Datasheet
LTC2369-18
12
236918fa
Table 1. SNR, THD vs R
IN
for ±10V Input Signal
R
IN
(Ω)
R1
(Ω)
R2
(Ω)
R3
(Ω)
R4
(Ω)
SNR
(dB)
THD
(dB)
2k 499 499 2k 402 96.1 –97.3
10k 2.49k 2.49k 10k 2k 96 –92
100k 24.9k 24.9k 100k 20k 93.8 –93.5
APPLICATIONS INFORMATION
ADC REFERENCE
The LTC2369-18 requires an external reference to define
its input range. A low noise, low temperature drift refer-
ence is critical to achieving the full datasheet performance
of the ADC. Linear Technology offers a portfolio of high
performance references designed to meet the needs of
many applications. With its small size, low power and
high accuracy, the LTC6655-5 is particularly well suited for
use with the LTC2369-18. The LTC6655-5 offers 0.025%
(max) initial accuracy and 2ppm/°C (max) temperature
coefficient for high precision applications. The LTC6655-5
is fully specified over the H-grade temperature range and
complements the extended temperature operation of the
LTC2369-18 up to 125°C. We recommend bypassing the
LTC6655-5 with a 47μF ceramic capacitor (X5R, 0805 size)
close to the REF pin.
The REF pin of the LTC2369-18 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 LTC2369-18 is used to continuously sample a signal
at a constant rate, the LTC6655-5 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 LTC2369-18 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 6, I
REF
quickly goes from approximately
0μA to a maximum of 1.1mA 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
Figure 6. CNV Waveform Showing Burst Sampling
Figure 5a. LT6202 Converting a ±10V Bipolar Signal
to a 0V to 5V Input Signal
Figure 5b. 32k Point FFT Plot with f
IN
= 2kHz
for Circuit Shown in Figure 5a
–
+
1
3
4
LT6202
R4
402Ω
R1
499Ω
R
IN
2k
R3
2k
R2
499Ω
10μF
10V
–10V
0V
200pF
200pF
V
CM
= V
REF
/2
5V
0V
236918 F05a
FREQUENCY (kHz)
1
AMPLITUDE (dBFS)
–80
–60
–40
800
236918 F05b
–100
–120
–160
200
400
600
100
300
500
700
–140
0
–20
SNR = 96.1dB
THD = –97.3dB
SINAD = 92.7dB
SFDR = 97.5dB
CNV
IDLE
PERIOD
IDLE
PERIOD
236918 F06