LMP7715MFSLASHNOPB datasheet(17/27 Pages) TI

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IF RA < < RF

CFc = ¨

1 +

CF =

CIN

GBWP

2 S RF

CCM

IIN

VOUT

IIN

- RF

CIN = CD + CCM

LMP7715, LMP7716, LMP7716Q

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SNOSAV0E – MARCH 2006 – REVISED MARCH 2013

Figure 54. Transimpedance Amplifier

A feedback capacitance CF is usually added in parallel with RF to maintain circuit stability and to control the

frequency response. To achieve a maximally flat, 2nd order response, RF and CF should be chosen by using

Equation 3

(3)

Calculating CF from Equation 3 can sometimes result in capacitor values which are less than 2 pF. This is

especially the case for high speed applications. In these instances, it is often more practical to use the circuit

shown in Figure 55 in order to allow more sensible choices for CF. The new feedback capacitor, CF′, is (1+

RB/RA) CF. This relationship holds as long as RA << RF.

Figure 55. Modified Transimpedance Amplifier

SENSOR INTERFACE

The LMP7715/LMP7716/LMP7716Q have low input bias current and low input referred noise, which make them

ideal choices for sensor interfaces such as thermopiles, Infra Red (IR) thermometry, thermocouple amplifiers,

and pH electrode buffers.

Thermopiles generate voltage in response to receiving radiation. These voltages are often only a few microvolts.

As a result, the operational amplifier used for this application needs to have low offset voltage, low input voltage

noise, and low input bias current. Figure 56 shows a thermopile application where the sensor detects radiation

from a distance and generates a voltage that is proportional to the intensity of the radiation. The two resistors, RA

and RB, are selected to provide high gain to amplify this signal, while CF removes the high frequency noise.

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LMP7715MF/NOPB Datasheet(PDF) 17 Page - Texas Instruments