AD7886

–9–

REV. B

AD7886 DYNAMIC SPECIFICATIONS

The AD7886 is specified for dynamic performance specifica-

tions as well as traditional dc specifications such as integral and

differential nonlinearity. These ac specifications are required for

signal processing applications such as speech recognition, spec-

trum analysis and high speed modems. These applications require

information on the ADC’s effect on the spectral content of the

input signal. Hence, the parameters for which the AD7886 is

specified include SNR, harmonic distortion, intermodulation

distortion and peak harmonics. These terms are discussed in

more detail in the following 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 de-

pendent upon the number of quantization levels used in the

digitization process; the more levels, the smaller the quantiza-

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

input is given by

SNR = (6.02N + 1.76) dB

(1)

where N is the number of bits. Thus, for an ideal 12-bit con-

verter, SNR = 74 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 a 750 kHz sampling rate. A Fast Fourier Trans-

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

obtained. Figure 12 shows a typical 2048 point FFT plot with

an input signal of 100 kHz and a sampling frequency of 750 kHz.

Figure 12. AD7886 FFT Plot

The SNR obtained from this graph is 68 dB. It should be noted

that the harmonics are taken into account 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

obtain a measure of performance expressed in effective num-

ber 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 13 shows a typical plot of effective number of bits versus

frequency for a sampling frequency of 750 kHz. Input frequency

range for this particular graph was limited by the test equipment

to FS/4. The effective number of bits typically falls between

10.9 and 11.2, corresponding to SNR figures of 67.38 dB and

69.18 dB.

12

11.5

11

10.5

10

0

FS/4

INPUT FREQUENCY

SAMPLING FREQUENCY = 750kHz

T = 25 C

A

Figure 13. Effective Number of Bits vs. Frequency

Total Harmonic Distortion (THD)

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

tal. For the AD7886, THD is defined as

THD

= 20 log

3

4

5

6

2

(3)

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

the ADC output spectrum.

Intermodulation Distortion (IMD)

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).

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 intermodu-

lation distortion is 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 14 shows a typical IMD plot for the AD7886.

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 will be