Datasheet
23
LTC1293/LTC1294/LTC1296
129346fs
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
FREQUENCY (kHz)
0
EFFECTIVE NUMBER OF BITS
9.5
10.0
10.5
60
100
LT1293 F17
9.0
8.5
8.0
20 40 80
11.0
11.5
12.0
f
S
= 45.4kHz
±5V SUPPLIES
+5V SUPPLY
For +5V supplies the ENOB decreases more rapidly. This
is due predominantly to the 2nd harmonic distortion term.
Figure 18 shows a FFT plot of the output spectrum for two
tones applied to the input of the A/D. Nonlinearities in the
A/D will cause distortion products at the sum and differ-
ence frequencies of the fundamentals and products of the
fundamentals. This is classically referred to as
intermodulation distortion (IMD).
Overvoltage Protection
Applying signals to the LTC1293/4/6’s analog inputs that
exceed the positive supply or that go below V
–
will
degrade the accuracy of the A/D and possibly damage the
device. For example this condition would occur if a signal
is applied to the analog inputs before power is applied to
the LTC1293/4/6. Another example is the input source is
operating from different supplies of larger value than the
LTC1293/4/6. These conditions should be prevented ei-
ther with proper supply sequencing or by use of external
circuitry to clamp or current limit the input source. There
are two ways to protect the inputs. In Figure 19 diode
clamps from the inputs to V
CC
and V
–
are used. The
second method is to put resistors in series with the analog
inputs for current limiting. As shown in Figure 20a, a 1kΩ
resistor is enough to stand off ±15V (15mA for only one
channel). If more than one channel exceeds the supplies
than the following guidelines can be used. Limit the
current to 7mA per channel and 28mA for all channels.
FREQUENCY (kHz)
0
MAGNITUDE (dB)
–40
–20
0
15
1293 F16a
–60
–80
20
–100
–120
–140
2510
5
FREQUENCY (kHz)
0
MAGNITUDE (dB)
–40
–20
0
15
1293 F16b
–60
–80
20
–100
–120
–140
2510
5
FREQUENCY (kHz)
0
MAGNITUDE (dB)
–40
–20
0
15
1293 F8
–60
–80
20
–100
–120
–140
2510
5
Figure 16b. LTC1294 FFT Plot
f
IN
= 22kHz, f
S
= 45.4kHz,
SNR = 72.5dB with ±5V Supplies
Figure 16a. LTC1294 FFT Plot
f
IN
= 1kHz, f
S
= 45.4kHz,
SNR = 72.7dB with ±5V Supplies
Figure 17. LTC1294 ENOB vs Input Frequency
Figure 18. LTC1294 FFT Plot
f
IN
1 = 5.1kHz, f
IN
2 = 5.6kHz, f
S
= 45.4kHz
with ±5V Supplies