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OP295GS-REEL7 Datasheet(PDF) 9 Page - Analog Devices |
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OP295GS-REEL7 Datasheet(HTML) 9 Page - Analog Devices |
9 / 16 page OP295/OP495 Rev. E | Page 9 of 16 APPLICATIONS RAIL-TO-RAIL APPLICATION INFORMATION The OP295/OP495 have a wide common-mode input range extending from ground to within about 800 mV of the positive supply. There is a tendency to use the OP295/OP495 in buffer applications where the input voltage could exceed the common- mode input range. This can initially appear to work because of the high input range and rail-to-rail output range. But above the common-mode input range, the amplifier is, of course, highly nonlinear. For this reason, there must be some minimal amount of gain when rail-to-rail output swing is desired. Based on the input common-mode range, this gain should be at least 1.2. LOW DROP-OUT REFERENCE The OP295/OP495 can be used to gain up a 2.5 V or other low voltage reference to 4.5 V for use with high resolution ADCs that operate from 5 V only supplies. The circuit in Figure 18 supplies up to 10 mA. Its no-load drop-out voltage is only 20 mV. This circuit supplies over 3.5 mA with a 5 V supply. 4 2 6 – + + 1/2 OP295/OP495 VOUT =4.5V 5V 5V 16kΩ 10Ω 20kΩ 0.001µF REF43 1µF TO 10µF Figure 18. 4.5 V, Low Drop-Out Reference LOW NOISE, SINGLE-SUPPLY PREAMPLIFIER Most single-supply op amps are designed to draw low supply current at the expense of having higher voltage noise. This tradeoff may be necessary because the system must be powered by a battery. However, this condition is worsened because all circuit resistances tend to be higher; as a result, in addition to the op amp’s voltage noise, Johnson noise (resistor thermal noise) is also a significant contributor to the total noise of the system. The choice of monolithic op amps that combine the character- istics of low noise and single-supply operation is rather limited. Most single-supply op amps have noise on the order of 30 nV/√Hz to 60 nV/√Hz, and single-supply amplifiers with noise below 5 nV/√Hz do not exist. To achieve both low noise and low supply voltage operation, discrete designs may provide the best solution. The circuit in Figure 19 uses the OP295/OP495 rail-to-rail amplifier and a matched PNP transistor pair—the MAT03—to achieve zero- in/zero-out single-supply operation with an input voltage noise of 3.1 nV/√Hz at 100 Hz. R5 and R6 set the gain of 1000, making this circuit ideal for maximizing dynamic range when amplifying low level signals in single-supply applications. The OP295/OP495 provide rail-to- rail output swings, allowing this circuit to operate with 0 V to 5 V outputs. Only half of the OP295/OP495 is used, leaving the other uncommitted op amp for use elsewhere. 1 2 3 4 8 – + +– 26 5 3 7 1 Q1 Q2 MAT03 0.1µF R1 LED R4 R3 OP295/OP495 10µF R6 10Ω VOUT C2 10µF R5 10kΩ Q2 2N3906 R7 510Ω R2 27kΩ R8 100Ω C1 1500pF VIN Figure 19. Low Noise Single-Supply Preamplifier The input noise is controlled by the MAT03 transistor pair and the collector current level. Increasing the collector current reduces the voltage noise. This particular circuit was tested with 1.85 mA and 0.5 mA of current. Under these two cases, the input voltage noise was 3.1 nV/√Hz and 10 nV/√Hz, respectively. The high collector currents do lead to a tradeoff in supply current, bias current, and current noise. All of these parameters increase with increasing collector current. For example, typically the MAT03 has an hFE = 165. This leads to bias currents of 11 μA and 3 μA, respectively. Based on the high bias currents, this circuit is best suited for applications with low source impedance such as magnetic pickups or low impedance strain gauges. Furthermore, a high source impedance degrades the noise performance. For example, a 1 kΩ resistor generates 4 nV/√Hz of broadband noise, which is already greater than the noise of the preamp. The collector current is set by R1 in combination with the LED and Q2. The LED is a 1.6 V Zener diode that has a temperature coefficient close to that of the Q2 base-emitter junction, which provides a constant 1.0 V drop across R1. With R1 equal to 270 Ω, the tail current is 3.7 mA and the collector current is half that, or 1.85 mA. The value of R1 can be altered to adjust the collector current. When R1 is changed, R3 and R4 should also be adjusted. To maintain a common-mode input range that includes ground, the collectors of the Q1 and Q2 should not go 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. |
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