LTC2281 Просмотр технического описания (PDF) - Linear Technology

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LTC2281

Dual 10-Bit, 125Msps Low Power 3V ADC

Linear Technology 

LTC2281

APPLICATIONS INFORMATION

DYNAMIC PERFORMANCE

Signal-to-Noise Plus Distortion Ratio

The signal-to-noise plus distortion ratio [S/(N + D)] is

the ratio between the RMS amplitude of the fundamen-

tal input frequency and the RMS amplitude of all other

frequency components at the ADC output. The output is

band limited to frequencies above DC to below half the

sampling frequency.

Signal-to-Noise Ratio

The signal-to-noise ratio (SNR) is the ratio between the

RMS amplitude of the fundamental input frequency and

the RMS amplitude of all other frequency components

except the first five harmonics and DC.

2fa + fb, 2fb + fa, 2fa – fb and 2fb – fa. The intermodula-

tion distortion is defined as the ratio of the RMS value of

either input tone to the RMS value of the largest 3rd order

intermodulation product.

Spurious Free Dynamic Range (SFDR)

Spurious free dynamic range is the peak harmonic or spuri-

ous noise that is the largest spectral component excluding

the input signal and DC. This value is expressed in decibels

relative to the RMS value of a full-scale input signal.

Input Bandwidth

The input bandwidth is that input frequency at which the

amplitude of the reconstructed fundamental is reduced

by 3dB for a full scale input signal.

Total Harmonic Distortion

Total harmonic distortion 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
 (V22 + V32 + V42 + ...Vn2) / V1

where V1 is the RMS amplitude of the fundamental fre-

quency and V2 through Vn are the amplitudes of the second

through nth harmonics. The THD calculated in this data

sheet uses all the harmonics up to the fifth.

Intermodulation Distortion

If the ADC input signal consists of more than one spectral

component, the ADC transfer function nonlinearity can

produce intermodulation distortion (IMD) in addition to

THD. IMD is the change in one sinusoidal input caused

by the presence of another sinusoidal input at a different

frequency.

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 of mfa ± nfb, where m and n = 0,

1, 2, 3, etc. The 3rd order intermodulation products are

Aperture Delay Time

The time from when CLK reaches midsupply to the in-

stant that the input signal is held by the sample and hold

circuit.

Aperture Delay Jitter

The variation in the aperture delay time from conversion

to conversion. This random variation will result in noise

when sampling an AC input. The signal to noise ratio due

to the jitter alone will be:

SNRJITTER = –20log (2π • fIN • tJITTER)

Crosstalk

Crosstalk is the coupling from one channel (being driven

by a full-scale signal) onto the other channel (being driven

by a –1dBFS signal).

CONVERTER OPERATION

As shown in Figure 1, the LTC2281 is a dual CMOS

pipelined multistep converter. The converter has six

pipelined ADC stages; a sampled analog input will result

in a digitized value five cycles later (see the Timing Dia-

gram section). For optimal AC performance the analog

inputs should be driven differentially. For cost sensitive

applications, the analog inputs can be driven single-ended

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