Datasheet

LTC2381-16
13
238116f
APPLICATIONS INFORMATION
The REF pin of the LTC2381-16 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 LTC2381-16 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 LTC2381-16 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 285µA at 250ksps. 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 LTC2381-16 provides
guaranteed tested limits for both AC distortion and noise
measurements.
Figure 10. 32k Point FFT of the LTC2381-16
FREQUENCY (kHz)
0 25 50 75 100 125
–180
AMPLITUDE (dBFS)
–60
–40
–20
–80
–100
–120
–140
–160
0
238116 F10
SNR = 91.8dB
THD = –106dB
SINAD = 91.6dB
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 LTC2381-16 achieves
a typical SINAD of 92dB at a 250kHz sampling rate with
a 20kHz input.
CNV
IDLE
PERIOD
IDLE
PERIOD
238116 F08
238116 F09
LTC2381-16
1
47µF
LTC6655-2.5
REF
V
OUT_S
V
OUT_F
Figure 8. CNV Waveform Showing Burst Sampling
Figure 9. LTC6655-2.5 Driving REF of LTC2381-16