AD688

Rev. B | Page 11 of 16

DYNAMIC PERFORMANCE

The output buffer amplifiers (A3 and A4) are designed to

provide the AD688 with static and dynamic load regulation

superior to less complete references.

Many A/D and D/A converters present transient current loads

to the reference, and poor reference response can degrade the

converter’s performance.

Figure 20, Figure 21, and Figure 22 display the characteristic of

the AD688 output amplifier driving a 0 mA to 10 mA load.

10V

A3 OR A4

1k

Ω

10V

0V

Figure 20. Transient Load Test Circuit

200mV

5V

500ns

100

90

10

Figure 21. Large-Scale Transient Response

100

90

10

0%

1mV

5V

2

µs

Figure 22. Fine-Scale Settling for Transient Load

Figure 23 and Figure 24 display the output amplifier

characteristic driving a 5 mA to 10 mA load, a common

situation found when the reference is shared among multiple

converters or is used to provide bipolar offset current.

10V

A3 OR A4

10V

0V

+

–

2k

Ω

2k

Ω

Figure 23. Transient and Constant Load Test Circuit

100

90

10

0%

1mV

200mV

1

µs

200mV/

CM

1mV/CM

5V

Figure 24. Transient Response 5 mA to 10 mA Load

In some applications, a varying load may be both resistive and

capacitive in nature, or may be connected to the AD688 by a

long capacitive cable. Figure 25 and Figure 26 display the output

amplifier characteristics driving a 1000 pF, 0 mA to 10 mA load.

10V

10V

0V

1k

Ω

1000pF

Figure 25. Capacitive Load Transient Response Test Circuit

100

90

10

0%

5V

200mV

1

µs

1000pF

Figure 26. Output Response with Capacitive Load

Figure 27 and Figure 28 display the crosstalk between output

amplifiers. The top trace shows the output of A4, dc-coupled

and offset by 10 V, while the output of A3 is subjected to a 0 mA

to 10 mA load current step. The transient at A4 settles in about

1 µs, and the load-induced offset is about 100 µV.