LMP7731 datasheet(14/26 Pages) TI

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INPUT STAGE

IBIAS CANCELLATION CIRCUIT

LMP7731

SNOSAT6E – JULY 2007 – REVISED MARCH 2013

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APPLICATION INFORMATION

LMP7731

The LMP7731 is a single, low noise, rail-to-rail input and output, and low voltage amplifier.

The low input voltage noise of only 2.9 nV/

√Hz with a 1/f corner at 3 Hz makes the LMP7731 ideal for sensor

applications where DC accuracy is of importance.

The LMP7731 has a high gain bandwidth of 22 MHz. This wide bandwidth enables use of the amplifier at higher

gain settings while retaining usable bandwidth for the application. This is particularly beneficial when system

designers need to use sensors with very limited output voltage range as it allows larger gains in one stage which

in turn increases the signal to noise ratio.

The LMP7731 has proprietary input bias cancellation circuitry on the input stages. This allows the LMP7731 to

have only about 1.5 nA bias current with a bipolar input stage. This low input bias current, paired with the

inherent lower input voltage noise of bipolar input stages makes the LMP7731 an excellent choice for precision

applications. The combination of low input bias current, and low input voltage noise enables the user to achieve

unprecedented accuracy and higher signal integrity.

Texas Instruments is heavily committed to precision amplifiers and the market segment they serve. Technical

support and extensive characterization data are available for sensitive applications or applications with a

constrained error budget.

The LMP7731 is offered in the space saving 5-Pin SOT-23 and 8-Pin SOIC packages. These small packages are

ideal solutions for area constrained PC boards and portable electronics.

INPUT BIAS CURRENT CANCELLATION

The LMP7731 has proprietary input bias current cancellation circuitry on their input stages.

The LMP7731 has rail-to-rail input. This is achieved by having two input stages in parallel. Figure 44 shows only

one of the input stages as the circuitry is symmetrical for both stages.

Figure 44 shows that as the common mode voltage gets closer to one of the extreme ends, current I1

significantly increases. This increased current shows as an increase in voltage drop across resistor R1 equal to

I1*R1 on IN

+ of the amplifier. This voltage contributes to the offset voltage of the amplifier. When common mode

voltage is in the mid-range, the transistors are operating in the linear region and I1 is significantly small. The

voltage drop due to I1 across R1 can be ignored as it is orders of magnitude smaller than the amplifier's input

offset voltage.

As the common mode voltage gets closer to one of the rails, the offset voltage generated due to I1 increases and

becomes comparable to the amplifiers offset voltage.

Figure 44. Input Bias Current Cancellation

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