14

LTC1287

1287fa

S

APPLICATI

I FOR ATIO

curve of Noise Error vs Reference Voltage shows the

LSB contribution of this 200

µV of noise.

For operation with a 2.5V reference, the 200

µV noise is

only 0.32LSB peak-to-peak. Here the LTC1287 noise

will contribute virtually no uncertainty to the output

code. For reduced references, the noise may become a

significant fraction of an LSB and cause undesirable

jitter in the output code. For example, with a 1.25V

reference, this 200

µV noise is 0.64LSB peak-to-peak.

This will reduce the range of input voltages over which

a stable output code can be achieved by 0.64LSB. Now

averaging readings may be necessary.

This noise data was taken in a very clean test fixture.

reference voltage used, the more critical it becomes to

have a noise-free setup.

Overvoltage Protection

Applying signals to the LTC1287’s analog inputs that

exceed the positive supply or that go below ground 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 LTC1287. Another example is the input

source operating from different supplies of larger value

than the LTC1287. These conditions should be pre-

vented 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

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 200

Ω when clocked at its maximum

clock frequency of 500kHz. If the LTC1287 is clocked at

the maximum clock frequency and 200

Ω is not enough

to current limit the input source then the clamp diodes

are recommended (Figures 16 and 17). The reason for

the limit on the resistor value is 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 curve of Maximum CLK

Frequency vs Source Resistance).

mended (see Figure 18).

Because a unique input protection structure is used on

the digital input pins, the signal levels on these pins can

Figure 17. Overvoltage Protection for Inputs

+3V

LTC1287 F17

+IN

GND

CLK

CS

–IN

LTC1287

1N4148 DIODES

1k

Figure 16. Overvoltage Protection for Inputs

+3V

LTC1287 F16

+IN

GND

CLK

CS

–IN

200

Ω

1k

LTC1287

Figure 15. Overvoltage Protection for Inputs

+3V

LTC1287 F15

+IN

GND

CLK

CS

–IN

1N4148 DIODES

LTC1287