LTC6078 datasheet(15/20 Pages) LINER

15 / 20 page

LTC2641/LTC2642

26412f

APPLICATIONS INFORMATION

such as the LTC6078 is suitable, if the application does not

require linear operation very near to GND, or zero scale

(Figure 2). The LTC6078 typically swings to within 1mV

of GND if it is not required to sink any load current. For an

LSB size of 38μV, 1mV represents 26 missing codes near

zero scale. Linearity will be degraded over a somewhat

larger range of codes above GND. It is also unavoidable

that settling time and transient performance will degrade

whenever a single supply ampliﬁer is operated very close

to GND, or to the positive supply rail.

The small LSB size of a 16-bit DAC, coupled with the tight

accuracy speciﬁcations on the LTC2641/LTC2642, means

that the accuracy and input speciﬁcations for the external

op amp are critical for overall DAC performance.

Op Amp Speciﬁcations and Unipolar DAC Accuracy

Most op amp accuracy speciﬁcations convert easily to

DAC accuracy.

Op amp input bias current on the noninverting (+) input is

equivalent to an IL loading the DAC VOUT pin and therefore

produces a DAC zero-scale error (ZSE) (see Unbuffered

Operation):

ZSE = –IB(IN+) • ROUT [Volts]

In 16-bit LSBs:

ZSE

I IN

LSB

REF

()

⎛

⎝⎜

⎞

⎠⎟ []

–• .

•

Op amp input impedance, RIN, is equivalent to an RL

loading the LTC2641/LTC2642 VOUT pin, and produces a

gain error of:

LSB

⎛

⎝⎜

⎞

⎠⎟

[]

–

Op amp offset voltage, VOS, corresponds directly to DAC

zero code offset error, ZSE:

ZSE

LSB

REF

[]

•

Temperature effects also must be considered. Over the

–40°C to 85°C industrial temperature range, an offset

voltage temperature coefﬁcient (referenced to 25°C) of

0.6μV/°C will add 1LSB of zero-scale error. Also, IBIAS and

the VOFFSET error it causes, will typically show signiﬁcant

relative variation over temperature.

Op amp open-loop gain, AVOL, contributes to DAC gain

error (GE):

LSB

VOL

[]

Op amp input common mode rejection ratio (CMRR) is an

input-referred error that corresponds to a combination of

gain error (GE) and INL, depending on the op amp archi-

tecture and operating conditions. A conservative estimate

of total CMRR error is:

Error

CMRR

CMRR RANGE

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝⎜

⎞

⎠⎟

•

kLSB

⎛

⎝⎜

⎞

⎠⎟

[]

•66

where VCMRR_RANGE is the voltage range that CMRR (in

dB) is speciﬁed over. Op amp Typical Performance Charac-

teristics graphs are useful to predict the impact of CMRR

errors on DAC performance. Typically, a precision op amp

will exhibit a fairly linear CMRR behavior (corresponding

to DAC gain error only) over most of the common mode

input range (CMR), and become nonlinear and produce

signiﬁcant errors near the edge of the CMR.

Rail-to-rail input op amps are a special case, because they

have 2 distinct input stages, one with CMR to GND and

the other with CMR to V+. This results in a “crossover”

CM input region where operation switches between the

two input stages.

The LTC6078 rail-to-rail input op amp typically exhibits

remarkably low crossover linearity error, as shown in the

VOS vs VCM Typical Performance Characteristics graphs

(see the LTC6078 data sheet). Crossover occurs at CM

inputs about 1V below V+, and an LTC6078 operating as

a unipolar DAC buffer with VREF = 2.5V and V+ = 5V will

typically add only about 1LSB of GE and almost no INL

error due to CMRR. Even in a full rail-to-rail application,

with VREF = V+ = 5V, a typical LTC6078 will add only about

1LSB of INL at 16-bits.

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