AD7871KN View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AD7871KN

LC2MOS Complete 14-Bit, Sampling ADCs

Analog Devices 

AD7871/AD7872

DYNAMIC SPECIFICATIONS

The AD7871/AD7872 is specified and tested for dynamic per-

formance specifications as well as traditional dc specifications

such as Integral and Differential Nonlinearity. These ac specifi-

cations are required for signal processing applications such as

Speech Recognition, Spectrum Analysis and High Speed

Modems. These applications require information on the effects

on the spectral content of the input signal. Hence, the param-

eters for which the AD7871/AD7872 is specified include SNR,

Harmonic Distortion, Intermodulation Distortion and Peak

Harmonics. These terms are discussed in more detail in the fol-

lowing sections.

Signal-to-Noise Ratio (SNR)

SNR is the measured signal-to-noise ratio at the output of the

ADC. The signal is the rms magnitude of the fundamental.

Noise is the rms sum of all the nonfundamental signals up to

half the sampling frequency (fs/2) excluding dc. SNR is depen-

dent upon the number of quantization levels used in the digiti-

zation process; the more levels, the smaller the quantization

noise. The theoretical signal to noise ratio for a sine wave input

is given by:

SNR(dB) = (6.02N + 1.76)

(1)

where N is the number of bits in the ADC. Thus for an ideal

14-bit converter, SNR = 86 dB.

The output spectrum from the ADC is evaluated by applying a

sine wave signal of very low distortion to the VIN input, which is

sampled at an 83 kHz sampling rate. A Fast Fourier Transform

(FFT) plot is generated from which the SNR data can be ob-

tained. Figure 13 shows a typical 2048 point FFT plot of the

AD7871/AD7872, with an input signal of 10 kHz and a sam-

pling frequency of 83 kHz. The SNR obtained from this graph

is

80 dB. It should be noted that the harmonics are included when

calculating the SNR.

Effective Number of Bits

The formula given in Equation 1 relates the 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).

N

=

SNR –1.76

6.02

(2)

The effective number of bits for a device can be calculated di-

rectly from its measured SNR. Figure 14 shows a typical plot of

effective number of bits versus frequency for the AD7871/

AD7872 with a sampling frequency of 60 kHz.

Figure 14. Effective Number of Bits vs. Frequency

Harmonic Distortion

Harmonic Distortion is the ratio of the rms sum of harmonics to

the fundamental. For the AD7871/AD7872, Total Harmonic

Distortion (THD) is defined as

THD(dB) = 20 log √V22+V32+V42+V52+V62

V1

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. Figure 15 shows how the THD var-

ies with input frequency.

Figure 13. Fast Fourier Transform Plot

Figure 15. Total Harmonic Distortion vs. Frequency

Intermodulation Distortion

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

fb, any active device with nonlinearities will create distortion

products at sum and difference frequencies of mfa ± nfb where

m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for which

neither m nor n are equal to zero. For example, the second or-

der terms include (fa+fb) and (fa–fb), while the third order

terms include (2fa+fb), (2fa–fb), (fa+2fb) and (fa–2fb).

–10–

REV. D

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