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LT1078IN8 Datasheet(PDF) 11 Page - Linear Technology |
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LT1078IN8 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LT1078/LT1079 APPLICATIONS INFORMATION difference is critical because in many applications these competing devices cannot be operated as micropower op amps and swing to ground simultaneously. As an example, consider the instrumentation amplifier shown on the front page. When the common mode signal is low and the output is high, amplifier A has to sink current. When the common mode signal is high and the output low, amplifier B has to sink current. The competing devices require a 12k pull-down resistor at the output of amplifier A and a 15k at the output of B to handle the specified signals. (The LT1078 does not need pull-down resistors.) When the common mode input is high and the output is high these pull-down resistors draw 300 µA (150µA each), which is excessive for micropower applications. The instrumentation amplifier is by no means the only application requiring current sinking capability. In seven of the nine single supply applications shown in this data sheet the op amps have to be able to sink current. In two of the applications the first amplifier has to sink only the 6nA input bias current of the second op amp. The compet- ing devices, however, cannot even sink 6nA without a pull- down resistor Since the output of the LT1078/LT1079 cannot go exactly to ground, but can only approach ground to within a few millivolts, care should be exercised to ensure that the output is not saturated. For example, a 1mV input signal will cause the amplifier to set up in its linear region in the gain 100 configuration shown in Figure 1a, but is not Single supply operation can also create difficulties at the input. The driving signal can fall below 0V — inadvertently or on a transient basis. If the input is more than a few hundred millivolts below ground, two distinct problems can occur on previous single supply designs, such as the LM124, LM158, OP-20, OP-21, OP-220, OP-221, OP-420 (1 and 2), OP-90/290/490 (2 only): 1. When the input is more than a diode drop below ground, unlimited current will flow from the substrate (V – terminal) to the input. This can destroy the unit. On the LT1078/LT1079, resistors in series with the input protect the devices even when the input is 5V below ground. 2. When the input is more than 400mV below ground (at 25 °C), the input stage saturates and phase reversal occurs at the output. This can cause lockup in servo systems. Due to a unique phase reversal protection cir- cuitry, the LT1078/LT1079 output does not reverse, as illustrated in Figure 2, even when the inputs are at –1V. enough to make the amplifier function properly in the voltage follower mode, Figure 1b. Figure 1a. Gain 100 Amplifier + – 5V 1mV R 99R 100mV LT1078/79 • F01a + – 5V 1mV OUTPUT SATURATED ≈ 3.5mV LT1078/79 • F01b Figure 1b. Voltage Follower Figure 2. Voltage Follower with Input Exceeding the Negative Common Mode Range (VS = 5V, 0V) 2V 0V 6VP-P INPUT 1ms/DIV –1V TO 5V LT1078/79 • F02a 1ms/DIV LT1078/LT1079 NO PHASE REVERSAL LT1078/79 • F02C 2V 0V 4V 1ms/DIV OP-90 EXHIBITS OUTPUT PHASE REVERSAL LT1078/79 • F02b 2V 0V 4V 4V |
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