AD712KR-REEL Ver la hoja de datos (PDF) - Analog Devices

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AD712KR-REEL

Dual Precision, Low Cost,High Speed, BiFET Op Amp

Analog Devices 

AD712

100

90

PD711 BUFF

5V

100

90

1␮s

10

10

0%

0%

500mV –10V ADC IN

200ns

a. Source Current = 2 mA

PD711 BUFF

100

90

10

0%

500mV –5V ADC IN

200ns

b. Sink Current = 1 mA

Figure 35. ADC Input Unity Gain Buffer Recovery Times

DRIVING A LARGE CAPACITIVE LOAD

The circuit in Figure 36 employs a 100 Ω isolation resistor

which enables the amplifier to drive capacitive loads exceeding

1500 pF; the resistor effectively isolates the high frequency

feedback from the load and stabilizes the circuit. Low frequency

feedback is returned to the amplifier summing junction via the

low pass filter formed by the 100 Ω series resistor and the load

capacitance, CL. Figure 37 shows a typical transient response

for this connection.

Figure 37. Transient Response RL = 2 kΩ, CL = 500 pF

ACTIVE FILTER APPLICATIONS

In active filter applications using op amps, the dc accuracy of

the amplifier is critical to optimal filter performance. The

amplifier’s offset voltage and bias current contribute to output

error. Offset voltage will be passed by the filter and may be

amplified to produce excessive output offset. For low frequency

applications requiring large value input resistors, bias currents

flowing through these resistors will also generate an offset voltage.

In addition, at higher frequencies, an op amp’s dynamics must

be carefully considered. Here, slew rate, bandwidth, and

open-loop gain play a major role in op amp selection. The slew

rate must be fast as well as symmetrical to minimize distortion.

The amplifier’s bandwidth in conjunction with the filter’s gain

will dictate the frequency response of the filter.

The use of a high performance amplifier such as the AD712 will

minimize both dc and ac errors in all active filter applications.

4.99k⍀

30pF

4.99k⍀

INPUT

TYPICAL CAPACITANCE

LIMIT FOR VARIOUS

LOAD RESISTORS

R1

2k⍀

10k⍀

20⍀

C1 UP TO

1500pF

1500pF

1000pF

+VIN

0.1␮F

+–

1/2

AD712

0.1␮F

–+

–VIN

100⍀

C1

OUTPUT

R1

Figure 36. Circuit for Driving a Large Capacitive Load

–12–

REV. B

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