AD668 datasheet(10/16 Pages) AD

10 / 16 page

AD668

REV. A

–10–

Figure 11. AD586 Driving the AD668

mode (Pin 22 grounded, input into Pin 21). Fine adjustment of

the gain is provided by both the AD586 external trim resistor

and the 100

Ω potentiometer in series with the reference input.

The resistive divider at the reference input will draw approxi-

mately 1 mA from the AD586, leaving plenty of driving current

for other loads in the system.

AC HOOKUP: 1.25 V AC FULL SCALE, 2.5 V DC FULL

SCALE

The circuit shown in Figure 12 allows separate setting of dc ref-

erence bias point on a 2.5 V scale and capacitively coupled ac

signal on a 1.25 V scale. The basic reference input is configured

in the 1.25 V mode (Pin 21 grounded, Pin 22 used as the refer-

ence input.) The 2.5 V dc range is achieved by using an external

Table II. AD668 Topology Variations

OUTPUT LEVELS

Nominal

Analog

Input

0 V to 1 V

–500 mV to +500 mV

0 V to –10 V

+5 V to –5 V

–1 V to +1 V

Unipolar

Bipolar

Unipolar

Bipolar

1 V

Unbuffered VOUT

Buffered VOUT

Unbuffered VOUT

AIN = Pins 21 + 22

External Amplifier

RL (Pin 19) Tied

(See Figure 10)

To IOUT (Pin 20)

Unipolar

Bipolar

Unipolar

Bipolar

Unbuffered VOUT

Buffered VOUT

Unbuffered VOUT

1.25 V

AIN = Pin 22

Pin 21 Grounded

(See Figure 8)

External Amplifier

RL (Pin 19) Tied

To IOUT (Pin 20)

Unipolar

Bipolar

Unipolar

Bipolar

Unbuffered VOUT

Buffered VOUT

Unbuffered VOUT

5 V

AIN = Pin 21

Pin 22 Grounded

(See Figure 7)

External Amplifier

RL (Pin 19) Tied

To IOUT (Pin 20)

(See Figure 9)

5k series resistor in the dc path. Note that because of the rela-

tively wide tolerance (

±20%) in the absolute value of the

AD668’s internal input divider resistors, substantial gain range

adjustment should be provided in the external series resistance.

Figure 12. AC Hookup

DAC DRIVE: THE AD568 DRIVING THE AD668

The circuit shown in Figure 13 produces an analog output pro-

portional to the product of two digital inputs. The AD568 has

an on-board fixed reference and generates a full-scale output

voltage of 1.024 V (just as the AD668 does in its unbuffered

voltage output mode). This output voltage can be used to di-

rectly drive the AD668 in the 1 V reference input mode. Note

that in this case, the lower 410 codes of the AD568 are out-of-

bounds; they produce an undervoltage condition at the AD668

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