LTC2379-18
APPLICATIONS INFORMATION
0μA to a maximum of 1.3mA at 1.6Msps. This step in DC
currentdrawtriggersatransientresponseinthereference
that must be considered since any deviation in the refer-
ence output voltage will affect the accuracy of the output
code. ꢁn applications where the transient response of the
reference is important, the fast settling LTC6655-5 refer-
ence is also recommended.
Signal-to-Noise Ratio ꢂSNRx
The signal-to-noise ratio (SIR) 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 SIR of 101.2dꢀ
at a 1.6MHz sampling rate with a 2kHz input.
DYNAMIC PERFORMANCE
Total Harmonic Distortion ꢂTHDx
Fast Fourier Transform (FFT) techniques are used to test
the ADC’s frequency response, distortion and noise at the
rated throughput. ꢀy 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.
TotalHarmonicDistortion(THD)istheratiooftheRMSsum
ofallharmonicsoftheinputsignaltothefundamentalitself.
The out-of-band harmonics alias into the frequency band
between DC and half the sampling frequency (f
THD is expressed as:
/2).
SMPL
V22 + V32 + V42 +…+ VI2
THD=20log
V1
Signal-to-Noise and Distortion Ratio ꢂSINADx
where V1 is the RMS amplitude of the fundamental fre-
quencyandV2throughV aretheamplitudesofthesecond
I
The signal-to-noise and distortion ratio (SꢁIAD) is the
ratiobetweentheRMSamplitudeofthefundamentalinput
frequency and the RMS amplitude of all other frequency
components at the A/D output. The output is band-limited
tofrequenciesfromaboveDCandbelowhalfthesampling
frequency. Figure 11 shows that the LTC2379-18 achieves
a typical SꢁIAD of 101dꢀ at a 1.6MHz sampling rate with
a 2kHz input.
through Ith harmonics.
POWER CONSIDERATIONS
The LTC2379-18 provides two power supply pins: the
2.5V power supply (V ), and the digital input/output
DD
interface power supply (OV ). The flexible OV supply
DD
DD
allows the LTC2379-18 to communicate with any digital
logic operating between 1.8V and 5V, including 2.5V and
3.3V systems.
0
SIR = 101.2dꢀ
–20
–40
THD = –120dꢀ
SꢁIAD = 101.1dꢀ
SFDR = 121dꢀ
Power Supply Sequencing
–60
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
–80
–100
–120
–140
–160
–180
0
100 200 300 400 500 600 700 800
FREQUEICY (kHz)
237918 F11
Figure 11. 32k Point FFT with fIN = 2kHz of the ꢀTC2379-18
237918fa
14
Linear Systems [ Linear Systems ]
