LTC2633

21

2633fc

For more information www.linear.com/LTC2633

Figure 5. 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

2633 F05

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

OPERATION

internal reference command (0110b), any command in

software that powers up the DACs will also power up the

integrated reference.

Voltage Output

The LTC2633’s integrated rail-to-rail amplifier has guaran-

teed load regulation when sourcing or sinking up to 10mA

at 5V, and 5mA at 3V.

Load regulation is a measure of the amplifier’s ability to

maintain the rated voltage accuracy over a wide range of

load current. The measured change in output voltage per

change in forced load current is expressed in LSB/mA.

DC output impedance is equivalent to load regulation, and

may be derived from it by simply calculating a change

in units from LSB/mA to Ω. The amplifier’s DC output

impedance is 0.1Ω when driving a load well away from

the rails.

When drawing a load current from either rail, the output

voltage headroom with respect to that rail is limited by

the 50Ω typical channel resistance of the output devices

(e.g., when sinking 1mA, the minimum output voltage is

50Ω • 1mA, or 50mV). See the graph Headroom at Rails

vs Output Current in the Typical Performance Character-

istics section.

The amplifier is stable driving capacitive loads of up to

500pF.

Rail-to-Rail Output Considerations

Inanyrail-to-railvoltageoutputdevice,theoutputislimited

to voltages within the supply range.

SincetheanalogoutputoftheDACcannotgobelowground,

it may limit for the lowest codes as shown in Figure 5b.

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.