AD7863

Rev. B | Page 14 of 24

AD7863 DYNAMIC SPECIFICATIONS

The AD7863 is specified and tested for dynamic performance as

well as traditional dc specifications such as integral and

differential nonlinearity. These ac specifications are required for

the signal processing applications such as phased array sonar,

adaptive filters, and spectrum analysis. These applications

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

of the input signal. Hence, the parameters for which the

AD7863 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

dependent upon the number of quantization levels used in the

digitization 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 14-bit converter, SNR = 86.04 dB.

Figure 12 shows a histogram plot for 8192 conversions of a dc

input using the AD7863 with 5 V supply. The analog input was

set at the center of a code transition. It can be seen that the

codes appear mainly in the one output bin, indicating very good

noise performance from the ADC.

746

747

748

749

750

751

752

753

754

755

CODE

8000

7000

6000

5000

4000

3000

2000

1000

0

Figure 12. Histogram of 8192 Conversions of a DC Input

The output spectrum from the ADC is evaluated by applying

which is sampled at a 175 kHz sampling rate. A fast fourier

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

be obtained. Figure 13 shows a typical 8192 point FFT plot of

the AD7863 with an input signal of 10 kHz and a sampling

frequency of 175 kHz. The SNR obtained from this graph is

−80.72 dB. It should be noted that the harmonics are taken into

account when calculating the SNR.

0

1020

304050

607080

90

FREQUENCY (kHz)

0

–130

–120

–110

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

–140

–150

SNR = +80.72dB

THD = –92.96dB

Figure 13. AD7863 FFT Plot

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 number

of bits (N).

02

.

6

76

.

1

−

=

SNR

N

(2)

The effective number of bits for a device can be calculated

directly from its measured SNR.

Figure 14 shows a typical plot of effective numbers of bits vs.

frequency for an AD7863-2 with a sampling frequency of

175 kHz. The effective number of bits typically falls between

13.11 and 11.05 corresponding to SNR figures of 80.68 dB

and 68.28 dB.

0

200

400

600

800

1000

FREQUENCY (kHz)

14.0

10.5

11.0

11.5

12.0

12.5

13.0

13.5

10.0

Figure 14. Effective Numbers of Bits vs. Frequency