SM73304 Datasheet, PDF(17/22 Page) Texas Instruments – Dual and Single Precision, 17 MHz, Low Noise, CMOS Input Amplifiers with Enable

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SM73304 Datasheet, PDF (17/22 Pages) Texas Instruments – Dual and Single Precision, 17 MHz, Low Noise, CMOS Input Amplifiers with Enable

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for current detection. This current needs to be amplified be-

fore it can be further processed. This amplification is per-

formed using a current-to-voltage converter configuration or

transimpedance amplifier. The signal of interest is fed to the

inverting input of an op amp with a feedback resistor in the

current path. The voltage at the output of this amplifier will be

equal to the negative of the input current times the value of

the feedback resistor. Figure 6 shows a transimpedance am-

plifier configuration. CD represents the photodiode parasitic

capacitance and CCM denotes the common-mode capaci-

tance of the amplifier. The presence of all of these capaci-

tances at higher frequencies might lead to less stable

topologies at higher frequencies. Care must be taken when

designing a transimpedance amplifier to prevent the circuit

from oscillating.

With a wide gain bandwidth product, low input bias current

and low input voltage and current noise, the SM73304/

SM73305 are ideal for wideband transimpedance applica-

tions.

30159431

FIGURE 7. Modified Transimpedance Amplifier

SENSOR INTERFACE

The SM73304/SM73305 have low input bias current and low

input referred noise, which make them ideal choices for sen-

sor interfaces such as thermopiles, Infra Red (IR) thermom-

etry, thermocouple amplifiers, and pH electrode buffers.

Thermopiles generate voltage in response to receiving radi-

ation. 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 applica-

tion 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 pro-

vide high gain to amplify this signal, while CF removes the high

frequency noise.

30159469

FIGURE 6. Transimpedance Amplifier

A feedback capacitance CF is usually added in parallel with

RF to maintain circuit stability and to control the frequency re-

sponse. 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 ca-

pacitor 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 CF. The new feedback ca-

pacitor, Câ²F, is (1+ RB/RA) CF. This relationship holds as long

as RA << RF.

30159427

FIGURE 8. Thermopile Sensor Interface

PRECISION RECTIFIER

Rectifiers are electrical circuits used for converting AC signals

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

VIN is in its positive half cycle then only the output of the bot-

tom 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 VIN is in its negative half

cycle then the output of the top amplifier will be positive, re-

sulting in the diode on the output of the top amplifier conduct-

ing and, delivering the signal on the amplifier's output to the

circuits output.

For R2/ R1 â¥ 2, the resistor values can be found by using the

equation shown in Figure 9. If R2/ R1 = 1, then R3 should be

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