AD7868

–9–

REV. B

AD7868 DYNAMIC SPECIFICATIONS

The AD7868 is specified and 100% tested for dynamic perfor-

mance specifications 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, spectrum analysis, and high-speed modems. These

applications require information on the converter’s effect on the

spectral content of the input signal. Hence, the parameters for

which the AD7868 is specified include SNR, harmonic distor-

tion and peak harmonics. These terms are discussed in more de-

tail in the following sections.

Signal-to-Noise Ratio (SNR)

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

ADC or DAC. The signal is the rms magnitude of the funda-

mental. Noise is the rms sum of all the nonfundamental signals

up to half the sampling frequency (fs/2) excluding dc. SNR is

dependent upon the number of levels used in the quantization

process; the more levels, the smaller the quantization 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.

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.

Harmonic Distortion

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

the fundamental. For the AD7868, total harmonic distortion

(THD) is defined as

THD

= 20 log

3

4

5

6

2

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

of the ADC or DAC output spectrum.

ADC Testing

The output spectrum from the ADC is evaluated by applying a

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 9 shows a typical 2048 point FFT plot of the

AD7868BQ ADC with an input signal of 10 kHz and a sam-

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

is 73 dB. It should be noted that the harmonics are taken into

account when calculating the SNR.

Figure 10 shows a typical plot of effective number of bits versus

frequency for an AD7868BQ with a sampling frequency of

83 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.

Figure 9. AD7868, ADC FFT Plot

12

11.5

11

10.5

10

INPUT FREQUENCY – kHz

SAMPLE FREQUENCY = 83 kHz

0

41.5

Figure 10. Effective Number of Bits vs. Frequency for the

ADC

DAC Testing

A simplified diagram of the method used to test the dynamic

performance specifications of the DAC is outlined in Figure 11.

Data is loaded to the DAC under control of the microcontroller

and associated logic. The output of the DAC is applied to a 9th

order low-pass filter whose cutoff frequency corresponds to the

Nyquist limit. The output of the filter is in turn applied to a

16-bit accurate digitizer. This digitizes the signal and the micro-

controller generates an FFT plot from which the dynamic per-

formance of the DAC can be evaluated.