Datasheet LT1880 (Analog Devices) - 8

LT1880
APPLICATIONS INFORMATION
The LT1880 single op amp features exceptional input
Input Protection
precision with rail-to-rail output swing. Slew rate and small The inverting and noninverting input pins of the LT1880 signal bandwidth are superior to other amplifi ers with have limited on-chip protection. ESD protection is provided comparable input precision. These characteristics make to prevent damage during handling. The input transistors the LT1880 a convenient choice for precision low voltage have voltage clamping and limiting resistors to protect systems and for improved AC performance in higher voltage against input differentials up to 10V. Short transients precision systems. Obtaining benefi cial advantage of the above this level will also be tolerated. If the input pins can precision inherent in the amplifi er depends upon proper see a sustained differential voltage above 10V, external applications circuit design and board layout. limiting resistors should be used to prevent damage to the amplifi er. A 1k resistor in each input lead will provide
Preserving Input Precision
protection against a 30V differential voltage. Preserving the input voltage accuracy of the LT1880 requires that the applications circuit and PC board layout
Capacitive Loads
do not introduce errors comparable to or greater than the The LT1880 can drive capacitive loads up to 600pF in unity 40μV offset. Temperature differentials across the input gain. The capacitive load driving capability increases as connections can generate thermocouple voltages of 10’s the amplifi er is used in higher gain confi gurations, see the of microvolts. PC board layouts should keep connections graph labled Capacitive Load Response. Capacitive load to the amplifi er’s input pins close together and away from driving may be increased by decoupling the capacitance heat dissipating components. Air currents across the board from the output with a small resistance. can also generate temperature differentials.
Capacitance Load Response
The extremely low input bias currents, 150pA, allow high accuracy to be maintained with high impedance sources and 30 VS = ±15V feedback networks. The LT1880’s low input bias currents TA = 25°C 25 are obtained by using a cancellation circuit on-chip. This causes the resulting I + – BIAS and IBIAS to be uncorrelated, as 20 implied by the lOS specifi cation being comparable to IBIAS. 15 The user should not try to balance the input resistances in AV = 1 each input lead, as is commonly recommended with most OVERSHOOT (%) 10 amplifi ers. The impedance at either input should be kept as small as possible to minimize total circuit error. 5 AV = 10 PC board layout is important to insure that leakage currents 010 100 1000 10000 do not corrupt the low IBIAS of the amplifi er. In high CAPACITIVE LOAD (pF) precision, high impedance circuits, the input pins should 1880 G25 be surrounded by a guard ring of PC board interconnect,
Getting Rail-to-Rail Operation without Rail-to-Rail
with the guard driven to the same common mode voltage
Inputs
as the amplifi er inputs. The LT1880 does not have rail-to-rail inputs, but for most
Input Common Mode Range
inverting applications and noninverting gain applications, this is largely inconsequential. Figure 1 shows the basic The LT1880 output is able to swing nearly to each power op amp configurations, what happens to the op amp supply rail, but the input stage is limited to operating inputs, and whether or not the op amp must have rail- between V– + 1V and V+ – 1.2V. Exceeding this common to-rail inputs. mode range will cause the gain to drop to zero, however no gain reversal will occur. 1880fa 8