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LTC2314-14

231414f

For more information www.linear.com/2314-14

applicaTions inForMaTion

Intermodulation Distortion (IMD)

If the ADC input signal consists of more than one spectral

component, the ADC transfer function nonlinearity can pro-

duce intermodulation distortion (IMD) in addition to THD.

IMD is the change in one sinusoidal input caused by the

presenceofanothersinusoidalinputatadifferentfrequency.

If two pure sine waves of frequencies fa and fb are ap-

plied to the ADC input, nonlinearities in the ADC transfer

function can create distortion products at the sum and

difference frequencies m • fa ± n • fb, where m and n = 0,

1, 2, 3, etc. For example, the 2nd order IMD terms include

(fa±fb).Ifthetwoinputsinewavesareequalinmagnitude,

the value (in decibels) of the 2nd order IMD products can

be expressed by the following formula:

IMD(fa ± fb) = 20 • log[VA (fa ± fb)/VA (fa)]

The LTC2314-14 has excellent IMD as shown in Figure 15.

Spurious Free Dynamic Range (SFDR)

The spurious free dynamic range is the largest spectral

componentexcludingDC,theinputsignalandtheharmon-

ics included in the THD. 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

theamplitudeofthereconstructedfundamentalisreduced

by 3dB for a full-scale input signal.

The full-linear bandwidth is the input frequency at which

the SINAD has dropped to 74dB (12 effective bits). The

LTC2314-14 has been designed to optimize the input band-

width, allowing the ADC to under-sample input signals with

frequencies above the converter’s Nyquist frequency. The

noise floor stays very low at high frequencies and SINAD be-

comesdominatedbydistortionatfrequenciesbeyondNyquist.

Recommended Layout

To obtain the best performance from the LTC2314-14 a

printedcircuitboardisrequired.Layoutfortheprintedcircuit

board(PCB)shouldensurethedigitalandanalogsignallines

areseparatedasmuchaspossible. Inparticular,careshould

be taken not to run any digital clocks or signals alongside

Figure 13. 32k Point FFT of the LTC2314-14 at fIN = 500 kHz

Figure 14. LTC2314-14 ENOB/SINAD vs fIN

Figure 15. LTC2314-14 IMD Plot

231414 F13

FREQUENCY (kHz)

2250

1750 2000

750 1000

500

250

1250 1500

–20

–40

–60

–80

–100

–120

–160

–140

VDD = 5V

SNR = 77.5dBFS

SINAD = 76.9dBFS

THD = 84.9dB

SFDR = 88.1dB

INPUT FREQUENCY (kHz)

11.50

11.67

11.83

12.00

12.17

12.33

12.50

250 500 750 1000 1250 1500 1750 2000 2250

231414 F14

VDD = 5V

VDD = 3V

INPUT FREQUENCY (kHz)

–160

–120

–140

–80

–100

–60

–40

–20

500

1000

1500

2000

2500

231414 F15

VDD = 5V

fS = 4.5Msps

fa = 471.421kHz

fb = 531.421kHz

IMD2 (fb + fa) = –79.4dBc

IMD3 (2fb –fa) = –90.8dBc

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