AD712

REV. B

–12–

100

10

0%

90

200ns

500mV

PD711 BUFF

–10V ADC IN

a. Source Current = 2 mA

100

10

0%

200ns

90

500mV

PD711 BUFF

–5V ADC IN

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

for this connection.

1/2

AD712

0.1 F

0.1 F

INPUT

TYPICAL CAPACITANCE

LIMIT FOR VARIOUS

LOAD RESISTORS

2k

1500pF

10k

1500pF

20

1000pF

4.99k

4.99k

30pF

OUTPUT

100

+

–

–

+

Figure 36. Circuit for Driving a Large Capacitive Load

100

90

10

0%

1 s

5V

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.