LSP2961 Datasheet, PDF (1/6 Pages) Agere Systems – Precision Log Ratio Amplifier

Advance Data Sheet
September 2001
LSP2961
Precision Log Ratio Amplifier
Features
I Seven decades (1 nA to 10 mA) typical input range
I Internal scale factor K = 1 ± 2% (K externally
adjustable):
OUTPUT = K × log ^I---^R_I---_I-^E-_N---^F--
(eq. 1)
I Internal trimmed reference current
(IREF = 1 µA ± 5%, externally adjustable)
I Maximum ±100 mV output offset (externally
adjustable)
I Minimal external components
I ±5 V power supply
I TSSOP small package
Applications
I Optical light density measurement
I Log, log ratio computation
I 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-
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 (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 µA.
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.