LTC1293 datasheet(23/28 Pages) LINER

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LTC1293/LTC1294/LTC1296

APPLICATI

I FOR ATIO

FREQUENCY (kHz)

9.5

10.0

10.5

100

LT1293 F17

9.0

8.5

8.0

11.0

11.5

12.0

fS = 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 VCC 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 1k

Ω

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)

–40

–20

1293 F16a

–60

–80

–100

–120

–140

FREQUENCY (kHz)

–40

–20

1293 F16b

–60

–80

–100

–120

–140

FREQUENCY (kHz)

–40

–20

1293 F8

–60

–80

–100

–120

–140

Figure 16b. LTC1294 FFT Plot

fIN = 22kHz, fS = 45.4kHz,

SNR = 72.5dB with

±5V Supplies

Figure 16a. LTC1294 FFT Plot

fIN = 1kHz, fS = 45.4kHz,

SNR = 72.7dB with

±5V Supplies

Figure 17. LTC1294 ENOB vs Input Frequency

Figure 18. LTC1294 FFT Plot

fIN1 = 5.1kHz, fIN2 = 5.6kHz, fS = 45.4kHz

with

±5V Supplies

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