LTC1292DI datasheet(20/24 Pages) LINER

20 / 24 page

LTC1292/LTC1297

APPLICATI

I FOR ATIO

Figure 20a. fIN = 1kHz, fS = 58.8kHz, SNR = 73.0dB

Figure 20b. fIN = 28kHz, fS = 58.8kHz, SNR = 61.5dB

Figure 21. LTC1292 ENOB vs Input Frequency

Figure 22. fIN1 = 5.1kHz, fIN2 = 5.6kHz, fS = 58.8kHz

Overvoltage Protection

Applying signals to the LTC1292/LTC1297’s analog

inputs that exceed the positive supply or that go below

ground will degrade the accuracy of the A/D and possi-

bly damage the devices. For example this condition

would occur if a signal is applied to the analog inputs

before power is applied to the LTC1292/LTC1297. An-

other example is the input source is operating from

different supplies of larger value than the LTC1292/

LTC1297. These conditions should be prevented either

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 23

diode clamps from the inputs to VCC and GND are used.

The second method is to put resistors in series with the

analog inputs for current limiting. Limit the current to

15mA per channel. The +IN input can accept a resistor

value of 1k but the –IN input cannot accept more than

250

Ω when clocked at its maximum clock frequency of

1MHz. If the LTC1292/LTC1297 are clocked at the

maximum clock frequency and 250

Ω is not enough to

current limit the input source, then the clamp diodes are

recommended (Figures 24a and 24b). The reason for

the limit on the resistor value is that the MSB bit test is

affected by the value of the resistor placed at the –IN

input (see discussion on Analog Inputs and the typical

performance characteristics Maximum CLK Frequency

vs Source Resistance).

FREQUENCY (kHz)

–60

–40

–20

LTC1292/7 F20a

–80

–100

510

–120

–140

FREQUENCY (kHz)

–60

–40

–20

LTC1292/7 F22

–80

–100

510

–120

–140

FREQUENCY (kHz)

–60

–40

–20

LTC1292/7 F20b

–80

–100

510

–120

–140

FREQUENCY (kHz)

9.5

10.0

10.5

100

LT1292/7 F21

9.0

8.5

8.0

11.0

11.5

12.0

fS = 58.8kHz

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