AD7821

REV. B

–5–

TERMINOLOGY

LEAST SIGNIFICANT BIT (LSB)

An ADC with 8-bit resolution can resolve one part in 2

of full scale). For the AD7821 operating in either the unipolar

or bipolar input range with 5 V full scale, one LSB is 19.53 mV.

TOTAL UNADJUSTED ERROR

This is a comprehensive specification which includes relative

accuracy, offset error, and full-scale error.

SLEW RATE

Slew rate is the maximum allowable rate of change of input

signal such that the digital sample values are not in error.

TOTAL HARMONIC DISTORTION (THD)

Total harmonic distortion is the ratio of the square root of the

sum of the squares of the rms value of the harmonics to the rms

value of the fundamental. For the AD7821, total harmonic dis-

tortion is defined as

20

2

2

3

2

4

2

5

2

6

2

1

log

VVV

VV

V

dB

++

++

()





















INTERMODULATION DISTORTION

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

fb, any active device with nonlinearities will create 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 (2fa + fb), (2fa – fb), (fa + 2fb) and

(fa – 2fb). For the AD7821 intermodulation distortion is calcu-

lated separately for both the second and third order terms.

SIGNAL-TO-NOISE RATIO (SNR)

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

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

Noise is the rms sum of all nonfundamental signals (excluding

dc) up to half the sampling frequency. SNR is dependent on the

number of quantization levels used in the digitization process.

The theoretical SNR for a sine wave input is given by:

SNR

N

dB

=+

()

602

176

..

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

8-bit ADC, SNR = 50 dB.

PEAK HARMONIC OR SPURIOUS NOISE

Peak harmonic or spurious noise is the rms value of the largest

nonfundamental frequency (excluding dc) up to half the sam-

pling frequency to the rms value of the fundamental.