Datasheet

LTC2315-12
17
231512fa
For more information www.linear.com/LTC2315-12
Figure 13. 16k Point FFT of the LTC2315-12 at f
IN
= 500 kHz
Figure 14. LTC2315-12 ENOB/SINAD vs f
IN
Figure 15. LTC2315-12 IMD Plot
231512 F13
FREQUENCY (kHz)
AMPLITUDE (dBFS)
0 25002000
1000500
1500
0
–20
–40
–60
–80
–100
–120
–140
V
DD
= 5V
SNR = 73.1dBFS
SINAD = 72.6dBFS
THD = –84dB
SFDR = 87dBc
INPUT FREQUENCY (kHz)
0
69
70
SINAD (dBFS)
ENOB
71
72
73
74
11.2
11.3
11.5
11.7
11.8
12.0
500 1000
1500 2000 2500
231512 F14
V
DD
= 5V
V
DD
= 3V
INPUT FREQUENCY (kHz)
0
–160
–120
–140
MAGNITUDE (dB)
–80
–100
–60
–40
0
–20
500 1000
1500 2000 2500
231512 F15
V
DD
= 5V
f
S
= 5Msps
f
a
= 461.421kHz
f
b
= 541.421kHz
IMD
2
(f
b
+ f
a
) = –77.4dBc
IMD
3
(2fb –f
a
) = –89.4dBc
APPLICATIONS INFORMATION
If two pure sine waves of frequencies f
a
and f
b
are ap-
plied to the ADC input, nonlinearities in the ADC transfer
function
can create distortion products at the sum and
difference frequencies m • f
a
± n • f
b
, where m and n = 0,
1, 2, 3, etc. For example, the 2nd order IMD terms include
(f
a
± f
b
). If the two input sine waves are equal in magnitude,
the value (in decibels) of the 2nd order IMD products can
be expressed by the following formula:
IMD(f
a
± f
b
) = 20 • log[V
A
(f
a
± f
b
)/V
A
(f
a
)]
The LTC2315-12 has excellent IMD as shown in Figure 15.
Spurious Free Dynamic Range (SFDR)
The spurious free dynamic range is the largest spectral
component excluding DC, the input signal and the harmon
-
ics included
in the THD. This value is expressed in decibels
relative to the RMS value of a full-scale input signal.
Full-Power and Full-Linear Bandwidth
The
full-power bandwidth is the input frequency at which
the amplitude of the reconstructed fundamental is reduced
by 3dB for a full-scale input signal.
The full-linear bandwidth is the input frequency at which
the SINAD has dropped to 68dB (11
effective bits).
The
LTC2315-12 has been designed to optimize the input
bandwidth, allowing the ADC to under-sample input signals
with frequencies above the converter’s Nyquist frequency.
The noise floor stays very low at high frequencies and
SINAD becomes dominated by distortion at frequencies
beyond Nyquist.
Recommended Layout
To obtain the best performance from the LTC2315-12 a
printed circuit board is required. Layout for the printed
circuit board (PCB) should ensure the digital and analog
signal lines are separated as much as possible. In particu
-
lar, care should be taken not to run any digital clocks or
signals alongside analog signals or underneath the ADC.
The following is an example of a recommended PCB layout.
A single solid ground plane is used. Bypass capacitors to
the supplies are placed as close as possible to the supply
pins. Low impedance common returns for these bypass
capacitors are essential to the low noise operation of the