AD688SQ AD [Analog Devices], AD688SQ Datasheet - Page 6

• Manufacturer Part Number: AD688SQ
• Description: High Precision -+10 V Reference
• Manufacturer: AD [Analog Devices]

AD688
Duplication of these results requires a combination of high
accuracy and stable temperature control in a test system.
Evaluation of the AD688 will produce curves similar to those in
Figure 7, but output readings may vary depending on the test
methods and equipment utilized.
KELVIN CONNECTIONS
Force and sense connections, also referred to as Kelvin connec-
tions, offer a convenient method of eliminating the effects of
voltage drops in circuit wires. As seen in Figure 9a, the load
DYNAMIC PERFORMANCE
The output buffer amplifiers (A3 and A4) are designed to pro-
vide 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 10 displays the characteristic of the AD688 output
amplifier driving a 0-to-10 mA load.
Input Noise Voltage Spectral
Density
A3, A4 Open-Loop Frequency
Response
Figure 9. Advantage of Kelvin Connection
A3, A4 CMR vs. Frequency
Unity-Gain Follower Pulse
Response (Large Signal)
–6–
current and wire resistance produce an error (V
at the load. The Kelvin connection of Figure 9b overcomes the
problem by including the wire resistance within the forcing loop
of the amplifier and sensing the load voltage. The amplifier
corrects for any errors in the load voltage. In the circuit shown,
the output of the amplifier would actually be at 10 volts + V
and the voltage at the load would be the desired 10 volts.
The AD688 has three amplifiers which can be used to imple-
ment Kelvin connections. Amplifier A2 is dedicated to the
ground force-sense function while uncommitted amplifiers A3
and A4 are free for other force-sense chores.
In some applications, one amplifier may be unused. In such
cases, the unused amplifier should be connected as a unity-gain
follower (force and sense pins tied together) and the input
should be connected to ground.
An unused amplifier may be used for other circuit functions as
well. The curves on this page show the typical performance of
A3 and A4.
Figure 10a. Transient Load Test Circuit
A3, A4 PSR vs. Frequency
Unity-Gain Follower Pulse
Response {Small Signal)
ERROR
= R
REV. A
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