LT2078 데이터 시트보기 (PDF) - Linear Technology

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LT2078

Micropower, Dual and Quad, Single Supply, Precision Op Amps

Linear Technology 

LT2078/LT 2079

APPLICATIONS INFORMATION

The LT2078/LT2079 devices are fully specified with

V + = 5V, V – = 0V, VCM = 0.1V. This set of operating

conditions appears to be the most representative for

battery powered micropower circuits. Offset voltage is

internally trimmed to a minimum value at these supply

voltages. When 9V or 3V batteries or ±2.5V dual supplies

are used, bias and offset current changes will be minimal.

Offset voltage changes will be just a few microvolts as

given by the PSRR and CMRR specifications. For example,

if PSRR = 114dB (=2µV/V), at 9V the offset voltage change

will be 8µV. Similarly, VS = ±2.5V, VCM = 0V is equivalent

to a common mode voltage change of 2.4V or a VOS

change of 7µV if CMRR = 110dB (3µV/V).

A full set of specifications is also provided at ±15V supply

voltages for comparison with other devices and for com-

pleteness.

Single Supply Operation

The LT2078/LT2079 is quite tolerant of power supply

bypassing. In some applications requiring faster settling

time the positive supply pin of the LT2078/LT2079 should

be bypassed with a small capacitor (about 0.1µF). The

same is true for the negative supply pin when using split

supplies.

The LT2078/LT2079 are fully specified for single supply

operation, i.e., when the negative supply is 0V. Input

common mode range goes below ground and the output

swings within a few millivolts of ground while sinking

current. All competing micropower op amps either cannot

swing to within 600mV of ground (OP-20, OP-220, OP-

420) or need a pull-down resistor connected to the output

to swing to ground (OP-90, OP-290, OP-490, HA5141/42/

44). This difference is critical because in many applica-

tions 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 LT2078 does not need pull-down

resistors.) When the common mode input is high and the

output is high these pull-down resistors draw 300µA (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 LT2078/LT2079 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 1, but is not

enough to make the amplifier function properly in the

voltage follower mode.

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):

10

5V

R

–

1mV +

99R

100mV

LT2078/79 • F02a

Figure 1a. Gain 100 Amplifier

5V

–

1mV +

OUTPUT

SATURATION

≈ 3.5mV

LT2078/79 • F02b

Figure 1b. Voltage Follower

20789fa

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