Datasheet

LTC2378-20
15
237820f
For more information www.linear.com/LTC2378-20
Figure 13. 128k Point FFT Plot with f
IN
= 2kHz of the LTC2378-20
FREQUENCY (kHz)
0 100 200 300 500400
–180
AMPLITUDE (dBFS)
–60
–40
–20
–80
–100
–120
–140
–160
0
237820 F13
SNR = 104dB
THD = –128dB
SINAD = 104dB
SFDR = 132dB
Signal-to-Noise Ratio (SNR)
The signal-to-noise ratio (SNR) is the ratio between the
RMS amplitude of the fundamental input frequency and
the RMS amplitude of all other frequency components
except the first five harmonics and DC. Figure 13 shows
that the LTC2378-20 achieves a typical SNR of 104dB at
a 1MHz sampling rate with a 2kHz input.
applicaTions inForMaTion
Power Supply Sequencing
The LTC2378-20 does not have any specific power supply
sequencing requirements. Care should be taken to adhere
to the maximum voltage relationships described in the
Absolute Maximum Ratings section. The LTC2378-20
has a power-on-reset (POR) circuit that will reset the
LTC2378-20 at initial power-up or whenever the power
supply voltage drops below 1V. Once the supply voltage
re-enters the nominal supply voltage range, the POR will
reinitialize the ADC. No conversions should be initiated
until 200µs after a POR event to ensure the reinitialization
period has ended. Any conversions initiated before this
time will produce invalid results.
TIMING AND CONTROL
CNV Timing
The LTC2378-20 conversion is controlled by CNV. A ris-
ing edge on CNV will start a conversion and power up
the LTC2378-20. Once a conversion has been initiated,
it
cannot be restarted until the conversion is complete.
For optimum performance, CNV should be driven by a
clean low jitter signal. Converter status is indicated by the
BUSY output which remains high while the conversion is
in progress. To ensure that no errors occur in the digitized
results, any additional transitions on CNV should occur
within 40ns from the start of the conversion or after the
conversion has been completed. Once the conversion has
completed, the LTC2378-20 powers down and begins
acquiring the input signal.
Internal Conversion Clock
The LTC2378-20 has an internal clock that is trimmed to
achieve a maximum conversion time of 675ns. With a min-
imum acquisition time of 312ns, throughput performance
of 1Msps is guaranteed without any external adjustments.
Auto Power-Down
The LTC2378-20 automatically powers down after a
conversion has been completed and powers up once a
new conversion is initiated on the rising edge of CNV.
During power down, data from the last conversion can
be clocked out. To minimize power dissipation during
Total Harmonic Distortion (THD)
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 (f
SMPL
/2).
THD is expressed as:
THD= 20log
V2
2
+ V3
2
+ V4
2
+…+ V
N
2
V1
where V1 is the RMS amplitude of the fundamental fre-
quency and V2 through V
N
are the amplitudes of the second
through Nth harmonics.
POWER CONSIDERATIONS
The LTC2378-20 provides two power supply pins: the
2.5V power supply (V
DD
), and the digital input/output
interface power supply (OV
DD
). The flexible OV
DD
supply
allows the LTC2378-20 to communicate with any digital
logic operating between 1.8V and 5V, including 2.5V and
3.3V systems.