AD688

REV. A

–6–

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

Figure 9. Advantage of Kelvin Connection

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,

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.

A3, A4 CMR vs. Frequency

A3, A4 Open-Loop Frequency

Response

A3, A4 PSR vs. Frequency

Input Noise Voltage Spectral

Density

Unity-Gain Follower Pulse

Response (Large Signal)

Unity-Gain Follower Pulse

Response {Small Signal)

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.

Figure 10a. Transient Load Test Circuit