AD7870LP datasheet(17/28 Pages) AD

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AD7870/AD7875/AD7876

Rev. C | Page 17 of

Fourier transform (FFT) plot is generated from which the SNR

data can be obtained. Figure 18 shows a typical 2048 point FFT

plot of the AD7870KN/AD7875KN with an input signal of 25 kHz

and a sampling frequency of 100 kHz. The SNR obtained from

this graph is 72.6 dB. It should be noted that the harmonics are

taken into account when calculating the SNR.

–30

–60

–90

–120

–140

FREQUENCY (kHz)

INPUT FREQUENCY = 25kHz

SAMPLE FREQUENCY = 100kHz

SNR = 72.6dB

TA = 25°C

Figure 18. FFT Plot

Effective Number of Bits

The formula given in Equation 1 relates SNR to the number of

bits. Rewriting the formula, as in Equation 2, it is possible to get

a measure of performance expressed in effective number of

bits (N).

−

SNR

(2)

The effective number of bits for a device can be calculated

directly from its measured SNR.

Figure 19 shows a typical plot of effective number of bits vs.

frequency for an AD7870KN/AD7875KN with a sampling

frequency of 100 kHz. The effective number of bits typically

falls between 11.7 and 11.85 corresponding to SNR figures of

72.2 and 73.1 dB.

12.0

11.5

11.0

10.5

10.0

12.5

18.75

6.25

25.0

31.25

37.5

43.75

50.0

INPUT FREQUENCY (kHz)

SAMPLE FREQUENCY = 100kHz

TA = 25°C

Figure 19. Effective Number of Bits vs. Frequency

Total Harmonic Distortion (THD)

THD is the ratio of the rms sum of harmonics to the rms

value of the fundamental. For the AD7870/AD7875, THD is

defined as

log

THD

where V1 is the rms amplitude of the fundamental and V2, V3,

V4, V5 and V6 are the rms amplitudes of the second through the

sixth harmonic. The THD is also derived from the FFT plot of

the ADC output spectrum.

Intermodulation Distortion

With inputs consisting of sine waves at two frequencies, fa and

fb, any active device with nonlinearities creates distortion

products at sum and difference frequencies of mfa ± nfb where

m, n = 0, 1, 2, 3, and so on. Intermodulation terms are those

for which neither m nor n are equal to zero. For example, the

second order terms include (fa + fb) and (fa − fb), while the

third order terms include (2fa + fb), (2fa − fb), (fa + 2fb) and

(fa − 2fb).

Using the CCIF standard, where two input frequencies near the

top end of the input bandwidth are used, the second and third

order terms are of different significance. The second order

terms are usually distanced in frequency from the original sine

waves while the third order terms are usually at a frequency

close to the input frequencies. As a result, the second and third

order terms are specified separately. The calculation of the

intermodulation distortion is as per the THD specification

where it is the ratio of the rms sum of the individual distortion

products to the rms amplitude of the fundamental expressed in

dBs. In this case, the input consists of two, equal amplitude, low

distortion sine waves. Figure 20 shows a typical IMD plot for

the AD7870/AD7875.

Peak Harmonic or Spurious Noise

Peak harmonic or spurious noise is defined as the ratio of the

rms value of the next largest component in the ADC output

spectrum (up to FS/2 and excluding dc) to the rms value of the

fundamental. Normally, the value of this specification is deter-

mined by the largest harmonic in the spectrum, but for parts

where the harmonics are buried in the noise floor the peak is a

noise peak.

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