AD668

REV. A

–5–

ANALOG OFFSET ERROR: The analog offset is defined as

the offset of the analog amplifier channel, referred to the analog

input. Ideally, this would be measured with the analog input at

0 V and the digital input at full scale. Since a 0 V analog input

voltage constitutes an undervoltage condition, this specification

is determined through linear extrapolation, as indicated in

Figure 1.

Figure 1. Derivation of Analog Offset Voltage

GLITCH IMPULSE: Asymmetrical switching times in a DAC

may give rise to undesired output transients which are quanti-

fied by their glitch impulse. It is specified as the net area of the

glitch in pV-sec.

Figure 2. AD668 Major Carry Glitch

FUNCTIONAL DESCRIPTION

The AD668 is designed to combine excellent performance with

maximum flexibility. The functional block diagram and the

simple transfer functions provided below will provide the user

with a basic grasp of the AD668’s operation. Examples of typi-

cal circuit configurations are provided in the section APPLY-

ING THE AD668. Subsequent sections contain more detailed

information useful in optimizing DAC performance in high

speed, high resolution applications.

DAC Transfer Function

The AD668 may be used either in a current output mode (DAC

output connected to a virtual ground) or a voltage output mode

(DAC output connected to a resistive load).

In current output mode:

Unipolar Mode

I

V

IN

V

NOM

×

DAC code

4096

× 10.24 mA

Bipolar Mode

I

V

IN

V

NOM

×

DAC code

4096

× 10.24 mA –

V

IN

V

NOM

× 5.12 mA

In voltage output mode:

V

(for both unipolar and bipolar modes)

where:

1.25 V, or 5 V, determined by the wiring configuration of Pins

21 and 22. (See APPLYING THE AD668.)

DAC code – the numerical representation of the DAC’s digital

inputs; a number between 0 and 4095.

this can be is 200

Ω (the internal DAC ladder resistance). The

on-board load resistor (Pin 19) has been trimmed so that its

parallel combination with the DAC ladder resistance is 100

Ω

(

±1%)

Bipolar mode – produces a bipolar analog output from the digital

input by offsetting the normal output current with a precision

current source. This offset is achieved by connecting Pin 16 to

the DAC output. In the unipolar mode, Pin 16 should be

grounded.

If the dc errors are included, the transfer function becomes

somewhat more complex:

I

V

IN

V

NOM

+ OFFSET

ANALOG









×

DAC code

4096

× (1 + E ) × 10.24 mA

+ OFFSET

DIGITAL

×

V

IN

V

NOM

× 10.24 mA

–

V

IN

V

NOM

+ OFFSET

ANALOG









× (5.12 mA + [OFFSET

×

where:

E = the gain error, expressed fractionally.

Operating Limits: