LTC1605
13
1605fd
For more information www.linear.com/LTC1605
a read. Again it is recommended that both R/C and CS
return high within 3µs after the start of the conversion.
Output Data
The output data can be read as a 16-bit word or it can be
read as two 8-bit bytes. The format of the output data is
two’s complement. The digital input pin BYTE is used to
control the two byte read. With the BYTE pin low the first
eight MSBs are output on the D15 to D8 pins and the
eight LSBs are output on the D7 to D0 pins. When the
BYTE pin is taken high the eight LSBs replace the eight
MSBs (Figure 10).
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 FFT plot which yields a SINAD of 87.5dB
and THD of –102dB.
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 87.5dB 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
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, 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. Pay particular attention to the design
of the analog and digital ground planes. The DGND pin
of the LTC1605 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 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. 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.
APPLICATIONS INFORMATION
