM = 2.0V 0.15V ≤ VCM ≤ 0.7V 1.5V ≤ VCM ≤ 2.35V PSRR Power Supply Rejection Ratio 2.5V ≤ V+ ≤ 5V 1.8V ≤ V+ ≤ 5.5V CMVR AVOL Common Mode Voltage Range Open Loop Voltage Gain Large Signal CMRR ≥ 80 dB RL = 10 kΩ to VOUT = 0.5V to 2.0V V+/2 RL = 2 kΩ to V+/2 VOUT = 0.5V to 2.0V 0 112 104 109 90 130 119 dB 0.23V ≤ VCM ≤ 0.7V 1.5V ≤ VCM ≤ 2.27V 101 89 105 99 111 105 Min Typ Max (Note 6) (Note 5) (Note 6) ±9 ±9 ±0.5 ±0.2 0.35 ±1 ±12 ±1 ±11 0.0474 120 dB ±30 ±45 ±50 ±75 ±50 ±75 ±60 ±80 ±50 ±120 ±40 ±100 ±1.0 ±0.8 Units
μV
μV/°C μV/month
nA
nA
nA/°C
129
129 dB 117 2.5 V
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LMP7731
Symbol VOUT
Parameter Output Voltage Swing High
Conditions RL = 10 kΩ to V+/2 RL = 2 kΩ to V+/2
Min Typ Max (Note 6) (Note 5) (Note 6) 4 13 6 9 22 12 15 10 31 44 2.0 2.3 2.7 3.4 3.1 3.9 50 75 50 75 50 75 50 75
Units
Output Voltage Swing Low
RL = 10 kΩ to V+/2 RL = 2 kΩ to V+/2
mV from either rail
IOUT
Output Current
Sourcing, VOUT = V+/2 VIN (diff) = 100 mV Sinking, VOUT = V+/2 VIN (diff) = −100 mV
mA
IS
Supply Current (Per Channel)
VCM = 2.0V VCM = 0.5V
mA
SR GBW GM ΦM RIN THD+N en
Slew Rate Gain Bandwidth Gain Margin Phase Margin Input Resistance Total Harmonic Distortion + Noise Input Referred Voltage Noise Density Input Voltage Noise
AV = +1, CL = 10 pF, RL = 10 kΩ to V+/2, VO = 2 VPP CL = 20 pF, RL = 10 kΩ to V+/2 CL = 20 pF, RL = 10 kΩ to V+/2 CL = 20 pF, RL = 10 kΩ to V+/2 Differential Mode Common Mode AV = 1, f = 1 kHz, Amplitude = 1V f = 1 kHz, VCM = 2.0V f = 1 kHz, VCM = 0.5V 0.1 Hz to 10 Hz f = 1 kHz, VCM = 2.0V f = 1 kHz, VCM = 0.5V
2.4 21 14 60 38 151 0.002 3 3 75 1.1 2.3
V/μs MHz dB deg kΩ MΩ % nV/ nVPP pA/
in
Input Referred Current Noise Density
3.3V Electrical Characteristics
Symbol VOS Parameter Input Offset Voltage (Note 7)
(Note 4)
Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = 3.3V, V− = 0V, VCM = V+/2, RL > 10 kΩ to V+/2. Boldface limits apply at the temperature extremes. Conditions VCM = 2.5V VCM = 0.5V TCVOS Input Offset Voltage Temperature Drift VCM = 2.5V VCM = 0.5V Input Offset Voltage Time Drift IB Input Bias Current VCM = 0.5V and VCM = 2.5V VCM = 2.5V VCM = 0.5V IOS Input Offset Current VCM = 2.5V VCM = 0.5V Min Typ Max (Note 6) (Note 5) (Note 6) ±6 ±6 ±0.5 ±0.2 0.35 ±1.5 ±13 ±1 ±11 ±30 ±45 ±50 ±77 ±50 ±70 ±60 ±80 ±50 ±120 ±40 ±100 ±1.0 ±0.8 Units
μV
μV/°C μV/month
nA
nA
3
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LMP7731
Symbol TCIOS CMRR
Parameter Input Offset Current Drift Common Mode Rejection Ratio
Conditions VCM = 0.5V and VCM = 2.5V 0.15V ≤ VCM ≤ 0.7V 1.5V ≤ VCM ≤ 3.15V 0.23V ≤ VCM ≤ 0.7V 1.5V ≤ VCM ≤ 3.07V
Min Typ Max (Note 6) (Note 5) (Note 6) 0.048 101 89 105 99 111 105 0 112 104 110 92 130 119 5 14 9 13 28 22 25 20 45 48 2.1 2.4 2.4 22 14 62 38 151 0.002 2.9 2.9 65 1.1 2.1 2.8 3.5 3.2 4.0 50 75 50 75 50 75 50 75 120
Units nA/°C
130
dB
PSRR
Power Supply Rejection Ratio
2.5V ≤ V+ ≤ 5.0V 1.8V ≤ V+ ≤ 5.5V
129 dB 117 3.3 V
CMVR AVOL
Common Mode Voltage Range Open Loop Voltage Gain
Large Signal CMRR ≥ 80 dB RL = 10 kΩ to VOUT = 0.5V to 2.8V V+/2 RL = 2 kΩ to V+/2 VOUT = 0.5V to 2.8V
dB
VOUT
Output Voltage Swing High
RL = 10 kΩ to V+/2 RL = 2 kΩ to V+/2
Output Voltage Swing Low
RL = 10 kΩ to
V+/2
mV from either rail
RL = 2 kΩ to V+/2 IOUT Output Current Sourcing, VOUT = V+/2 VIN (diff) = 100 mV Sinking, VOUT = V+/2 VIN (diff) = -100 mV IS Supply Current (Per Channel) VCM = 2.5V VCM = 0.5V SR GBW GM ΦM RIN THD+N en Slew Rate Gain Bandwidth Gain Margin Phase Margin Input Resistance Total Harmonic Distortion + Noise Input Referred Voltage Noise Density Input Voltage Noise in Input Referred Current Noise Density AV = +1, CL = 10 pF, RL = 10 kΩ to V+/2, VOUT = 2 VPP CL = 20 pF, RL = 10 kΩ to V+/2 CL = 20 pF, RL = 10 kΩ to V+/2 CL = 20 pF, RL = 10 kΩ to Differential Mode Common Mode AV = 1, f = 1 kHz, Amplitude = 1V, f = 1 kHz, VCM = 2.5V f = 1 kHz, VCM = 0.5V 0.1 Hz to 10 Hz f = 1 kHz, VCM = 2.5V f = 1 kHz, VCM = 0.5V V+/2
mA
mA
V/μs MHz dB deg kΩ MΩ % nV/ nVPP pA/
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5V Electrical Characteristics
Symbol VOS Parameter Input Offset Voltage (Note 7)
(Note 4)
Unless otherwise specified, all limits are guaranteed for TA = 25°C, V+ = 5V, V− = 0V, VCM = V+/2, RL > 10 kΩ to V+/2. Boldface limits apply at the temperature extremes. Conditions VCM = 4.5V VCM = 0.5V TCVOS Input Offset Voltage Temperature Drift VCM = 4.5V VCM = 0.5V Input Offset Voltage Time Drift IB Input Bias Current VCM = 0.5V and VCM = 4.5V VCM = 4.5V VCM = 0.5V IOS Input Offset Current VCM = 4.5V VCM = 0.5V TCIOS CMRR Input Offset Current Drift Common Mode Rejection Ratio VCM = 0.5V and VCM = 4.5V 0.15V ≤ VCM ≤ 0.7V 1.5V ≤ VCM ≤ 4.85V PSRR Power Supply Rejection Ratio 2.5V ≤ V+ ≤ 5V 1.8V ≤ V+ ≤ 5.5V CMVR AVOL Common Mode Voltage Range Open Loop Voltage Gain Large Signal CMRR ≥ 80 dB RL = 10 kΩ to VOUT = 0.5V to 4.5V V+/2 RL = 2 kΩ to V+/2 VOUT = 0.5V to 4.5V VOUT Output Voltage Swing High RL = 10 kΩ to V+/2 RL = 2 kΩ to V+/2 Output Voltage Swing Low RL = 10 kΩ to V+/2 RL = 2 kΩ to V+/2 IOUT Output Current Sourcing, VOUT = V+/2 VIN (diff) = 100 mV Sinking, VOUT = V+/2 VIN (diff) = -100 mV IS Supply Current (Per Channel) VCM = 4.5V VCM = 0.5V SR GBW Slew Rate Gain Bandwidth AV = +1, CL = 10 pF, RL = 10 kΩ to V+/2, VOUT = 2 VPP CL = 20 pF, RL = 10 kΩ to V+/2
5
Min Typ Max (Note 6) (Note 5) (Note 6) ±6 ±6 ±0.5 ±0.2 0.35 ±1.5 ±14 ±1 ±11 0.0482 101 89 105 99 111 105 0 112 104 110 94 130 119 8 24 9 23 33 27 30 25 47 49 2.2 2.5 3.0 3.7 3.4 4.2 50 75 50 75 50 75 50 75 120 ±30 ±50 ±50 ±85 ±50 ±70 ±65 ±80 ±50 ±120 ±40 ±100 ±1.0 ±0.8
Units
μV μV/°C μV/month
nA
nA
nA/°C
0.23V ≤ VCM ≤ 0.7V 1.5V ≤ VCM ≤ 4.77V
130
dB
129 dB 117 5 V
dB
mV from either rail
mA
mA
2.4 22
V/μs MHz
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LMP7731
Symbol GM ΦM RIN THD+N en Gain Margin Phase Margin
Parameter
Conditions CL = 20 pF, RL = 10 kΩ to V+/2 CL = 20 pF, RL = 10 kΩ to Differential Mode Common Mode AV = 1, f = 1 kHz, Amplitude = 1V f = 1 kHz, VCM = 4.5V f = 1 kHz, VCM = 0.5V 0.1 Hz to 10 Hz f = 1 kHz, VCM = 4.5V f = 1 kHz, VCM = 0.5V V+/2
Min Typ Max (Note 6) (Note 5) (Note 6) 12 65 38 151 0.001 2.9 2.9 78 1.1 2.2
Units dB deg kΩ MΩ % nV/ nVPP pA/
Input Resistance Total Harmonic Distortion + Noise Input Referred Voltage Noise Density Input Voltage Noise
in
Input Referred Current Noise Density
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, applicable std. MIL-STD-883, Method 3015.7. Machine Model, applicable std. JESD22-A115-A (ESD MM std. of JEDEC) Field-Induced Charge-Device Model, applicable std. JESD22-C101-C (ESD FICDM std. of JEDEC). Note 3: The maximum power dissipation is a function of TJ(MAX), θJA. The maximum allowable power dissipation at any ambient temperature is PD = (TJ(MAX) – TA)/ θJA. 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 such that TJ = TA. No guarantee of parametric performance is indicated in the electrical tables under conditions of internal self-heating where TJ > TA. Absolute maximum Ratings indicate junction temperature limits beyond which the device maybe permanently degraded, either mechanically or electrically. Note 5: Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary over time and will also depend on the application and configuration. The typical values are not tested and are not guaranteed on shipped production material. Note 6: All limits are guaranteed by testing, statistical analysis or design. Note 7: Ambient production test is performed at 25°C with a variance of ±3°C.
Connection Diagrams
5-Pin SOT-23 8-Pin SOIC
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Top View
Top View
Ordering Information
Package 5-Pin SOT-23 Part Number LMP7731MF LMP7731MFE LMP7731MFX 8-Pin SOIC LMP7731MA LMP7731MAX LMP7731MA AY3A Package Marking Transport Media 1k Units Tape and Reel 250 Units Tape an Reel 3k Units Tape and Reel 95 Units/Rail 2.5k Tape and Reel M08A MF05A NSC Drawing
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LMP7731
Typical Performance Characteristics
Offset Voltage Distribution
Unless otherwise noted: TA = 25°C, RL > 10 kΩ, VCM = VS/2. TCVOS Distribution
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Offset Voltage Distribution
TCVOS Distribution
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Offset Voltage Distribution
TCVOS Distribution
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LMP7731
Offset Voltage Distribution
TCVOS Distribution
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Offset Voltage vs. Temperature
Offset Voltage vs. Temperature
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PSRR vs. Frequency
CMRR vs. Frequency
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LMP7731
Offset Voltage vs. Supply Voltage
Offset Voltage vs. VCM
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Offset Voltage vs. VCM
Offset Voltage vs. VCM
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Input Offset Voltage Time Drift
Slew Rate vs. Supply Voltage
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Input Voltage Noise vs. Frequency
Input Current Noise vs. Frequency
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Time Domain Voltage Noise
Time Domain Voltage Noise
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Time Domain Voltage Noise
Output Voltage vs. Output Current
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Input Bias Current vs. VCM
Input Bias Current vs. VCM
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Input Bias Current vs. VCM
Open Loop Frequency Response Over Temperature
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Open Loop Frequency Response
Open Loop Frequency Response
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THD+N vs. Frequency
THD+N vs. Output Voltage
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Large Signal Step Response
Small Signal Step Response
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Large Signal Step Response
Small Signal Step Response
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Supply Current vs. Supply Voltage
Output Swing High vs. Supply Voltage
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Output Swing Low vs. Supply Voltage
Sinking Current vs, Supply Voltage
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Sourcing Current vs. Supply Voltage
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LMP7731
Application Information
LMP7731 The LMP7731 is a single, low noise, low offset voltage, railto-rail input and output, and low voltage precision amplifier. with a 1/f corThe low input voltage noise of only 2.9 nV/ ner at 3 Hz makes the LMP7731 ideal for sensor applications where DC accuracy is of importance. The LMP7731 has a very low guaranteed offset voltage of only ±40 µV. This low offset voltage along with the very low input voltage noise allows higher signal integrity and higher signal to noise ratios as the error contribution by the amplifier is at a minimum. The LMP7731 has a high gain bandwidth of 22 MHz. This wide bandwidth enables use of the amplifier at higher gain settings while retaining usable bandwidth for the application. This is particularly beneficial when system designers need to use sensors with very limited output voltage range as it allows larger gains in one stage which in turn increases the signal to noise ratio. The LMP7731 has proprietary input bias cancellation circuitry on the input stages. This allows the LMP7731 to have only about 1.5 nA bias current with a bipolar input stage. This low input bias current, paired with the inherent lower input voltage noise of bipolar input stages makes the LMP7731 an excellent choice for precision applications. The combination of low input bias current, low input offset voltage, and low input voltage noise enables the user to achieve unprecedented accuracy and higher signal integrity. National Semiconductor is heavily committed to precision amplifiers and the market segment they serve. Technical support and extensive characterization data are available for sensitive applications or applications with a constrained error budget. The LMP7731 is offered in the space saving 5-Pin SOT-23 and 8-Pin SOIC packages. These small packages are ideal solutions for area constrained PC boards and portable electronics. INPUT BIAS CURRENT CANCELLATION The LMP7731 has proprietary input bias current cancellation circuitry on their input stages.
The LMP7731 has rail-to-rail input. This is achieved by having two input stages in parallel. Figure 1 shows only one of the input stages as the circuitry is symmetrical for both stages. Figure 1 shows that as the common mode voltage gets closer to one of the extreme ends, current I1 significantly increases. This increased current shows as an increase in voltage drop across resistor R1 equal to I1*R1 on IN+ of the amplifier. This voltage contributes to the offset voltage of the amplifier. When common mode voltage is in the mid-range, the transistors are operating in the linear region and I1 is significantly small. The voltage drop due to I1 across R1 can be ignored as it is orders of magnitude smaller than the amplifier's input offset voltage. As the common mode voltage gets closer to one of the rails, the offset voltage generated due to I1 increases and becomes comparable to the amplifiers offset voltage.
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FIGURE 1. Input Bias Current Cancellation INPUT VOLTAGE NOISE MEASUREMENT The LMP7731 has very low input voltage noise. The peak-topeak input voltage noise of the LMP7731 can be measured using the test circuit shown in Figure 2
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FIGURE 2. 0.1 Hz to 10 Hz Noise Test Circuit The frequency response of this noise test circuit at the 0.1 Hz corner is defined by only one zero. The test time for the 0.1
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Hz to 10 Hz noise measurement using this configuration should not exceed 10 seconds, as this time limit acts as an
LMP7731
additional zero to reduce or eliminate the noise contributions of noise from frequencies below 0.1 Hz. Figure 3 shows typical peak-to-peak noise for the LMP7731 measured with the circuit in Figure 2 for the LMP7731.
During the peak-to-peak noise measurement, the LMP7731 must be shielded. This prevents offset variations due to airflow. Offset can vary by a few nV due to this airflow and that can invalidate measurements of input voltage noise with a magnitude which is in the same range. For similar reasons, sudden motions must also be restricted in the vicinity of the test area. The feed-through which results from this motion could increase the observed noise value which in turn would invalidate the measurement. DIODES BETWEEN THE INPUTS The LMP7731 has a set of anti-parallel diodes between the input pins as shown in Figure 5. These diodes are present to protect the input stage of the amplifier. At the same time, they limit the amount of differential input voltage that is allowed on the input pins. A differential signal larger than the voltage needed to turn on the diodes might cause damage to the diodes. The differential voltage between the input pins should be limited to ±3 diode drops or the input current needs to be limited to ±20 mA.
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FIGURE 3. 0.1 Hz to 10 Hz Input Voltage Noise Measuring the very low peak-to-peak noise performance of the LMP7731, requires special testing attention. In order to achieve accurate results, the device should be warmed up for at least five minutes. This is so that the input offset voltage of the op amp settles to a value. During this warm up period, the offset can typically change by a few µV because the chip temperature increases by about 30°C. If the 10 seconds of the measurement is selected to include this warm up time, some of this temperature change might show up as the measured noise. Figure 4 shows the start-up drift of five typical LMP7731 units.
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FIGURE 5. Anti-Parallel Diodes between Inputs
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FIGURE 4. Start-Up Input Offset Voltage Drift
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LMP7731
DRIVING AN ADC Analog to Digital Converters, ADCs, usually have a sampling capacitor on their input. When the ADC's input is directly connected to the output of the amplifier a charging current flows from the amplifier to the ADC. This charging current causes a momentary glitch that can take some time to settle. There are different ways to minimize this effect. One way is to slow down the sampling rate. This method gives the amplifier sufficient time to stabilize its output. Another way to minimize the glitch caused by the switch capacitor is to have an external capacitor connected to the input of the ADC. This capacitor is chosen so that its value is much larger than the internal switching capacitor and it will hence provide the voltage needed to quickly and smoothly charge the ADC's sampling capacitor. Since this large capacitor will be loading the output of the amplifier as well, an isolation resistor is needed between the output of the amplifier and this capacitor. The isolation resistor, RISO, separates the additional load capacitance from the output of the amplifier and will also form a low-pass filter and can be designed to provide noise reduction as well as anti-aliasing. The drawback to having RISO is that it reduces signal swing since there is some voltage drop across it. Figure 6 (a) shows the ADC directly connected to the amplifier. To minimize the glitch in this setting, a slower sample rate needs to be used. Figure 6 (b) shows RISO and an external capacitor used to minimize the glitch.
THERMOPILE AMPLIFIER Thermopile Sensors Thermopiles are arrays of interconnected thermocouples which can detect the surface temperature of an object through radiation rather than direct contact. The hot and cold junctions of the thermocouples are thermally isolated. The hot junctions are exposed to IR radiation emitted from the measurement surface and the cold junctions are connected to a heat sink. The incident IR changes the temperature of the hot junctions of the thermopile and produces an output voltage proportional to this change. The hot junction of the thermopile is covered with a highly emissive coating. The IR radiation incident to this highly emissive material changes the temperature of this coating. The temperature change is converted to a voltage by the thermopile. Emissivity represents the radiation or absorption efficiency of a material relative to a black body. An ideal black body has an emissivity of 1.0. Excluding shiny metals, most objects have emissivities above 0.85. As a practical matter, shiny metals are not good candidates for IR sensing because of their low emissivity. The low emissivity means that the material is highly reflective. Reflective materials often “reflect” the temperature of their surrounding environment rather than their own heat radiation. This makes them not suitable for thermopile applications. The output voltage of a thermopile is related to temperature and emissivity by the following formula:
Where: VOUT : Output voltage of the thermopile K : Proportionality constant εOBJ: Emissivity of object being measured TOBJ: Temperature of object being measured δ : Correction factor. (This is needed since thermopile filters do not allow all wavelengths to enter the sensor.) εTP: Emissivity of the thermopile TTP: Temperature of the thermopile As mentioned above, the IR radiation generates a static voltage across the pyroelectric material. If the illumination is constant, the signal level detection declines. This is why the radiation needs to be periodically refreshed. This task is usually achieved by the means of a mechanical chopper in front of the detector. Thermopiles offer much faster response time compared to other temperature measurement devices. Packaged thermistors and thermocouples have response times that can range up to a few seconds, whereas packaged thermopiles can easily achieve response times in the order of tens of milliseconds. Thermopiles also provide superior thermal isolation compared to their contact temperature measurement counterparts. Physical contact disturbs the systems temperature and also creates temperature gradients. Figure 7 shows a simplified schematic of a thermopile. The cold junctions are connected to a heat sink, and the absorber material covers the hot junction. The output voltage resulting from the temperature difference between the two junctions is measured at the two ends of the array of thermocouples. As is evident in Figure 7, increasing the number of thermocouples in a thermopile increases the output voltage range. This also increases the active area of the thermopile sensor.
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FIGURE 6. Driving an ADC
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LMP7731
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FIGURE 7. Thermopile Thermopiles have very wide temperature ranges of -100°C to 1000°C When choosing a thermopile for a certain application, one must pay attention to several parameters. Thermopiles' sensitivity, or responsivity, is determined by the ratio of output voltage to the absorbed input signal power and is usually specified in V/W. Typical sensitivity of thermopiles ranges from 10s of V/W to about 100 V/W. Generally, higher values of sensitivity are desirable. Sensitivity is dependent on the absorber's area and number of thermocouples used in the sensor. Sensitivity is often represented by S where: S = VOUT/PIN The sensitivity of a thermopile varies with change in temperature. This change is usually specified as the Temperature Coefficient, TC, of sensitivity. Lower numbers are desired for this parameter. Resistance of the thermopiles is usually specified in the datasheet. This is the impedance which will be seen by the input of the amplifier used to process the thermopile's output signal. Typical values for thermopile resistance, RTP, range from 10s of kilo-ohms to about 100 kΩ. This resistance is also a function of temperature. The temperature coefficient of the resistance is usually specified in a thermopile's datasheet. As with any other parameter, minimum variation with temperature is desired. The dominant noise source for a thermopile is its resistance. The noise spectral density of a resistor is calculated by:
As shown above, the NEP of two thermopiles cannot be compared without considering the corresponding active areas. A better way to compare thermopiles is to look at their specific detectivity, D*. Specific detectivity includes both the device noise and its sensitivity. It is normalized with respect to the detector's active area and also noise bandwidth. D* is given by:
The unit of D* is cm / W. Typical values for specific detectivity range from 108 to 3*108 cm / W. After receiving radiation, the thermopile takes some time before it comes to thermal equilibrium. The time it takes for the sensor to achieve this equilibrium is called response time or time constant of the sensor. Clearly, lower time constants are very desirable. Precision Amplifier Since the output of thermopiles is usually very small and at most in the order of only a few millivolts, the first part of the signal conditioning path should involve amplification. In choosing an amplifier for this purpose, a few different sensor characteristics and the way they interface with the amplifier should be considered. Sensor's Impedance and Op Amp's Input Bias Current: The input bias current causes a voltage drop across the sensor and the amount of this voltage is equal to the sensor's impedance multiplied by the magnitude of bias current. The higher the sensor's input impedance, the more accentuated the effect of the amplifier's input bias current will be. For very high impedance sensors, it is imperative that op amps with very low input bias currents be used. Thermopiles have input impedances in the range of 100 kΩ, so input bias current is not as critical as in some other applications. Sensor's Output Voltage Range: The output signal of the sensor is fed into the op amp where it will be amplified or otherwise conditioned, e.g. level shifted, buffered. It is important to pay attention to different parameters of this output signal. The lowest expected level of the sensor's output is very important. It is necessary to compare this level with the different parameters contributing to the amplifier's total input noise. If the sensor's output level is in the same order of magnitude or smaller than the op amp's total input noise, then signal integrity at the op amp's output and the ADC's input will be compromised.
Where k is the Boltzman constant and T is absolute temperature. The unit of noise spectral density is: V/ For the thermopile sensor, this noise is usually represented by VNOISE where:
A typical value for this voltage noise is in the order of a few . tens of nV/ The Noise Equivalent Power, NEP, is often used to specify the minimum detectable signal level per square root bandwidth. A smaller NEP is desired, however NEP is dependent on the thermopile active area, AD. For a thermopile
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LMP7731
20175260
FIGURE 8. Thermopile Amplifier Figure 8 shows the LMP7731 used as a thermopile amplifier. The LMP7731 is a great choice for use with thermopile sensors. The LMP7731 provides unprecedented accuracy and precision because of its very low input voltage noise and the very low 1/f corner frequency. The 1/f noise is one of the main sources of error in DC operating mode. Since thermopiles and most other sensors operate on DC signals, signal integrity at the DC level is very important. The LMP7731 also has very low offset voltage and offset voltage drift which greatly reduces the effects of input offset voltage of the amplifier on the thermopile signal. The thermopile used in this circuit is TPS332 from PerkinElmer Optoelectronics, PKI. This thermopile has an internal resistance, RTP, of 75 kΩ. The output voltage of the thermopile is represented with a DC voltage source. The TPS332 has a thermistor integrated in the package. The thermistor is used to measure the ambient temperature of the thermopile at the time of measurement. The thermistor's resistance at room temperature is 30 kΩ. More information about this thermopile and other sensors from PKI can be found on http://www.perkinelmer.com/ The circuit in Figure 8 shows how the LMP7731 is connected to the thermopile. This circuit is comprised of two LMP7731 amplifiers, the LM4140A-2.5 which is a precision voltage reference, the ADC122S021 which is a 2 channel Analog to Digital converter, and the thermopile sensor. Note that the two amplifiers used in this circuit are numbered for ease of reference. The LMP7731 amplifiers are referred to as amplifier 1 and amplifier 2 per Figure 8. In Figure 8 the LM4140A is providing a precision voltage reference of 2.5V. This reference voltage is applied to the thermistor via the 30 kΩ resistor. The thermistor's resistance is converted to a voltage using this set up. This voltage is fed into the ADC's channel one. The ADC uses this voltage and the thermistor's look up table to convert this voltage to temperature. The 2.5V reference is also fed into amplifier 1, which is configured as a buffer. This LMP7731 transfers the 2.5V signal to both inputs of amplifier 2. This means the 2.5V will show up on the output of amplifier 2. Having an output level that is mid-supply is important since the thermopile sensor has a bipolar output signal and this way the amplifier can accurately gain the thermopile voltage, whether its polarity is positive or negative. It is also important because the output signal of amplifier 2 is only positive. ADCs can only handle positive signals on their inputs. Amplifier 2 is used to gain and filter the thermopile signal. The low pass filter ensures that AC noise will not be gained up and as a result the output signal will be cleaner. The output of amplifier 2 is fed into the ADC's channel 0. The ADC uses the ambient temperature, which was calculated using the voltage on Channel 1 and the thermistor's look up table, along with the thermopiles' gained output voltage available on channel 0 and the thermopile's look up table to determine the object's temperature.
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18
LMP7731
Physical Dimensions inches (millimeters) unless otherwise noted
5-Pin SOT-23 NS Package Number MF05A
8-Pin SOIC NS Package Number M08A
19
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LMP7731, Low Noise, Precision, RRIO Amplifier
Notes
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