LTC2636

21

Rev D

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OPERATION

2636 F04

INPUT CODE

(b)

OUTPUT

VOLTAGE

NEGATIVE

OFFSET

0V

0V

2,048

0

4,095

INPUT CODE

OUTPUT

VOLTAGE

(a)

(c)

INPUT CODE

OUTPUT

VOLTAGE

POSITIVE

FSE

Figure 4. Effects of Rail-to-Rail Operation On a DAC Transfer Curve (Shown for 12 Bits).

(a) Overall Transfer Function

(b) Effect of Negative Offset for Codes Near Zero

(c) Effect of Positive Full-Scale Error for Codes Near Full-Scale

Rails vs. Output Current” in the Typical Performance

Characteristics section.

The amplifier is stable driving capacitive loads of up to

500pF.

Rail-to-Rail Output Considerations

In any rail-to-rail voltage output device, the output is lim-

ited to voltages within the supply range.

Since the analog output of the DAC cannot go below

ground, it may limit for the lowest codes as shown in

Figure 4b. Similarly, limiting can occur near full-scale

DAC full-scale error (FSE) is positive, the output for the

Offset and linearity are defined and tested over the region

of the DAC transfer function where no output limiting can

occur.

Board Layout

The PC board should have separate areas for the analog

and digital sections of the circuit. A single, solid ground

plane should be used, with analog and digital signals care-

fully routed over separate areas of the plane. This keeps

digital signals away from sensitive analog signals and

minimizes the interaction between digital ground currents

and the analog section of the ground plane. The resistance

from the LTC2636 GND pin to the ground plane should

be as low as possible. Resistance here will add directly to

the effective DC output impedance of the device (typically

0.1Ω). Note that the LTC2636 is no more susceptible to

this effect than any other parts of this type; on the con-

trary, it allows layout-based performance improvements

to shine rather than limiting attainable performance with

excessive internal resistance.

Another technique for minimizing errors is to use a sepa-

rate power ground return trace on another board layer.

The trace should run between the point where the power

supply is connected to the board and the DAC ground pin.

Thus the DAC ground pin becomes the common point for

analog ground, digital ground, and power ground. When

the LTC2636 is sinking large currents, this current flows

out the ground pin and directly to the power ground trace

without affecting the analog ground plane voltage.

It is sometimes necessary to interrupt the ground plane

to confine digital ground currents to the digital portion of

the plane. When doing this, make the gap in the plane only

as long as it needs to be to serve its purpose and ensure

that no traces cross over the gap.