Datasheet LT2078, LT2079 (Analog Devices) - 10

LT2078/LT 2079
U U W U APPLICATIONS INFORMATION
The LT2078/LT2079 devices are fully specified with As an example, consider the instrumentation amplifier V + = 5V, V – = 0V, VCM = 0.1V. This set of operating shown on the front page. When the common mode signal conditions appears to be the most representative for is low and the output is high, amplifier A has to sink battery powered micropower circuits. Offset voltage is current. When the common mode signal is high and the internally trimmed to a minimum value at these supply output low, amplifier B has to sink current. The competing voltages. When 9V or 3V batteries or ±2.5V dual supplies devices require a 12k pull-down resistor at the output of are used, bias and offset current changes will be minimal. amplifier A and a 15k at the output of B to handle the Offset voltage changes will be just a few microvolts as specified signals. (The LT2078 does not need pull-down given by the PSRR and CMRR specifications. For example, resistors.) When the common mode input is high and the if PSRR = 114dB (=2µV/V), at 9V the offset voltage change output is high these pull-down resistors draw 300µA (150µA will be 8µV. Similarly, VS = ±2.5V, VCM = 0V is equivalent each), which is excessive for micropower applications. to a common mode voltage change of 2.4V or a VOS The instrumentation amplifier is by no means the only change of 7µV if CMRR = 110dB (3µV/V). application requiring current sinking capability. In seven A full set of specifications is also provided at ±15V supply of the nine single supply applications shown in this data voltages for comparison with other devices and for com- sheet the op amps have to be able to sink current. In two pleteness. of the applications the first amplifier has to sink only the 6nA input bias current of the second op amp. The compet-
Single Supply Operation
ing devices, however, cannot even sink 6nA without a The LT2078/LT2079 is quite tolerant of power supply pull-down resistor bypassing. In some applications requiring faster settling Since the output of the LT2078/LT2079 cannot go exactly time the positive supply pin of the LT2078/LT2079 should to ground, but can only approach ground to within a few be bypassed with a small capacitor (about 0.1µF). The millivolts, care should be exercised to ensure that the same is true for the negative supply pin when using split output is not saturated. For example, a 1mV input signal supplies. will cause the amplifier to set up in its linear region in the The LT2078/LT2079 are fully specified for single supply gain 100 configuration shown in Figure 1, but is not operation, i.e., when the negative supply is 0V. Input enough to make the amplifier function properly in the common mode range goes below ground and the output voltage follower mode. swings within a few millivolts of ground while sinking Single supply operation can also create difficulties at the current. All competing micropower op amps either cannot input. The driving signal can fall below 0V — inadvertently swing to within 600mV of ground (OP-20, OP-220, OP- or on a transient basis. If the input is more than a few 420) or need a pull-down resistor connected to the output hundred millivolts below ground, two distinct problems to swing to ground (OP-90, OP-290, OP-490, HA5141/42/ can occur on previous single supply designs, such as the 44). This difference is critical because in many applica- LM124, LM158, OP-20, OP-21, OP-220, OP-221, OP-420 tions these competing devices cannot be operated as (1 and 2), OP-90/290/490 (2 only): micropower op amps and swing to ground simultaneously. 5V 5V R – 99R – OUTPUT 100mV SATURATION ≈ 3.5mV 1mV + 1mV + LT2078/79 • F02a LT2078/79 • F02b
Figure 1a. Gain 100 Amplifier Figure 1b. Voltage Follower
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