AD713
Figure 12. Buffer Recovery Time Source Current = 2 mA
Figure 15. Transient Response, RL = 2 kΩ, CL = 500 pF
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 kΩ and 33 kΩ. Therefore, with the DAC’s
internal feedback resistance of 11 kΩ, 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.
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.
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 14. Circuit for Driving a Large Capacitance Load
Table II. Recommended Trim Resistor Values vs.
Grades for AD7545 for VD = 5 V
Trim
Resistor SD
JN/AQ/
KN/BQ/ LN/CQ/
GLN/GCQ/
GUD
TD
UD
R1
R2
500 Ω
150 Ω
200 Ω
68 Ω
100 Ω
33 Ω
20 Ω
6.8 Ω
Figure 17. Bipolar Operation
–10–
REV. C
ADI [ ADI ]
