ADS820 datasheet(14/21 Pages) TI1

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ADS820

SBAS037B

PC-BOARD LAYOUT AND BYPASSING

A well-designed, clean PC-board layout will assure proper

operation and clean spectral response. Proper grounding

and bypassing, short lead lengths, and the use of ground

planes are particularly important for high frequency circuits.

Multilayer PC boards are recommended for best perfor-

mance but if carefully designed, a two-sided pc board with

large, heavy ground planes can give excellent results. It is

recommended that the analog and digital ground pins of the

ADS820 be connected directly to the analog ground plane. In

our experience, this gives the most consistent results. The

A/D converter power-supply commons should be tied to-

gether at the analog ground plane. Power supplies should be

bypassed with 0.1

µF ceramic capacitors as close to the pin

as possible.

DYNAMIC PERFORMANCE TESTING

The ADS820 is a high-performance converter and careful

attention to test techniques is necessary to achieve accurate

results. Highly accurate phase-locked signal sources allow

high-resolution FFT measurements to be made without using

data windowing functions. A low jitter signal generator such

as the HP8644A for the test signal, phase-locked with a low

jitter HP8022A pulse generator for the A/D converter clock,

gives excellent results. Low-pass filtering (or bandpass filter-

ing) of test signals is absolutely necessary to test the low

distortion of the ADS820. Using a signal amplitude slightly

lower than full scale will allow a small amount of “headroom”

so that noise or DC-offset voltage will not overrange the

A/D converter and cause clipping on signal peaks.

DYNAMIC PERFORMANCE DEFINITIONS

1. Signal-to-Noise-and-Distortion Ratio (SINAD):

log

Sinewave Signal Power

Noise

Harmonic Power first

harmonics

(

)

2. Signal-to-Noise Ratio (SNR):

10 log

Sinewave Signal Power

Noise Power

3. Intermodulation Distortion (IMD):

log

Highest IMD

oduct Power to th order

Sinewave Signal Power

(

)

IMD is referenced to the larger of the test signals f1 or f2. Five

“bins” either side of peak are used for calculation of funda-

mental and harmonic power. The “0” frequency bin (DC) is

not included in these calculations, as it is of little importance

in dynamic signal processing applications.

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