Datasheet

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
V
OUT
I
L
1kΩ
10V
0V
V
L
00815-021
Figure 20. Transient Load Test Circuit
00815-022
V
OUT
V
L
200mV
5V
500ns
100
90
10
Figure 21. Large-Scale Transient Response
00815-023
100
90
10
0%
1mV
5V
2µs
V
OUT
V
L
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
V
OUT
I
L
10V
0V
V
L
+
–
2k
Ω 2k Ω
00815-024
Figure 23. Transient and Constant Load Test Circuit
00815-025
100
90
10
0%
1mV
200mV
1µs
V
OUT
200mV/
CM
V
OUT
1mV/CM
5V
V
L
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
V
OUT
10V
0V
V
L
1kΩ
1000pF
C
L
00815-026
Figure 25. Capacitive Load Transient Response Test Circuit
00815-027
100
90
10
0%
5V
200mV
1µs
C
L
= 0
C
L
=
1000pF
V
L
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.