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OP176GP 데이터시트(PDF) 9 Page - Analog Devices |
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OP176GP 데이터시트(HTML) 9 Page - Analog Devices |
9 / 21 page OP176 REV. 0 –9– Noise The voltage noise density of the OP176 is below 6 nV/ √Hz from 30 Hz. This enables low noise designs to have good perfor- mance throughout the full audio range. Figure 27 shows a typical OP176 with a 1/f corner at 6 Hz. Figure 27. 1/f Noise Corner Noise Testing For audio applications the noise density is usually the most important noise parameter. For characterization the OP176 is tested using an Audio Precision, System One. The input signal to the Audio Precision must be amplified enough to measure accurately. For the OP176 the noise is gained by approximately 1020 using the circuit shown in Figure 28. Any readings on the Audio Precision must then be divided by the gain. In imple- menting this test fixture, good supply bypassing is essential. Figure 28. Noise Test Upgrading “5534‘’ Sockets The OP176 is a superior amplifier for upgrading existing designs using the industry standard 5534. In most application circuits, the OP176 can directly replace the 5534 without any modifications to the surrounding circuitry. Like the 5534, the OP176 follows the industry standard, single op amp pinout. The difference between these two devices is the location of the null pins and the 5534’s compensation capacitor. The 5534 normally requires a 22 pF capacitor between Pins 5 and 8 for stable operation. Since the OP176 is internally compensated for unity gain operation, it does not require external compensation. Nevertheless, if the 5534 socket already includes a capacitor, the OP176 can be inserted without removing it. Since the OP176’s Pin 8 is a “NO CONNECT’’ pin, there is no internal connection to that pin. Thus, the 22 pF capacitor would be electrically connected through Pin 5 to the internal nulling circuitry. With the other end left open, the capacitor should have no effect on the circuit. However, to avoid altogether any possibility for noise injection, it is recom- mended that the 22 pF capacitor be cut out of the circuit entirely. If the original 5534 socket includes offset nulling circuitry, one would find a 10 k Ω to 100 kΩ potentiometer connected between Pins 1 and 8 with said potentiometer’s wiper arm connected to V+. In order to upgrade the socket to the OP176, this circuit should be removed before inserting the OP176 for its offset nulling scheme uses Pins 1 and 5. Whereas the wiper arm of the 5534 trimming potentiometer is connected to the positive supply, the OP176’s wiper arm is connected to the negative supply. Directly substituting the OP176 into the original socket would inject a large current imbalance into its input stage. In this case, the potentiometer should be removed altogether, or, if nulling is still required, the trimming potentiometer should be rewired to match the nulling circuit as illustrated in Figure 29. Figure 29. Offset Voltage Nulling Scheme Input Overcurrent Protection The maximum input differential voltage that can be applied to the OP176 is determined by a pair of internal Zener diodes connected across its inputs. They limit the maximum differen- tial input voltage to ±7.5 V. This is to prevent emitter-base junction breakdown from occurring in the input stage of the OP176 when very large differential voltages are applied. However, in order to preserve the OP176’s low input noise voltage, internal resistances in series with the inputs were not used to limit the current in the clamp diodes. In small signal applications, this is not an issue; however, in applications where large differential voltages can be inadvertently applied to the device, large transient currents can flow through these diodes. Although these diodes have been designed to carry a current of ±5 mA, external resistors as shown in Figure 30 should be used in the event that the OP176’s differential voltage were to exceed ±7.5 V. Figure 30. Input Overcurrent Protection OP176 2 3 7 6 5 4 1 P1 –VS VOUT ΩP1 = 10kΩ VOS TRIM RANGE = ±2mV +VS OP176 1.4kΩ 1.4kΩ – + 2 3 6 50Hz / 300 mHz \ 0 Hz MKR: 5.4 Hz 10.0 µV /DIV CH A: 80.0 µV FS BW: MKR: 15.9 µV/ Hz OP176 OP37 OP37 OUTPUT 4.42k Ω 909 Ω 909 Ω 100 Ω 490 Ω 100 Ω |
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