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LTC1854/LTC1855/LTC1856

1854565af

applicaTions inFormaTion

SIGNAL-TO-NOISE AND DISTORTION RATIO

The Signal-to-Noise and Distortion Ratio (SINAD) is the

ratio between the RMS amplitude of the fundamental input

frequency to the RMS amplitude of all other frequency

components at the A/D output. The output is band limited

to frequencies from above DC and below half the sampling

frequency. Figure 12 shows a typical SINAD of 87dB with

a 100kHz sampling rate and a 1kHz input.

TOTAL HARMONIC DISTORTION

Total Harmonic Distortion (THD) is the ratio of the RMS

sum of all harmonics of the input signal to the fundamental

itself. The out-of-band harmonics alias into the frequency

band between DC and half the sampling frequency. THD

is expressed as:

THD = 20log

2 + V32 + V42...+ VN2

where V1 is the RMS amplitude of the fundamental fre-

quencyandV2throughVNaretheamplitudesofthesecond

through Nth harmonics.

BOARD LAYOUT, POWER SUPPLIES

AND DECOUPLING

Wire wrap boards are not recommended for high reso-

lution or high speed A/D converters. To obtain the best

performance from the LTC1854/LTC1855/LTC1856, a

printed circuit board is required. Layout for the printed

circuit board should ensure the digital and analog signal

lines are separated as much as possible. In particular, care

should be taken not to run any digital track alongside an

analog signal track or underneath the ADC. The analog

input should be screened by AGND.

InapplicationswheretheMUXisconnectedtotheADC,itis

possible to get noise coupling into the ADC from the trace

connecting the MUXOUT to the ADC. Therefore, reducing

thelengthofthetracesconnectingtheMUXOUTpins(Pins

10, 11) to the ADC pins (Pins 12, 13) can minimize the

problem. The unused MUX inputs should be grounded to

prevent noise coupling into the inputs.

Figure 13 shows the power supply grounding that will help

obtain the best performance from the 12-bit/14-bit/16-bit

ADCs. Pay particular attention to the design of the analog

and digital ground planes. The DGND pin of the LTC1854/

Figure 12. LTC1856 Nonaveraged 4096 Point FFT Plot

FREQUENCY (kHz)

185456 F12

5 10

40 45

–60

–40

–20

–80

–100

–70

–50

–30

–10

–90

–110

–130

–120

fSAMPLE = 100kHz

fIN = 1kHz

SINAD = 87dB

THD = –101dB

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