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LTC1275

12-Bit, 300ksps Sampling A/D Converters with Reference

Linear Technology 

LTC1273

LTC1275/LTC1276

APPLICATI S I FOR ATIO

Effective Number of Bits

The Effective Number of Bits (ENOBs) is a measurement of

the resolution of an ADC and is directly related to the

S/(N + D) by the equation:

N = [S/(N + D) – 1.76]/6.02

where N is the Effective Number of Bits of resolution and

S/(N + D) is expressed in dB. At the maximum sampling

rate of 300kHz the LTC1273/LTC1275/LTC1276 maintain

very good ENOBs up to the Nyquist input frequency of

150kHz. Refer to Figure 3.

12

74

11

68

10

62

9

56

8

50

7

6

5

4

3

2

1 fSAMPLE = 300kHz

0

10k

100k

INPUT FREQUENCY (Hz)

1M 2M

LTC1273/75/76 • F03

Figure 3. Effective Bits and Signal to (Noise + Distortion)

vs Input Frequency

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√V22

+

V32

+ V42

V1

...

+

VN2

where V1 is the RMS amplitude of the fundamental fre-

quency and V2 through VN are the amplitudes of the

second through Nth harmonics. THD versus input fre-

quency is shown in Figure 4. The LTC1273/LTC1275/

LTC1276 have good distortion performance up to Nyquist

and beyond.

0

fSAMPLE = 300kHz

– 10

– 20

– 30

THD

2nd HARMONIC

3rd HARMONIC

– 40

– 50

– 60

– 70

– 80

– 90

– 100

1k

10k

100k

1M

10M

INPUT FREQUENCY (Hz)

LTC1273/75/76 • F04

Figure 4. Distortion vs Input Frequency

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 applied

to the ADC input, nonlinearities in the ADC transfer func-

tion can create distortion products at sum and difference

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 the 3rd order IMD terms include (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)

=

20log

Amplitude at (fa

Amplitude at

±

fa

fb)

127356fa

11

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