13

LTC1606

1606fa

Dynamic Performance

FFT (Fast Fourier Transform) test techniques are used to

test the ADC’s frequency response, distortion and noise at

the rated throughput. By applying a low distortion sine

wave and analyzing the digital output using an FFT algo-

rithm, the ADC’s spectral content can be examined for

frequencies outside the fundamental.

Signal-to-Noise Ratio

The Signal-to-Noise and Distortion Ratio (SINAD) is the

ratio between the RMS amplitude of the fundamental input

frequency to the RMS amplitude of all other frequency

components at the A/D output. The output is band limited

to frequencies from above DC and below half the sampling

frequency. A typical LTC1606 has a SINAD of 90dB and

THD of –100dB with a 250kHz sampling rate and a 1kHz

input.

Total Harmonic Distortion

Total Harmonic Distortion (THD) is the ratio of the RMS

sum of all harmonics of the input signal to the fundamental

itself. The out-of-band harmonics alias into the frequency

band between DC and half the sampling frequency. THD is

expressed as:

THD

VVV

V

V

N

=

++

+

20

2

2

3

2

4

22

1

log

...

second through Nth harmonics.

Board Layout, Power Supplies and Decoupling

Wire wrap boards and molded sockets are not recom-

mended for high resolution or high speed A/D converters.

To obtain the best performance from the LTC1606, a

printed circuit board is required. Layout for the printed

circuit board should ensure the digital and analog signal

lines are separated as much as possible. In particular, care

should be taken not to run any digital track alongside an

analog signal track or underneath the ADC. The analog

input should be screened by AGND.

Pay particular attention to the design of the analog and

digital ground planes. The DGND pin of the LTC1606

should be tied to the analog ground plane. Placing the

bypass capacitor as close as possible to the power supply,

the reference and reference buffer output is very impor-

tant. Low impedance common returns for these bypass

capacitors are essential to low noise operation of the ADC,

and the foil width for these tracks should be as wide as

possible. Also, since any potential difference in grounds

between the signal source and ADC appears as an error

voltage in series with the input signal, attention should be

paid to reducing the ground circuit impedance as much as

possible.

APPLICATIO S I FOR ATIO