AD688

Rev. B | Page 7 of 16

APPLICATIONS

The AD688 can be configured to provide ±10 V reference

outputs as shown in Figure 4. The architecture of the AD688

provides ground sense and uncommitted output buffer

amplifiers which offer the user a great deal of functional

flexibility. The AD688 is specified and tested in the

configuration shown in Figure 4. The user may choose to take

advantage of other configuration options available with the

AD688; however performance in these configurations is not

guaranteed to meet the stringent data sheet specifications.

Unbuffered outputs are available at Pin 6 and Pin 8. Loading of

these unbuffered outputs will impair circuit performance.

Amplifiers A3 and A4 can be used interchangeably. However,

the AD688 is tested (and the specifications are guaranteed) with

the amplifiers connected as indicated in Figure 4. When either

A3 or A4 is unused, its output force and sense pins should be

connected and the input tied to ground.

Two outputs of the same voltage polarity may be obtained by

connecting both A3 and A4 to the appropriate unbuffered

output on Pin 6 or Pin 8. Performance in these dual output

configurations will typically meet data sheet specifications.

R3

R1

R2

R4

R5

R6

A1

A4

AD688

A3

A2

SYSTEM

GROUND

+10V

+15V SUPPLY

–10V

–15V SUPPLY

SYSTEM

GROUND

0.1

µF

0.1

µF

7

6

4

3

1

14

2

15

16

5

9

10

8

12

11

13

Figure 4. +10 V and −10 V Outputs

CALIBRATION

Generally, the AD688 will meet the requirements of a precision

system without additional adjustment. Initial output voltage

error of 2 mV and output noise specs of 6 µV p-p allow for

accuracies of 12 to 16 bits. However, in applications where an

even greater level of accuracy is required, additional calibration

may be called for. The provision for trimming has been made

through the use of the GAIN ADJ and BAL ADJ pins (Pin 5 and

Pin 12, respectively).

The AD688 provides a precision 20 V span with a center tap

which is used with the buffer and ground sense amplifiers to

achieve the ±10 V output configuration. GAIN ADJ and

BAL ADJ can be used to trim the magnitude of the 20 V span

voltage and the position of the center tap within the span. The

gain adjustment should be performed first. Although the trims

are not interactive within the device, the gain trim will move the

balance trim point as it changes the magnitude of the span.

Figure 5 shows the gain and balance trims of the AD688. A

100 kΩ 20-turn potentiometer is used for each trim. The

potentiometer for the gain trim is connected between Pin 6

(GAIN ADJ). The potentiometer is adjusted to produce exactly

20 V between Pin 1 and Pin 15, the amplifier outputs. The

balance potentiometer, also connected between Pin 6 and Pin 8

with the wiper to Pin 12 (BAL ADJ), is then adjusted to center

the span from +10 V to −10 V.

Input impedance on both the GAIN ADJ and the BAL ADJ pins

is approximately 150 kΩ. The gain adjustment trim network

effectively attenuates the 20 V across the trim potentiometer by

a factor of about 1150 to provide a trim range of –5.8 mV to

+12.0 mV with a resolution of approximately 900 µV/turn

(20-turn potentiometer). The balance adjustment trim network

attenuates the trim voltage by a factor of about 1250, providing

a trim range of ±8 mV with a resolution of 800 µV/turn.

Trimming the AD688 introduces no additional errors over

temperature, so precision potentiometers are not required.

When balance adjustment is not necessary, Pin 12 should be left

floating. If gain adjustment is not required, Pin 5 should also be

left floating.

R3

R1

R2

R4

R5

R6

A1

A4

AD688

A3

A2

SYSTEM

GROUND

+10V

+15V SUPPLY

–10V

–15V SUPPLY

SYSTEM

GROUND

0.1

µF

0.1

µF

100k

Ω

20T

BALANCE

ADJUST

100k

Ω

20T

GAIN ADJUST

+15V

NOISE

REDUCTION

1

µF

7

6

4

3

1

14

2

15

16

5

9

10

8

12

11

13

20k

Ω

Figure 5. Gain and Balance Adjustment with Noise Reduction