AD7823

–5–

REV. B

TERMINOLOGY

Signal to (Noise + Distortion) Ratio

This is the measured ratio of signal to (noise + distortion) at the

output of the A/D converter. The signal is the rms amplitude of

the fundamental. Noise is the rms sum of all nonfundamental

signals up to half the sampling frequency (f

The ratio is dependent upon the number of quantization levels

in the digitization process; the more levels, the smaller the

quantization noise. The theoretical signal to (noise + distortion)

ratio for an ideal N-bit converter with a sine wave input is given

by:

Signal to (Noise + Distortion) = (6.02N + 1.76) dB

Thus for an 8-bit converter, this is 50 dB.

Total Harmonic Distortion

Total harmonic distortion (THD) is the ratio of the rms sum of

harmonics to the fundamental. For the AD7823 it is defined as:

THD (dB)

= 20 log

3

4

5

6

2

sixth harmonics.

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

fundamental. Normally, the value of this specification is

determined by the largest harmonic in the spectrum, but for

parts where the harmonics are buried in the noise floor, it will

be a noise peak.

Intermodulation Distortion

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 order terms include (fa + fb) and (fa – fb), while the

third order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and

(fa – 2fb).

The AD7823 is tested using the CCIF standard where two

input frequencies near the top end of the input bandwidth are

used. In this case, 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 intermodulation

distortion is as 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.

Relative Accuracy

Relative accuracy or endpoint nonlinearity is the maximum

deviation from a straight line passing through the endpoints of

the ADC transfer function.

Differential Nonlinearity

This is the difference between the measured and the ideal

1 LSB change between any two adjacent codes in the ADC.

Offset Error

This is the deviation of the first code transition (0000 . . . 000)

to (0000 . . . 001) from the ideal, i.e., AGND + 1 LSB.

Gain Error

This is the deviation of the last code transition (1111 . . . 110)

offset error has been adjusted out.

Track/Hold Acquisition Time

Track/hold acquisition time is the time required for the output

of the track/hold amplifier to reach its final value, within

±1/2 LSB, after the end of conversion (the point at which the

track/hold returns to track mode). It also applies to situations

where there is a step input change on the input voltage applied

wait for the duration of the track/hold acquisition time, after the

starting another conversion to ensure that the part operates to

specification.

Typical Performance Characteristics

THROUGHPUT – kSPS

10

0

0.01

030

10

0.1

20

40

50

Figure 2. Power vs. Throughput

FREQUENCY BINS

0

–70

–100

–10

–60

–80

–90

–30

–50

–20

–40

AD7823

2048 POINT FFT

SAMPLING 136.054

Figure 3. AD7823 SNR