LMP7712MM datasheet(16/18 Pages) NSC

16 / 18 page

Application Notes (Continued)

A feedback capacitance C

F is usually added in parallel with

F to maintain circuit stability and to control the frequency

response. To achieve a maximally flat, 2

nd order response,

F and CF should be chosen by using Equation (3)

(3)

Calculating C

F 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, its

often more practical to use the circuit shown in Figure 7 in

order to allow more sensible choices for C

F. The new feed-

back capacitor, C'

F, is (1+ RB/RA)CF. This relationship holds

as long as R

A << RF.

SENSOR INTERFACE

The LMP7711/LMP7712 have low input bias current and low

input referred noise, which make them ideal choices for

sensor interfaces such as thermopiles, Infra Red (IR) ther-

mometry, thermocouple amplifiers, and pH electrode buffers.

Thermopiles generate voltage in response to receiving ra-

diation. 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 8 shows a thermopile

application where the sensor detects radiation from a dis-

tance and generates a voltage that is proportional to the

intensity of the radiation. The two resistors, R

A and RB, are

selected to provide high gain to amplify this signal, while C

removes the high frequency noise.

PRECISION RECTIFIER

Rectifiers are electrical circuits used for converting AC sig-

nals to DC signals. Figure 9 shows a full-wave precision

rectifier. Each operational amplifier used in this circuit has a

diode on its output. This means for the diodes to conduct, the

output of the amplifier needs to be positive with respect to

ground. If V

IN is in its positive half cycle then only the output

of the bottom amplifier will be positive. As a result, the diode

on the output of the bottom amplifier will conduct and the

signal will show at the output of the circuit. If V

IN is in its

negative half cycle then the output of the top amplifier will be

positive, resulting in the diode on the output of the top

amplifier conducting and, delivering the signal on the ampli-

fier’s output to the circuits output.

For R

2/R1

≥ 2, the resistor values can be found by using the

equation shown in Figure 9.IfR

2/R1 = 1, then R3 should be

left open, no resistor needed, and R

4 should simply be

shorted.

20150369

FIGURE 6. Transimpedance Amplifier

20150331

FIGURE 7. Modified Transimpedance Amplifier

20150327

FIGURE 8. Thermopile Sensor Interface

20150374

FIGURE 9. Precision Rectifier

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