Advance Data Sheet
September 2001
LSP2961
Precision Log Ratio Amplifier
Features
I
I
Seven decades (1 nA to 10 mA) typical input range
Internal scale factor K = 1 ± 2% (K externally
adjustable):
OUTPUT
=
K
×
log
I
REF
------------
-
I
IN
(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
µA
±
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 (V
CC
– 1 V) to
(V
EE
+ 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
µA.
This reference current can be changed using an
external resistor connecting to the I
REF_ADJ
pin, and
either V
CC
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 V
OS_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 (V
EE
) and one positive 5 V power
supply (V
CC
). Refer to Table 1 for detailed pin
information. Figure 3 references the external pin
connections.
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Internal trimmed reference current
(I
REF
= 1 µA ± 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 (I
REF
) over the input current (I
IN
).
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-
Advance Data Sheet
September 2001
LSP2961
Precision Log Ratio Amplifier
Pin Information
IN
1
8
V
OS_ADJ
SCALE
FACTOR
OUTPUT
V
CC
2311(F)
GND
I
REF_ADJ
V
EE
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
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 V
CC
.
–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 V
CC
and V
EE
.
Input
Ground
I
REF_ADJ
Sets the Reference Current to
Other than 1 µA
V
EE
Power Supply
Power Supply
V
CC
OUTPUT
Output Voltage
SCALE FACTOR Sets the Scale Factor to Other
than 1 V/decade
8
V
OS_ADJ
Offset Voltage Adjustment
1
Agere Systems Inc.
3
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
V
CC
V
EE
Operating Temperature (T
A
)
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
Ω,
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%.
4
Agere Systems Inc.
Advance Data Sheet
September 2001
LSP2961
Precision Log Ratio Amplifier
Electrical Characteristics
Table 3. Electrical Characteristics
T
A
= 25 °C, V
CC
= 5 V, V
EE
= –5 V, no external components for performance adjustment.
Parameter
Total
Conformity
Error
I
REF
K
×
log -----------
–
V
OUT
-
I
IN
--------------------------------------------------------
×
100
%
-
V
FULLSCALE
Symbol
—
Conditions
T
A
= 25 °C
IN = 1 nA to 1 mA
T
A
= 25 °C
IN = 1 nA to 10 mA
T
A
= 25 °C
IN = 100 pA to 10 mA
Temperature
Coefficient
1, 2
Scale Factor
3
Scale Factor
Temperature Coefficient
2
Reference
Accuracy
3
Output Offset
Voltage
4
3 dB Bandwidth
5, 6
Min
—
—
—
—
0.98
—
0.95
—
–100
—
—
V
EE
+ 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
—
—
V
CC
– 1
1.3
0.3
5.5
–5.5
Unit
%
—
K
—
—
—
V
OS
—
f
3dB
—
—
–5 °C to 70 °C
2
T
A
= 25 °C
IN = 1 nA to 1 mA
–5 °C to 70 °C
1
T
A
= 25 °C
IN = 1 nA to 1 mA
–5 °C to 70 °C
1
T
A
= 25 °C
IN = 1 nA to 1 mA
–5 °C to 70 °C
C = 10 pF, IN = 1 µA
—
R
LOAD
> 1 MΩ
IN = 1 nA to 1 mA
–5 °C to 70 °C
1
IN
≤
1 µA
8
—
—
%/°C
V/decade
%/°C
µA
%/°C
mV
%/°C
kHz
V
mA
Reference
Accuracy
Temperature Coefficient
2
—
Temperature Coefficient
2
Output Voltage Range
Supply Current
7
Power Supply Operation Range
2
1.
2.
3.
4.
5.
6.
7.
8.
V
CC
V
EE
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 µA.
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.
5
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