Datasheet LT6018 (Analog Devices) - 13

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applicaTions inForMaTion High Dynamic Input Impedance
150 Op amps often have protection diodes clamping the two 120 inputs within a diode voltage of each other as seen in 90 LT6018 I LT6018 I B– B+ 60 Figure 2. During large voltage transitions on the input, 30 these diodes can conduct, since the output cannot respond 0 instantaneously. This can cause circuitry in front of the –30 amplifier as well as the amplifier’s own output stage to –60 get overloaded. Often there may be series input resistors INPUT BIAS CURRENT (µA) –90 TYPICAL (either integrated or discrete) to limit this current, but TYPICAL –120 OP AMP IB+ OP AMP IB– on extremely low noise parts such as the LT6018, that –150 is not desirable. –6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6 DIFFERENTIAL INPUT VOLTAGE (V) 6018 F04 The unique input circuitry of the LT6018 does not have
Figure 4. Typical Op Amp vs LT6018 Input Protection
this typical diode configuration, but rather a series Zener diode configuration as shown in Figure 3. For 5V input
Shutdown Operation
steps, the LT6018 has much higher impedance during The LT6018 shutdown function has been designed to transients and allows the user to reduce or eliminate the be easily controlled from single supply logic or micro- current limiting resistors, preserving low noise. Figure 4 controllers. To enable the LT6018 when V shows how input bias current increases with differential DGND = 0V the enable pin must be driven above 1.7V. Conversely, to input voltage for the traditional protection scheme and the enter the low power shutdown mode the enable pin must LT6018 protection scheme. be driven below 0.8V. In a ±15V dual supply application where V CURRENT LIMITING RESISTOR NEEDED DGND = –15V, the enable pin must be driven above –13.3V to enable the LT6018. If the enable pin is driven RF below –14.2V the LT6018 enters the low power shutdown mode. Note that to enable the LT6018 the enable pin volt- – age can range from –13.3V to 15V whereas to disable the OUTPUT LT6018 the enable pin can range from –15V to –14.2V. 6018 F02 5V INPUT STEP Figure 5 shows examples of enable pin control. While in + shutdown, the output of the LT6018 is not high imped-
Figure 2. Typical Op Amp Diode Input Protection
ance. The LT6018 is typically capable of coming out of shutdown within 25μs. This is useful in power sensitive applications where duty cycled operation is employed. In CURRENT LIMITING RESISTOR NOT NEEDED these applications the system is in low power mode the LT6018 majority of the time, but then needs to wake up quickly – and settle for an acquisition before being powered back OUTPUT 6018 F03 down to save power. 5V INPUT STEP +
Figure 3. LT6018 Series Zener Diode Input Protection
6018fa For more information www.linear.com/LT6018 13 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Simplified Schematic Applications Information Typical Applications Package Description Revision History Typical Applications Related Parts