REV. C

AD713

–10–

Figure 12. Buffer Recovery Time Source Current = 2 mA

Figure 13. Buffer Recovery Time Sink Current = 1 mA

Driving A Large Capacitive Load

The circuit of Figure 14 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, C1.

Figure 15 shows a typical transient response for this connection.

Figure 14. Circuit for Driving a Large Capacitance Load

Table II. Recommended Trim Resistor Values vs.

Trim

JN/AQ/

KN/BQ/

LN/CQ/

GLN/GCQ/

Resistor

SD

TD

UD

GUD

R1

500

Ω

200

Ω

100

Ω

20

Ω

R2

150

Ω

68

Ω

33

Ω

6.8

Ω

CMOS DAC APPLICATIONS

The AD713 is an excellent output amplifier for CMOS DACs.

It can be used to perform both 2 and 4 quadrant operation. The

output impedance of a DAC using an inverted R-2R ladder

approaches R for codes containing many “1”s, 3R for codes con-

taining a single “1” and infinity for codes containing all zeros.

For example, the output resistance of the AD7545 will modu-

late between 11 k

Ω and 33 kΩ. Therefore, with the DAC’s

internal feedback resistance of 11 k

Ω, the noise gain will vary

from 2 to 4/3. This changing noise gain modulates the effect of

the input offset voltage of the amplifier, resulting in nonlinear

DAC amplifier performance. The AD713, with its guaranteed

1.5 mV input offset voltage, minimizes this effect achieving

12-bit performance.

Figures 16 and 17 show the AD713 and a 12-bit CMOS DAC,

the AD7545, configured for either a unipolar binary (2-quadrant

multiplication) or bipolar (4-quadrant multiplication) operation.

Capacitor C1 provides phase compensation which reduces over-

shoot and ringing.

Figure 16. Unipolar Binary Operation

Figure 17. Bipolar Operation