AN1315: Rail-to-Rail Amplifier Extends Resolution and

Rail-to-Rail Amplifier Extends Resolution and
Range of Light Sensor
®
Application Note
May 8, 2007
AN1315.0
By Mike Wong and Tamara Papalias
For example, consider the circuit in Figure 1. The light
sensor affects a current at the inverting input of the amplifier.
The non-inverting input of a standard amplifier is typically
biased half-way between the supply rails. In a 3V
single-supply configuration, the non-inverting input would be
biased at 1.5V. The output voltage of the circuit is in
Equation 1:
V OUT = V REF – IR
(and a bit beyond.) For maximum resolution on the light
sensor, the non-inverting terminal is biased at the supply
voltage. This accommodates the complete response
(Figure 2) of the light sensor, improving the output dynamic
range as much as 45%.
800
700
LIGHT SENSOR RANGE
WITH STANDARD
OP AMP
600
IOUT (µA)
Light sensors are appearing in every sort of consumer
application, aiding in aspects like power conservation. Since
a wide range of products are portable, they utilize battery
power and are therefore configured for single-supply
operation. Single-supply operation introduces limitations on
analog circuitry. One of the most obvious trade-offs is
reduced input voltage range. Upgrading to a rail-to-rail
amplifier will restore this sacrifice.
500
400
LIGHT SENSOR RANGE
WITH RAIL-TO-RAIL
OP AMP
300
200
100
(EQ. 1)
0
0
Notice the response curve of our example light sensor, the
EL7900, in Figure 2. The sensor produces varying amounts of
negative current (drawn into the sensor) for varying illumination.
Since this characteristic has a single polarity, it behooves the
designer to adjust the reference voltage on the non-inverting
input (Figure 1) to accommodate a greater range of input
values from the sensor. The output voltage is only reduced by
the sensor's signal, so VREF should be raised.
How far can this reference voltage be raised? That depends
on the topology of the amplifier used. Standard op amps
have an input voltage range limited by the bias circuitry
supporting the input differential pair. This reduction may be
0.5V to 1.5V approaching one or both power supply rails.
With the shrinking power supplies offered by portable
devices, this reduction can severely limit performance.
5000
10000
15000
20000
ILLUMINANCE (LUX)
FIGURE 2. OUTPUT OF LIGHT SENSOR vs ILLUMINANCE
OF LIGHT
NOTE: Ranges use Circuit 1 in Figure 1 with R = 4.7kΩ
TABLE 1. CIRCUIT CHARACTERISTICS WITH STANDARD OP
AMP AND RAIL-TO-RAIL OP AMP
OP AMP WITH/
STANDARD INPUT
(EL5144)
OP AMP WITH/
RAIL-TO-RAIL
(EL8178)
3.3V
3.3V
VREF
1.5V up to 1.8v
3.3V
VOUT
Limited by (VS+ - VREF)
Full power supply
range
Supply Voltage (VS+)
Rail-to-rail op amps, like the EL8178, include dual input pairs
to allow the input signal to travel the full power supply range
VS+ = +3.3V
R
3
1
VS+
2
5
EL7900
-
6
IIN
3
VOUT = VREF - IR
+
EL81XX
2
0V
FIGURE 1. LIGHT SENSOR CIRCUIT
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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Application Note 1315
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2
AN1315.0
May 8, 2007