Datasheet

13
LTC1609
1609fa
APPLICATIO S I FOR ATIO
WUUU
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
algorithm, the ADC’s spectral content can be examined
for frequencies outside the fundamental. Figure 6 shows
a typical LTC1609 FFT plot which yields a SINAD of
87.2dB and THD of –100dB.
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 6 shows a typical SINAD of 87.2dB with
a 200kHz sampling rate and a 1kHz input.
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.
Internal Voltage Reference
The LTC1609 has an on-chip, temperature compensated,
curvature corrected, bandgap reference, which is factory
trimmed to 2.50V. The full-scale range of the ADC scales
with V
REF
. The output of the reference is connected to the
input of a unity-gain buffer through a 4k resistor (see
Figure 7). The input to the buffer or the output of the
reference is available at REF. The internal reference can be
overdriven with an external reference if more accuracy is
needed. The buffer output drives the internal DAC and is
available at CAP. The CAP pin can be used to drive a steady
DC load of less than 2mA. Driving an AC load is not
recommended because it can cause the performance of
the converter to degrade.
For minimum code transition noise the REF pin and the
CAP pin should each be decoupled with a capacitor to
filter wideband noise from the reference and the buffer
(2.2µF tantalum).
Figure 6. LTC1609 Nonaveraged 4096 Point FFT Plot
S
S
–
+
1609 F07
INTERNAL
CAPACITOR
DAC
BANDGAP
REFERENCE
V
ANA
4k
2.2µF
CAP
(2.5V)
2.2µF
REF
(2.5V)
6
7
Figure 7. Internal or External Reference Source
FREQUENCY (kHz)
0
–60
–40
0
75
1609 F06
–80
–100
25 50 100
–120
–130
–20
MAGNITUDE (dB)
f
SAMPLE
= 200kHz
f
IN
= 1kHz
SINAD = 87.2dB
THD = –100.1dB
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
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