LTC2636

21

2636f

OPERATION

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 when the REF

(FSE) is positive, the output for the highest codes limits

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 carefully

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.

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

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