A feedback capacitance C

R

response. To achieve a maximally flat, 2

R

(3)

Calculating C

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

back capacitor, C'

as long as R

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

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

F

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

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

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, the resistor values can be found by using the

equation shown in Figure 9.IfR

left open, no resistor needed, and R

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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