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OP777 Datasheet(PDF) 10 Page - Analog Devices |
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OP777 Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page OP777 –10– REV. 0 BASIC OPERATION The OP777 amplifier uses a precision Bipolar PNP input stage coupled with a high-voltage CMOS output stage. This enables this amplifier to feature an input voltage range which includes the negative supply voltage (often ground-in single-supply applications) and also swing to within 1 mV of the output rails. Additionally, the input voltage range extends to within 1 V of the positive supply rail. The epitaxial PNP input structure provides high breakdown voltage, high gain, and input bias current figure comparable to that obtained with “Darlington” input stage amplifier but without the drawbacks (i.e., severe penalties for input voltage range, offset, drift and noise). PNP input structure also greatly lowers the noise and reduces the dc input error terms. Supply Voltage The amplifiers are fully specified with a single 5 V supply and, due to design and process innovations, can also operate with a supply voltage from 2.7 V up to 30 V. This allows operation from most split supplies used in current industry practice, with the advantage of substantially increased input and output voltage ranges over conventional split-supply amplifiers. The OP777 series is specified with (VSY = 5 V, V– = 0 V and VCM = 2.5 V which is most suitable for single supply application. With PSRR of 130 dB (0.3 µV/V) and CMRR of 110 dB (3 µV/V) offset is minimally affected by power supply or common-mode voltages. Dual supply, ±15 V operation is also fully specified. Input Common-Mode Voltage Range The OP777 is rated with an input common-mode voltage which extends from minus supply to 1 V of the Positive supply. However, the amplifier can still operate with input voltages slightly below VEE. In Figure 43, OP777 is configured as a difference amplifier with a single supply of 2.7 V and negative dc common-mode volt- ages applied at the inputs terminals. A 400 mV p-p input is then applied to the noninverting input. It can be seen from the graph below that the output does not show any distortion. Micropower operation is maintained by using large input and feedback resistors. TIME – 0.2ms/DIV VIN VOUT 0V Figure 42. Input and Output Signals with VCM < 0 V +3V OP777 100k 100k 100k 100k 0.1V VIN = 1kHz at 400mV p-p 0.27V Figure 43. OP777 Configured as a Difference Amplifier Operating at VCM < 0 V Input Over Voltage Protection When the input of an amplifier is more than a diode drop below VEE, large currents will flow from the substrate (V– pin) to the input pins which can destroy the device. In the case of OP777, differential voltage equal to the supply voltage will not cause any problem (see Figure 44). OP777 has built in 500 Ω internal current limiting resistors, in series with the inputs, to minimize the chances of damage. It is a good practice to keep the current flowing into the inputs below 5 mA. In this context it should also be noted that the high breakdown of the input transistors removes the necessity for clamp diodes between the inputs of the amplifier; a feature that is mandatory on many precision op amps. Unfortunately, such clamp diodes greatly interfere with many application circuits such as precision rectifiers and comparators. The OP777 series is free from such limitations. 30V V p-p = 32 V OP777 Figure 44a. Unity Gain Follower TIME – 400 s/DIV VSY = 15V VIN VOUT Figure 44b. Input Voltage Can Exceed the Supply Voltage Without Damage |
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