AD7884/AD7885

REV. C

–7–

TERMINOLOGY

Integral Nonlinearity

This 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.

Bipolar Zero Error

This is the deviation of the midscale transition (all 0s to all 1s)

from the ideal (AGND).

Positive Gain Error

This is the deviation of the last code transition (01 . . . 110 to

Zero Error has been adjusted out.

Negative Gain Error

This is the deviation of the first code transition (10 . . . 000 to

Zero Error has been adjusted out.

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

The ratio is dependent upon the number of quantization levels

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

tization 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 ideal 16-bit converter, this is 98 dB.

Total Harmonic Distortion

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

harmonics to the fundamental. For the AD7884/AD7885, it is

defined as:

THD (dB)

= 20 log

V

2

2

+V

3

2

+V

4

2

+V

5

2

+V

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 deter-

mined 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 or 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 AD7884/AD7885 is tested using the CCIFF standard

where two input frequencies near the top end of the input band-

width 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 dis-

tortion is as per the THD specification where it is the ratio of

the rms sum of the individual distortion products to the rms am-

plitude of the fundamental expressed in dBs.

Power Supply Rejection Ratio

This is the ratio, in dBs, of the change in positive gain error to

OPERATIONAL DIAGRAM

An operational diagram for the AD7884/AD7885 is shown in

Figure 6. It is set up for an analog input range of

±5 V. If a

±3 V input range is required, A1 should drive ±3 V

±3 V

±5 V

±5 V

–5V

+5V

AD817

AD711

AD817

AGNDS

AGNDF

AD7884

AD7885

A1

A3

A4

AD845, AD817 OR

EQUIVALENT

NOTE: POWER SUPPLY DECOUPLING NOT SHOWN

A2

GND

DGND

DATA

OUTPUTS

CONTROL

INPUTS

AD845, AD817 OR

EQUIVALENT

AD780

2

6

8

4

10µF

Figure 6. AD7884/AD7885 Operational Diagram

The chosen input buffer amplifier (A1) should have low noise

and distortion and fast settling time for high bandwidth applica-

tions. Both the AD711 and the AD845 are suitable amplifiers.

A2 is the force, sense amplifier for AGND. The AGNDS pin

should be at zero potential. Therefore, the amplifier must have a

low input offset voltage and good noise performance. It must

also have the ability to deal with fast current transients on the

AGNDS pin. The AD817 has the required performance and is

the recommended amplifier.

If AGNDS and AGNDF are simply tied together to Star

Ground instead of buffering, the SNR and THD are not signifi-

cantly degraded. However, dc specifications like INL, Bipolar

Zero and Gain Error will be degraded.