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LTC1291

1291fa

APPLICATI

I FOR ATIO

Overvoltage Protection

Applying signals to the LTC1291’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 will occur if a signal is

applied to the analog inputs before power is applied to the

LTC1291. It can also happen if the input source is operat-

ing from supplies of larger value than the LTC1291 supply.

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 13

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 LTC1291 is 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 14 and 15). 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 Maxi-

mum CLK Rate vs Source Resistance).

Because a unique input protection structure is used on the

digital input pins, the signal levels on these pins can

exceed the device VCC without damaging the device.

LTC1291 F13

CH0

GND

DIN

DOUT

CLK

VCC (VREF)

CH1

1N4148 DIODES

LTC1291

LTC1291 F14

CH0

GND

DIN

DOUT

CLK

CH1

250

Ω

LTC1291

VCC (VREF)

LTC1291 F15

CH0

GND

DIN

DOUT

CLK

CH1

LTC1291

1N4148 DIODES

VCC (VREF)

Figure 13. Overvoltage Protection for Inputs

Figure 14. Overvoltage Protection for Inputs

Figure 15. Overvoltage Protection for Inputs

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