AD688

Rev. B | Page 9 of 16

TEMPERATURE PERFORMANCE

The AD688 is designed for precision reference applications

where temperature performance is critical. Extensive

temperature testing ensures that the device’s high level of

performance is maintained over the operating temperature

range.

Figure 11 shows the typical output voltage drift and illustrates

the test methodology. The box in Figure 11 is bounded on the

sides by the operating temperature extremes, and on top and

bottom by the maximum and minimum +10 V output error

voltages measured over the operating temperature range. The

slopes of the diagonals drawn for both the +10 V and –10 V

outputs determine the performance grade of the device.

6

–6

TEMPERATURE (

°C)

5

4

3

2

1

0

–1

–2

–3

–4

–5

–10V OUT

+10V OUT

SLOPE

–60 –50 –40 –30 –20 –10 0

10 20 30 40 50 60 70 80 90 100 110 120 130

+10V OUTPUT SLOPE = T.C. =

2.2mV – –3.2mV

(85

=

= 3ppm/

°C

Figure 11. Typical AD688AQ Temperature Drift

Each AD688A and B grade unit is tested at −40°C, −25°C, 0°C,

+25°C, +50°C, +70°C, and +85°C. This approach ensures that

the variations of output voltage that occur as the temperature

changes within the specified range will be contained within a

box whose diagonal has a slope equal to the maximum specified

drift. The position of the box on the vertical scale will change

from device to device as initial error and the shape of the curve

vary. Maximum height of the box for the appropriate

temperature range is shown in Figure 12.

MAXIMUM OUTPUT CHANGE (mV)

DEVICE GRADE

0 TO +70

°C

–40

°C TO +85°C

AD688AQ

AD688BQ

AD688ARWZ

1.40 (TYP)

3.75

3.75

1.05

4.0

Figure 12. Maximum + 10 V or −10 V Output Change

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 11, but output readings may vary depending on the test

methods and equipment utilized.

KELVIN CONNECTIONS

Force and sense connections, also referred to as Kelvin

connections, offer a convenient method of eliminating the

effects of voltage drops in circuit wires. As seen in Figure 13a,

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 V.

–

+

10V

R

R

R

V = 10V

i = 0

i = 0

a.

b.

R

Figure 13. Advantage of Kelvin Connection

The AD688 has three amplifiers which can be used to

implement 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. Figure 14 through Figure 19 show the typical performance

of A3 and A4.