Datasheet

LTC2379-18
14
237918fa
Figure 11. 32k Point FFT with f
IN
= 2kHz of the LTC2379-18
FREQUENCY (kHz)
0 100 200 300 400 800700600500
–180
AMPLITUDE (dBFS)
–60
–40
–20
–80
–100
–120
–140
–160
0
237918 F11
SNR = 101.2dB
THD = –120dB
SINAD = 101.1dB
SFDR = 121dB
APPLICATIONS INFORMATION
0µA to a maximum of 1.3mA at 1.6Msps. This step in DC
current draw triggers a transient response in the reference
that must be considered since any deviation in the refer-
ence output voltage will affect the accuracy of the output
code. In applications where the transient response of the
reference is important, the fast settling LTC6655-5 refer-
ence is also recommended.
DYNAMIC PERFORMANCE
Fast Fourier Transform (FFT) 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 frequen-
cies outside the fundamental. The LTC2379-18 provides
guaranteed tested limits for both AC distortion and noise
measurements.
Signal-to-Noise and Distortion Ratio (SINAD)
The signal-to-noise and distortion ratio (SINAD) is the
ratio between the RMS amplitude of the fundamental input
frequency and 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 that the LTC2379-18 achieves
a typical SINAD of 101dB at a 1.6MHz sampling rate with
a 2kHz input.
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 11 shows
that the LTC2379-18 achieves a typical SNR of 101.2dB
at a 1.6MHz sampling rate with a 2kHz input.
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 LTC2379-18 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 LTC2379-18 to communicate with any digital
logic operating between 1.8V and 5V, including 2.5V and
3.3V systems.
Power Supply Sequencing
The LTC2379-18 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 LTC2379-18
has a power-on-reset (POR) circuit that will reset the
LTC2379-18 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