LTC1605-1/LTC1605-2
13
160512fa
For more information www.linear.com/LTC1605-1
applicaTions inForMaTion
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
.
Figure 11 shows a
typical LTC1605-2 FFT plot which yields a SINAD of 87dB
and THD of –101.1dB.
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. Figure 11 shows a typical SINAD of 87dB with
a 100kHz 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 = 20log
V2
2
+ V3
2
+ V4
2
...+ V
N
2
FREQUENCY (kHz)
0
MAGNITUDE (dB)
5 10 15 20 25 30 35 40 45 50
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
–130
f
SAMPLE
= 100kHz
f
IN
= 1kHz
SINAD = 87dB
THD = 101.1dB
SNR = 87.2dB
Figure 11. LTC1605-2 Nonaveraged 4096-Point FFT Plot
where V1 is the RMS amplitude of the fundamental
frequency and V2 through V
N
are the amplitudes of the
second through Nth harmonics.
Board Layout, Power Supplies and Decoupling
Wire wrap boards are not recommended for high reso
-
lution or high speed A/D converters. To obtain the best
performance from the
LTC1605-1/LTC1605-2, 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.
Figures 12 through 15 show a layout for a suggested evalu
-
ation circuit which will help obtain the best performance
from the 16-bit ADC. Additional information regarding the
evaluation circuit and Gerber files for the PC board layout
are available from Linear Technology or your local sales
office. Pay particular attention to the design of the analog
and digital ground planes. The DGND pin of the LTC1605-1/
LTC1605-2 can 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
important. Low impedance common returns for these
bypass capacitors are essential to low noise operation of
the ADC, and the PC track width for these lines 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. The digital output latches and the
onboard sampling clock have been placed on the digital
ground plane. The two ground planes are tied together at
the power supply ground connection.
