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
...
where V
1
is the RMS amplitude of the fundamental fre-
quency and V
2
through V
N
are the amplitudes of the
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
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