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LTC2315-12

231512f

For more information www.linear.com/2315-12

applicaTions inForMaTion

with a 12-bit resolution ADC which can be measured with a

fixed DC signal applied to the input of the ADC. The resulting

outputcodesarecollectedoveralargenumberofconversions.

The shape of the distribution of codes will give an indication

of the magnitude of the transition noise. In Figure 12, the

distribution of output codes is shown for a DC input that has

been digitized 16,384 times. The distribution is Gaussian and

theRMScodetransitionnoiseis0.33LSB.Thiscorrespondsto

a noise level of 73dB relative to a full scale voltage of 4.096V.

Effective Number of Bits (ENOB)

The effective number of bits (ENOB) is a measurement of

the resolution of an ADC and is directly related to SINAD

by the equation where ENOB is the effective number of

bits of resolution and SINAD is expressed in dB:

ENOB = (SINAD – 1.76)/6.02

At the maximum sampling rate of 5MHz, the LTC2315-12

maintains an ENOB above 11.7 bits up to the Nyquist input

frequency of 2.5MHz. (Figure 14)

Signal-to-Noise Ratio (SNR)

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. Figure 13 shows

that the LTC2315-12 achieves a typical SNR of 73dB at a

5MHz sampling rate with a 500kHz input frequency.

Total Harmonic Distortion (THD)

TotalHarmonicDistortion(THD)istheratiooftheRMSsum

ofallharmonicsoftheinputsignaltothefundamentalitself.

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

between DC and half the sampling frequency (fSMPL/2).

THD is expressed as:

THD

=20log

V22

+V32 +V42 +VN2

where V1 is the RMS amplitude of the fundamental fre-

quencyandV2throughVNaretheamplitudesofthesecond

through Nth harmonics. THD versus Input Frequency is

shown in the Typical Performance Characteristics section.

The LTC2315-12 has excellent distortion performance up

to the Nyquist frequency.

Intermodulation Distortion (IMD)

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.

Figure 12. Histogram for 16384 Conversions

CODE

2047

2049

231512 F12

2048

2050

2051

σ = 0.33

10000

20000

30000

40000

60000

50000

Dynamic Performance

The LTC2315-12 has excellent high speed sampling

capability. Fast Fourier Transform (FFT) techniques are

used to test the ADC’s frequency response, distortion and

noise at the rated throughput. By applying a low distortion

sine wave and analyzing the digital output using an FFT

algorithm, the ADC’s spectral content can be examined

for frequencies outside the applied fundamental. The

LTC2315-12 provides guaranteed tested limits for both

AC distortion and noise measurements.

Signal-to-Noise and Distortion Ratio (SINAD)

The signal-to-noise and distortion ratio (SINAD) is the

ratio between the RMS amplitude of the fundamental input

frequency and 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 14 shows the LTC2315-12 maintains a

SINAD above 71dB up to the Nyquist input frequency of

2.5MHz.

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