Datasheet OP295, OP495 (Analog Devices) - 9

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OP295/OP495 APPLICATIONS RAIL-TO-RAIL APPLICATION INFORMATION
R5 and R6 set the gain of 1000, making this circuit ideal for maximizing dynamic range when amplifying low level signals in The OP295/OP495 have a wide common-mode input range single-supply applications. The OP295/OP495 provide rail-to- extending from ground to within about 800 mV of the positive rail output swings, allowing this circuit to operate with 0 V to supply. There is a tendency to use the OP295/OP495 in buffer 5 V outputs. Only half of the OP295/OP495 is used, leaving the applications where the input voltage could exceed the common- other uncommitted op amp for use elsewhere. mode input range. This can initially appear to work because of
0.1µF
the high input range and rail-to-rail output range. But above the common-mode input range, the amplifier is, of course, highly
LED R1 10µF
nonlinear. For this reason, there must be some minimal amount
Q2 + –
of gain when rail-to-rail output swing is desired. Based on the
2N3906
input common-mode range, this gain should be at least 1.2.
3 5 R6 VIN 10Ω LOW DROP-OUT REFERENCE 2 6 Q1 MAT03 Q2
The OP295/OP495 can be used to gain up a 2.5 V or other low
1 7 R5 C2 10kΩ 10µF 2 – 8
voltage reference to 4.5 V for use with high resolution ADCs
V R7 OUT 510Ω 1
that operate from 5 V only supplies. The circuit in Figure 19 supplies up to 10 mA. Its no-load drop-out voltage is only
3 4 C1 + OP295/OP495 1500pF
20 mV. This circuit supplies over 3.5 mA with a 5 V supply.
R2 R3 R4 27kΩ R8 100Ω 16kΩ
8
5V 0.001µF
01 31- 003
5V
Figure 20. Low Noise Single-Supply Preamplifier
20kΩ –
The input noise is controlled by the MAT03 transistor pair
V 2 10Ω OUT = 4.5V
and the collector current level. Increasing the collector current
6 REF43 + + 1µF TO
reduces the voltage noise. This particular circuit was tested
10µF 1/2
with 1.85 mA and 0.5 mA of current. Under these two cases,
4
7
OP295/OP495
01 1- the input voltage noise was 3.1 nV/√Hz and 10 nV/√Hz, respect- 33 00 ively. The high collector currents do lead to a tradeoff in supply Figure 19. 4.5 V, Low Drop-Out Reference current, bias current, and current noise. All of these parameters
LOW NOISE, SINGLE-SUPPLY PREAMPLIFIER
increase with increasing collector current. For example, typically Most single-supply op amps are designed to draw low supply the MAT03 has an hFE = 165. This leads to bias currents of 11 μA current at the expense of having higher voltage noise. This tradeoff and 3 μA, respectively. may be necessary because the system must be powered by a Based on the high bias currents, this circuit is best suited for battery. However, this condition is worsened because all circuit applications with low source impedance such as magnetic resistances tend to be higher; as a result, in addition to the op pickups or low impedance strain gauges. Furthermore, a high amp’s voltage noise, Johnson noise (resistor thermal noise) is source impedance degrades the noise performance. For also a significant contributor to the total noise of the system. example, a 1 kΩ resistor generates 4 nV/√Hz of broadband The choice of monolithic op amps that combine the character- noise, which is already greater than the noise of the preamp. istics of low noise and single-supply operation is rather limited. The collector current is set by R1 in combination with the LED Most single-supply op amps have noise on the order of 30 nV/√Hz and Q2. The LED is a 1.6 V Zener diode that has a temperature to 60 nV/√Hz, and single-supply amplifiers with noise below coefficient close to that of the Q2 base-emitter junction, which 5 nV/√Hz do not exist. provides a constant 1.0 V drop across R1. With R1 equal to To achieve both low noise and low supply voltage operation, 270 Ω, the tail current is 3.7 mA and the collector current is half discrete designs may provide the best solution. The circuit in that, or 1.85 mA. The value of R1 can be altered to adjust the Figure 20 uses the OP295/OP495 rail-to-rail amplifier and a collector current. When R1 is changed, R3 and R4 should also matched PNP transistor pair—the MAT03—to achieve zero- be adjusted. To maintain a common-mode input range that in/zero-out single-supply operation with an input voltage noise includes ground, the collectors of the Q1 and Q2 should not go of 3.1 nV/√Hz at 100 Hz. above 0.5 V; otherwise, they could saturate. Thus, R3 and R4 must be small enough to prevent this condition. Their values and the overall performance for two different values of R1 are summarized in Table 6. Rev. G | Page 9 of 16 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PIN CONFIGURATIONS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ELECTRICAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS APPLICATIONS RAIL-TO-RAIL APPLICATION INFORMATION LOW DROP-OUT REFERENCE LOW NOISE, SINGLE-SUPPLY PREAMPLIFIER DRIVING HEAVY LOADS DIRECT ACCESS ARRANGEMENT SINGLE-SUPPLY INSTRUMENTATION AMPLIFIER SINGLE-SUPPLY RTD THERMOMETER AMPLIFIER COLD JUNCTION COMPENSATED, BATTERY-POWERED THERMOCOUPLE AMPLIFIER 5 V ONLY, 12-BIT DAC THAT SWINGS 0 V TO 4.095 V 4 mA TO 20 mA CURRENT-LOOP TRANSMITTER 3 V LOW DROPOUT LINEAR VOLTAGE REGULATOR LOW DROPOUT, 500 mA VOLTAGE REGULATOR WITH FOLDBACK CURRENT LIMITING SQUARE WAVE OSCILLATOR SINGLE-SUPPLY DIFFERENTIAL SPEAKER DRIVER HIGH ACCURACY, SINGLE-SUPPLY, LOW POWER COMPARATOR OUTLINE DIMENSIONS ORDERING GUIDE