10 (X)

10

OUT

LMP8271

+

-

A1

GND

+IN

-IN

A2

GAIN = 2

A1

A2

100 k:

GAIN = 10

OFFSET

PROPRIETARY

LEVEL-SHIFT

CIRCUITRY

OBSOLETE

LMP8271

SNOSAP6H – JUNE 2005 – REVISED APRIL 2013

www.ti.com

APPLICATION NOTE

LMP8271

The LMP8271 is a single supply amplifier with a fixed gain of 20 and a common mode voltage range of

−2V to

16V. The fixed gain is achieved in two separate stages, a preamplifier with gain of +10 and a second stage

amplifier with gain of +2. A block diagram of the LMP8271 is shown in Figure 21.

Figure 21. LMP8271

The overall offset of the LMP8271 is minimized by trimming amplifier A1. This is done so that the output referred

Because of this offset voltage relationship, the offset of each individual amplifier stage may be more than the limit

specified for the overall system in the datasheet tables. Care must be given when pin 3 and 4, A1 and A2, are

connected to each other. If the signal going from A1 to A2 is amplified or attenuated (by use of amplifiers and

resistors), the overall LMP8271 offset will be affected as a result. Filtering the signal between A1 and A2 or

simply connecting the two pins will not change the offset of the LMP8271.

Referencing the input referred offset voltages of each stage, the following relationship holds:

(1)

If the signal on pin 3 is scaled, attenuated or amplified, by a factor X then the offset of the overall system will

become:

(2)

POWER SUPPLY DECOUPLING

In order to decouple the LMP8271 from AC noise on the power supply, it is recommended to use a 0.1

μF on the

supply pin. It is best to use a 0.1

μF capacitor in parallel with a 10 μF capacitor. This will generate an AC path to

ground for most frequency ranges and will greatly reduce the noise introduced by the power supply.

LIFETIME DRIFT

Input Offset Voltage is an electrical parameter which may drift over time. This drift, known as lifetime drift, is very

common in operational amplifiers; however, its effect is more evident in precision amplifiers. This is due to the

very low Input Offset Voltage specifications in these amplifiers.

8

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