LTC1289 Linear Technology, LTC1289 Datasheet - Page 21

• Manufacturer Part Number: LTC1289
• Description: 3 Volt Single Chip 12-Bit Data Acquisition System
• Manufacturer: Linear Technology

7. LTC1289 AC Characteristics
Two commonly used figures of merit for specifying the
dynamic performance of the A/D’s in digital signal pro-
cessing applications are the Signal-to-Noise Ratio (SNR)
and the “effective number of bits (ENOB).” SNR is defined
as the ratio of the RMS magnitude of the fundamental to
the RMS magnitude of all the nonfundamental signals up
to the Nyquist frequency (half the sampling frequency).
The theoretical maximum SNR for a sine wave input is
given by:
where N is the number of bits. Thus the SNR is a function
of the resolution of the A/D. For an ideal 12-bit A/D the SNR
is equal to 74dB. A Fast Fourier Transform(FFT) plot of the
A
PPLICATI
SNR = (6.02N + 1.76dB)
Figure 17b. f
Figure 17a. f
–100
–140
–120
–100
–140
–120
–20
–40
–60
–80
–20
–60
–80
–40
0
0
0
0
2
O
2
IN
IN
= 12kHz, f
U
= 1kHz, f
4
4
FREQUENCY (kHz)
S
FREQUENCY (kHz)
6
6
I FOR ATIO
U
S
S
= 25kHz, SNR = 72.92dB
8
= 25kHz, SNR = 72.23dB
8
10
10
W
LTC1289 F17a
12
LTC1289 F17b
12
14
14
U
output spectrum of the LTC1289 is shown in Figures 17a
and 17b. The input (f
with the sampling frequency (f
obtained from the plot are 72.92dB and 72.23dB.
Rewriting the SNR expression it is possible to obtain the
equivalent resolution based on the SNR measurement.
This is the so-called effective number of bits (ENOB). For
the example shown in Figures 17a and 17b, N = 11.8 bits
and 11.7 bits, respectively. Figure 18 shows a plot of ENOB
as a function of input frequency. The curve shows the A/
D’s ENOB remain in the range of 11.8 to 11.7 for input
frequencies up to f
N =
Figure 19. f
SNR – 1.76dB
Figure 18. LTC1289 ENOB vs Input Frequency
–140
–100
–120
–20
–40
–60
–80
12
11
10
9
8
7
6
0
6.02
0
0
IN
2
10
1 = 2.6kHz, f
S
/2
IN
4
FREQUENCY (kHz)
FREQUENCY (kHz)
) frequencies are 1kHz and 12kHz
20
6
IN
8
30
2 = 3.1kHz, f
S
10
) at 25kHz. The SNR
F
S
40
= 25kHz
LTC1289 • AIF18
12
LTC1289 F19
LTC1289
S
14
50
= 25kHz
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