AD7776/AD7777/AD7778

–10–

REV. A

Short Circuit Current

This is defined as the maximum current which flows either

into or out of the REFOUT pin if this pin is shorted to any

for up to 10 seconds provided that the power dissipation of the

package is not exceeded.

Signal-to-Noise and Distortion Ratio, S/(N+D)

Signal-to-noise and distortion ratio, S/(N+D), is the ratio of the

rms value of the measured input signal to the rms sum of all

other spectral components below the Nyquist frequency, includ-

ing harmonics, but excluding dc. The value for S/(N+D) is

given in decibels.

Total Harmonic Distortion, THD

Total harmonic distortion is the ratio of the rms sum of the first

five harmonic components to the rms value of a full-scale input

signal and is expressed in decibels. For the AD7776/AD7777/

AD7778, total harmonic distortion (THD) is defined as:

20

2

2

3

2

4

2

5

2

6

2

12

1

log

VVV

VV

V

++

++

()

harmonics.

Intermodulation Distortion, IMD

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

any active device with nonlinearities creates distortion products,

of order (m + n), at sum and difference frequencies of mfa + nfb,

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

m or n is not equal to zero. For example, the second order terms

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

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

Channel-to-Channel Isolation

Channel-to-channel isolation is a measure of the level of cross-talk

between channels. It is measured by applying a full-scale 100 kHz

sine wave signal to any one of the input channels and monitoring

the remaining channels. The figure given is the worst case across

all channels.

DIGITAL SIGNAL PROCESSING APPLICATIONS

In digital signal processing (DSP) application areas like voice

recognition, echo cancellation, and adaptive filtering, the dynamic

characteristics S/(N+D), THD, and IMD of the ADC are critical.

The AD7776/AD7777/AD7778 are specified dynamically as well

as with standard dc specifications. Because the track/hold

amplifier has a wide bandwidth, an antialiasing filter should be

placed on the analog inputs to avoid aliasing high frequency noise

back into the bands of interest.

The dynamic performance of the ADC is evaluated by applying a

sine wave signal of very low distortion to a single analog input

which is sampled at 380.95 kHz. A fast Fourier transform (FFT)

plot or histogram plot is then generated from which the signal to

noise and distortion, harmonic distortion, and dynamic differential

nonlinearity data can be obtained. Similarly, for intermodulation

distortion, an input signal consisting of two pure sine waves at

different frequencies is applied to the AD7776/AD7777/AD7778.

Figure 11 shows a 2048-point FFT plot for a single channel of

the AD7778 with an input signal of 99.88 kHz. The SNR is

58.71 dB. It can be seen that most of the harmonics are buried

in the noise floor. It should be noted that the harmonics are

taken into account when calculating the S/(N+D).

0

–90

–80

0

–60

–40

–20

99.88

FREQUENCY – kHz

INPUT FREQUENCY =

99.88kHz

SAMPLE FREQUENCY =

380.95kHz

SNR = 58.7dB

Figure 11. ADC FFT Plot

The relationship between S/(N+D) and resolution (n) is ex-

pressed by the following equation:

SN

D

n

dB

+

()

602

176

..

This is for an ideal part with no differential or integral linearity

errors. These errors cause a degradation in S/(N+D). By work-

ing backwards from the above equation, it is possible to get a

measure of ADC performance expressed in effective number

of bits (n).

n effective

SN

D

dB

+

602

.

.

The effective number of bits plotted versus frequency for a

single channel of the AD7778 is shown in Figure 12. The effec-

tive number of bits is typically 9.5.

10.0

7.5

189.2

9.0

8.0

8.5

0

9.5

INPUT FREQUENCY – kHz

SAMPLE FREQUENCY = 378.4kHz

Figure 12. Effective Number of Bits vs. Frequency