Datasheet

LTC2382-16
13
238216f
APPLICATIONS INFORMATION
The REF pin of the LTC2382-16 draws charge (Q
CONV
)
from the 47µF bypass capacitor during each conversion
cycle. The reference replenishes this charge with a DC
current, I
REF
= Q
CONV
/t
CYC
. The DC current draw of the
REF pin, I
REF
, depends on the sampling rate and output
code. If the LTC2382-16 is used to continuously sample
a signal at a constant rate, the LTC6652-2.5 will keep the
deviation of the reference voltage over the entire code
span to less than 0.5LSBs.
When idling, the REF pin on the LTC2382-16 draws only
a small leakage current (< 1µA). In applications where a
burst of samples is taken after idling for long periods as
shown in Figure 8, I
REF
quickly goes from approximately
0µA to a maximum of 495µA at 500ksps. This step in DC
current draw triggers a transient response in the reference
that must be considered since any deviation in the reference
output voltage will affect the accuracy of the output code. In
applications where the transient response of the reference
is important, the fast settling LTC6655-2.5 reference
is recommended. Inserting a 1 resistor between the
47µF bypass capacitor and reference output as shown in
Figure 9 helps to improve the transient settling time and
minimize the reference voltage deviation.
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 frequencies
outside the fundamental. The LTC2382-16 provides
guaranteed tested limits for both AC distortion and noise
measurements.
Figure 10. 32k Point FFT of the LTC2382-16
FREQUENCY (kHz)
0 50 100 150 200 250
–180
AMPLITUDE (dBFS)
–60
–40
–20
–80
–100
–120
–140
–160
0
238216 F10
SNR = 92.2dB
THD = –106dB
SINAD = 92dB
SFDR = 107dB
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 10 shows that the LTC2382-16 achieves
a typical SINAD of 92dB at a 500kHz sampling rate with
a 20kHz input.
CNV
IDLE
PERIOD
IDLE
PERIOD
238216 F08
Figure 8. CNV Waveform Showing Burst Sampling
238216 F09
LTC2382-16
1
47µF
LTC6655-2.5
V
OUT_S
V
OUT_F
Figure 9. LTC6655-2.5 Driving REF of LTC2382-16