9

LTC1401

Driving the Analog Input

The analog input of the LTC1401 is easy to drive. It draws

only one small current spike while charging the sample-

and-hold capacitor at the end of a conversion. During

conversion, the analog input draws only a small leakage

current. The only requirement is that the amplifier driving

the analog input must settle after the small current spike

before the next conversion starts. Any op amp that settles

in 315ns to small load current transients will allow maxi-

mum speed operation. If a slower op amp is used, more

settling time can be provided by increasing the time

between conversions. Suitable devices capable of driving

op amps.

The following list is a summary of the op amps that are

suitable for driving the LTC1401, more detailed informa-

tion is available in the Linear Technology databooks and

the LinearView

LT1215/LT1216: Dual and quad 23MHz, 50V/

µs single

supply op amps. Single 5V to

±15V supplies, 6.6mA

specifications, 90ns settling to 0.5LSB.

LT1229/LT1230: Dual and quad 100MHz current feedback

amplifiers.

±2V to ±15V supplies, 6mA supply current

each amplifier. Low noise. Good AC specs.

LT1498/LT1499: Dual or quad 10MHz, 6V/

µs, single

2.2V to

±15V supplies, 1.7mA supply current per ampli-

fier, input/output swings rail-to-rail. Excellent AC and DC

specs.

LT1630: Dual or quad 30MHz, 10V/

µs, single 2.7V to ±15V

supplies, 3.5mA supply current per amplifier, input/output

swings rail-to-rail. Good AC and DC specs.

Internal Reference

The LTC1401 has an on-chip, temperature compensated,

curvature corrected, bandgap reference, which is factory

trimmed to 1.20V. It is internally connected to the DAC and

ence frequencies of mfa

±nfb, where m and n = 0, 1, 2, 3,

etc. For example, the 2nd order IMD terms include (fa + fb)

and (fa – fb) while 3rd order IMD terms includes

(2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb). If the two input

sine waves are equal in magnitude, the value (in decibels)

of the 2nd order IMD products can be expressed by the

following formula.

IMD fa

fb

fa

fb

±

±

20log

Amplitude at (

)

Amplitude at fa

Figure 5 shows the IMD performance at a 50kHz input.

Figure 5. Intermodulation Distortion Plot

FREQUENCY (kHz)

020

40

50

70

90

10

30

60

80

100

LTC1401 • F05

0

–10

–20

–30

–40

–50

– 60

–70

–80

–90

–100

–110

–120

fa = 49.853kHz

fb = 53.076kHz

fa

fb

3fa

2fb – fa

2fa + fb

2fa – fb

2fb + fa

2fa

3fb

fa + fb

2fb

fb – fa

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 the input frequency at which

the amplitude of the reconstructed fundamental is reduced

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).

LinearView is a trademark of Linear Technology Corporation.