LTC1273ACSW datasheet(12/24 Pages) LINER

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LTC1273

LTC1275/LTC1276

127356fa

APPLICATI

I FOR ATIO

Figure 5 shows the IMD performance at a 30kHz input.

FREQUENCY (kHz)

–120

–100

–80

–60

–40

120

160

LTC1273/75/76 • F05

–20

100

140

fSAMPLE = 300kHz

fIN1 = 29.37kHz

fIN2 = 32.446kHz

Figure 5. Intermodulation Distortion Plot

the analog input must settle after the small current spike

before the next conversion starts. Any op amp that settles

in 600ns to small current transients will allow maximum

speed operation. If slower op amps are used, more settling

time can be provided by increasing the time between

conversions. Suitable devices capable of driving the ADCs’

AIN input include the LT1190/LT1191, LT1007, LT1220,

LT1223 and LT1224 op amps.

The analog input tolerates source resistance very well.

Here again, the only requirement is that the analog input

must settle before the next conversion starts. For larger

source resistance, full DC accuracy can be obtained if

more time is allowed between conversions. For more

information, see the Acquisition Time vs Source Resis-

tance curve in the Typical Performance Characteristics

section. For optimum frequency domain performance

[e.g., S/(N + D)], keep the source resistance below 100

Ω.

Internal Reference

The LTC1273/LTC1275/LTC1276 have an on-chip, tem-

perature compensated, curvature corrected, bandgap ref-

erence which is factory trimmed to 2.42V. It is internally

connected to the DAC and is available at pin 2 to provide

up to 1mA current to an external load.

For minimum code transition noise the reference output

should be decoupled with a capacitor to filter wideband

noise from the reference (10

µF tantalum in parallel with a

0.1

µF ceramic).

In the LTC1275, the VREF pin can be driven above its

normal value with a DAC or other means to provide input

span adjustment or to improve the reference temperature

drift. Figure 6 shows an LT1006 op amp driving the

VREF(OUT) ≥ 2.45V

Ω

INPUT RANGE

±1.033VREF(OUT)

–

LT1006

LTC1275

AIN

AGND

VREF

µF

LTC1273/75/76 • F06

Figure 6. Driving the VREF with the LT1006 Op Amp

Peak Harmonic or Spurious Noise

The peak harmonic or spurious noise is the largest spec-

tral component excluding the input signal and DC. This

value is expressed in decibels relative to the RMS value of

a full scale input signal.

Full Power and Full Linear Bandwidth

The full power bandwidth is that input frequency at which

the amplitude of the reconstructed fundamental is re-

duced by 3dB for a full scale input signal.

The full linear bandwidth is the input frequency at which

the S/(N + D) has dropped to 68dB (11 effective bits). The

LTC1273/LTC1275/LTC1276 have been designed to opti-

mize input bandwidth, allowing ADCs to undersample

input signals with frequencies above the converters’ Nyquist

Frequency. The noise floor stays very low at high frequen-

cies; S/(N + D) becomes dominated by distortion at

frequencies far beyond Nyquist.

Driving the Analog Input

The analog inputs of the LTC1273/LTC1275/LTC1276 are

easy to drive. They draw only one small current spike while

charging the sample-and-hold capacitor at the end of

conversion. During conversion the analog input draws no

current. The only requirement is that the amplifier driving

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