LTC1290BCSW datasheet(24/28 Pages) LINER

24 / 28 page

LTC1290

APPLICATI

I FOR ATIO

larger current limiting resistors. Use 1N4148 diode clamps

from the MUX inputs to VCC and V – if the value of the series

resistor will not allow the maximum clock speeds to be

used or if an unknown source is used to drive the LTC1290

MUX inputs.

How the various power supplies to the LTC1290 are

applied can also lead to overvoltage conditions. For single

supply operation (i.e., unipolar mode), if VCC and REF+ are

not tied together, then VCC should be turned on first, then

REF+. If this sequence cannot be met, connecting a diode

from REF+ to VCC is recommended (see Figure 21).

For dual supplies (bipolar mode) placing two Schottky

diodes from VCC and V– to ground (Figure 23) will prevent

power supply reversal from occuring when an input source

is applied to the analog MUX before power is applied to the

device. Power supply reversal occurs, for example, if the

input is pulled below V– then VCC will pull a diode drop

below ground which could cause the device not to power

up properly. Likewise, if the input is pulled above VCC then

V– will be pulled a diode drop above ground. If no inputs

are present on the MUX, the Schottky diodes are not

required if V– is applied first, then VCC.

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.

1290 F21

REF+

VCC

LTC1290

µF

1N4148

VREF

1290 F22

DGND

V –

AGND

VCC

LTC1290

–5V

0.1

µF

1N5817

Figure 21

Figure 22. Power Supply Reversal

APPLICATI

TYPICAL

A “Quick Look” Circuit for the LTC1290

Users can get a quick look at the function and timing of the

LTC1290 by using the following simple circuit. REF+ and

DIN are tied to VCC selecting a 5V input span, CH7 as a

single-ended input, unipolar mode, MSB-first format and

16-bit word length. ACLK and SCLK are tied together and

driven by an external clock. CS is driven at 1/128 the clock

rate by the CD4520 and DOUT outputs the data. All other

pins are tied to a ground plane. The output data from the

DOUT pin can be viewed on an oscilloscope which is set up

to trigger on the falling edge of CS.

A “Quick Look” Circuit for the LTC1290

1290 TA02

LTC1290

0.1

µF

CHO

CH1

CH2

CH3

CH4

CH5

CH6

CH7

COM

DGND

VIN

OSCILLOSCOPE

CD4520

CLK

RESET

VSS

VDD

RESET

CLK

CLOCK IN

2MHz MAX

VCC

ACLK

SCLK

DIN

DOUT

REF+

REF–

V–

AGND

f/128

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