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Advance Data Sheet September 2001
LSP2961 Precision Log Ratio Amplifier
Features
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Seven decades (1 nA to 10 mA) typical input range Internal scale factor K = 1 2% (K externally adjustable): OUTPUT = K x log IREF -----------IIN
(eq. 1)
decade (1 nA to 1 mA) minimum, with seven-decade (1 nA to 10 mA) and eight-decade (100 pA to 10 mA) operation available with increased conformity error. The amplifier has an internal trimmed reference 1 A 5% current source that is also externally adjustable using an external resistor. The output offset voltage is 100 mV maximum and can be nulled using an external potentiometer. The input current is applied to the IN pin and the corresponding output is taken from the OUTPUT pin. For a 1 V/decade application in a stable temperature environment, no external components are required to obtain the specified performance. For an application that a scale factor other than 1 V/decade is required, an external resistor connecting the SCALE FACTOR pin to either the OUTPUT pin or GND will result in less or more than 1 V/decade for the scale factor. The amplifier's output can range from (VCC - 1 V) to (VEE + 1 V). This is ideal for the control in optical density measurements, and in other situations such as precision logarithm calculations. Each amplifier has an internally trimmed reference current of 1 A. This reference current can be changed using an external resistor connecting to the IREF_ADJ pin, and either VCC or GND, depending on the need to increase or decrease the reference current. Additionally, the maximum output offset voltage is 100 mV. This can be nulled using an external potentiometer connected between the 5 V supplies and the wiper connected to the VOS_ADJ pin. The internal block diagram is shown in Figure 1. The LSP2961 is available in a 3 mm x 3 mm TSSOP package. The 8-pin LSP2961 requires one negative 5 V power supply (VEE) and one positive 5 V power supply (VCC). Refer to Table 1 for detailed pin information. Figure 3 references the external pin connections.
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Internal trimmed reference current (IREF = 1 A 5%, externally adjustable) Maximum 100 mV output offset (externally adjustable) Minimal external components 5 V power supply TSSOP small package
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Applications
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Optical light density measurement Log, log ratio computation Data linearization
Description
The LSP2961 is a single-channel precision logarithm and log ratio integrated circuit powered by +5 V and -5 V power supplies. The voltage output of the amplifier is a logarithmic function of the ratio of a reference current (IREF) over the input current (IIN). The scale factor is set internally to 1 V/decade, and is further adjustable by adding an external resistor. The input dynamic range for the amplifier is a six-
LSP2961 Precision Log Ratio Amplifier
Advance Data Sheet September 2001
Description (continued)
Block Diagram
IREF_ADJ (3) VCC (5)
REFERENCE CURRENT
IN (1) VOS_ADJ (8)
VIN- VIN+ 10.2 k
VIN+ OUTPUT (6) VIN- 160 k
GND (2)
SCALE FACTOR (7)
VEE (4)
2352.a(F)
Figure 1. LSP2961 Internal Block Diagram
Typical Application Circuit
VCC IREF_ADJ (3) 0.1 F
REFERENCE CURRENT
PIN VOS_ADJ (8)
VIN- VIN+ 10.2 k
VIN+ OUTPUT (6) VIN- 160 k
SCALE FACTOR (7) GND (2) 1 k (0.42%) THERMISTOR 16 k RESISTOR
2353.b(F)
VEE
0.1 F
Figure 2. Typical Application Circuit with External Temperature Compensation
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Agere Systems Inc.
Advance Data Sheet September 2001
LSP2961 Precision Log Ratio Amplifier
Pin Information
IN
1
8
VOS_ADJ SCALE FACTOR OUTPUT VCC
2311(F)
GND IREF_ADJ VEE
2 3 4
7 6 5
Figure 3. External Connections Table 1. LSP2961 Pin Description Pin Number 1 2 3 4 5 6 7 Pin Name IN GND IREF_ADJ Function Pin Total Counts 1 1 1 1 1 1 1 Description Input current. Analog ground. Used to set reference current with an external resistor to GND or VCC. -5 V power supply. 5 V power supply. Output voltage. Used to change the scale factor to other than 1 V/decade (either less than or greater than) with an external resistor connecting to the OUTPUT pin or GND. Used to null offset voltage by connecting to the wiper of an external potentiometer between VCC and VEE.
Input Ground Sets the Reference Current to Other than 1 A VEE Power Supply Power Supply VCC OUTPUT Output Voltage SCALE FACTOR Sets the Scale Factor to Other than 1 V/decade
8
VOS_ADJ
Offset Voltage Adjustment
1
Agere Systems Inc.
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LSP2961 Precision Log Ratio Amplifier
Advance Data Sheet September 2001
Absolute Maximum Ratings
Stresses in excess of the absolute maximum ratings can cause permanent damage to the device. These are absolute stress ratings only. Functional operation of the device is not implied at these or any other conditions in excess of those given in the operational sections of the data sheet. Exposure to absolute maximum ratings for extended periods can adversely affect device reliability. Table 2. Absolute Maximum Ratings Parameter Power Supply VCC VEE Operating Temperature (TA) Storage Temperature Lead Temperature (soldering 10 seconds) Min 4.0 -6.0 -40 -65 -- Typ 5.0 -5.0 25 25 -- Max 6.0 -4.0 85 150 300 Unit V V C C C
Handling Precautions
Although protection circuitry has been designed into this device, proper precautions should be taken to avoid exposure to electrostatic discharge (ESD) during handling and mounting. Agere Systems employs a human-body model (HBM) and charged-device model (CDM) for ESD-susceptibility testing and protection design evaluation. ESD voltage thresholds are dependent on the circuit parameters used in the defined model. No industrywide standard has been adopted for the CDM. However, a standard HBM (resistance = 1500 , capacitance = 100 pF) is widely used and, therefore, can be used for comparison purposes. The HBM ESD threshold (>1500 V) presented here was obtained by using these circuit parameters.
Application Considerations
A typical application is shown in Figure 2 for optical light intensity detection with temperature compensation by using a thermistor with positive temperature coefficient of 0.42%.
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Agere Systems Inc.
Advance Data Sheet September 2001
LSP2961 Precision Log Ratio Amplifier
Electrical Characteristics
Table 3. Electrical Characteristics TA = 25 C, VCC = 5 V, VEE = -5 V, no external components for performance adjustment. Parameter Total Conformity Error IREF K x log ----------- - VOUT IIN -------------------------------------------------------- x 100% VFULLSCALE Symbol -- Conditions TA = 25 C IN = 1 nA to 1 mA TA = 25 C IN = 1 nA to 10 mA TA = 25 C IN = 100 pA to 10 mA Temperature Coefficient1, 2 Scale Factor 3 Scale Factor Temperature Coefficient2 Reference Accuracy 3 Output Offset Voltage 4 3 dB Bandwidth 5, 6 Output Voltage Range Supply Current
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Min -- -- -- -- 0.98 -- 0.95 -- -100 -- -- VEE + 1 0.2 0.18 4.5 -4.5
Typ 1 TBD TBD 0.34 1 0.34 1 0.02 -10 0.20 100 -- 0.3 0.24 5.0 -5.0
Max TBD -- -- -- 1.02 -- 1.05 -- 100 -- -- VCC - 1 1.3 0.3 5.5 -5.5
Unit %
-- K -- -- -- VOS -- f3dB -- --
-5 C to 70 C2 TA = 25 C IN = 1 nA to 1 mA -5 C to 70 C1 TA = 25 C IN = 1 nA to 1 mA -5 C to 70 C1 TA = 25 C IN = 1 nA to 1 mA -5 C to 70 C C = 10 pF, IN = 1 A -- RLOAD > 1 M IN = 1 nA to 1 mA -5 C to 70 C1 IN 1 A 8 -- --
%/C V/decade %/C A %/C mV %/C kHz V mA
Reference Accuracy Temperature Coefficient2 -- Temperature Coefficient2
Power Supply Operation Range2
1. 2. 3. 4. 5. 6. 7. 8.
VCC VEE
V
Can be externally compensated using a thermistor and a resistor. Guaranteed by design. Externally adjustable using a resistor. Can be nulled using an external potentiometer. Transimpedance bandwidth. Tested by limited samples. Supply current at 1 nA < IN < 1 mA. Supply current at IN 1 A.
Note: Terms used for the specification of the analog log amplifier:
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Total conformity error--this is used to describe the log linearity of the log amplifier, which is the maximum deviation from the best-fit straight line of logarithm, allowing for offset error.
Agere Systems Inc.
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LSP2961 Precision Log Ratio Amplifier
Advance Data Sheet September 2001
LSP2961 Package Diagram
0.60
4 1 1
0.50
0.60
4.90 BSC
5
8
2.90/3.10 0.65 BSC
1.10 MAX 0.75/0.95 0.10 C SEATING PLANE 0.127/0.25 0/6 2.90/3.10 0.40/0.70
0790(F)
0.13/0.23
0.25/0.40
For additional information, contact your Agere Systems Account Manager or the following: INTERNET: http://www.agere.com E-MAIL: docmaster@agere.com N. AMERICA: Agere Systems Inc., 555 Union Boulevard, Room 30L-15P-BA, Allentown, PA 18109-3286 1-800-372-2447, FAX 610-712-4106 (In CANADA: 1-800-553-2448, FAX 610-712-4106) ASIA: Agere Systems Hong Kong Ltd., Suites 3201 & 3210-12, 32/F, Tower 2, The Gateway, Harbour City, Kowloon Tel. (852) 3129-2000, FAX (852) 3129-2020 CHINA: (86) 21-5047-1212 (Shanghai), (86) 10-6522-5566 (Beijing), (86) 755-695-7224 (Shenzhen) JAPAN: (81) 3-5421-1600 (Tokyo), KOREA: (82) 2-767-1850 (Seoul), SINGAPORE: (65) 778-8833, TAIWAN: (886) 2-2725-5858 (Taipei) EUROPE: Tel. (44) 7000 624624, FAX (44) 1344 488 045
Agere Systems Inc. reserves the right to make changes to the product(s) or information contained herein without notice. No liability is assumed as a result of their use or application.
Copyright (c) 2001 Agere Systems Inc. All Rights Reserved
September 2001 DS01-188ASP

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