Datasheet

20
LTC1197/LTC1197L
LTC1199/LTC1199L
Signal-to-Noise Ratio
T he signal-to-noise ratio (SNR) is the ratio between the
RMS amplitude of the fundamental input frequency to
the RMS amplitude of all other frequency components at
the A/D output. This includes distortion as well as noise
products and for this reason it is sometimes referred to
as signal-to-noise + distortion [S/(N + D)]. The output is
band limited to frequencies from DC to one half the
sampling frequency. Figure 9 shows spectral content
from DC to 250kHz which is 1/2 the 500kHz sampling
rate.
Effective Number of Bits
The effective number of bits (ENOBs) is a measurement of
the resolution of an ADC and is directly related to the
S/(N + D) by the equation:
ENOB = [S/(N + D) –1.76]/6.02
where S/(N + D) is expressed in dB. At the maximum
sampling rate of 500kHz, the LTC1197 maintains 9.5
ENOBs or better to 200kHz. Above 200kHz, the ENOBs
gradually decline, as shown in Figure 10, due to increasing
second harmonic distortion. The noise floor remains
approximately 100dB.
FREQUENCY (kHz)
0
AMPLITUDE (dB)
–40
–20
–30
–10
0
1197/99 G06
–60
–80
–50
–70
–90
–100
50
100 150 200 250
f
SMPL
= 500kHz
f
IN
= 97.045898kHz
Figure 9. This Clean FFT of a 97kHz Input Shows Remarkable
Performance for an ADC Sampling at the 500kHz Rate
FREQUENCY (kHz)
1
4
ENOBs
5
6
7
8
10 100 1000
1197/99 G07
3
2
1
0
9
10
V
CC
= 2.7V
f
SMPL
= 250kHz
V
CC
= 5V
f
SMPL
= 500kHz
Figure 10. Dynamic Accuracy is Maintained
Up to an Input Frequency of 200kHz for the
LTC1197 and 50kHz for the LTC1197L
APPLICATIO S I FOR ATIO
WUUU