LTC1289BCSW datasheet(22/28 Pages) LINER

22 / 28 page

LTC1289

1289fb

APPLICATI

I FOR ATIO

Figure 19 shows an 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).

8. Overvoltage Protection

Applying signals to the analog MUX that exceed the

positive or negative supply of the device 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 MUX before power is applied to the LTC1289.

Another example is the input source is operating from

different supplies of larger value than the LTC1289. These

conditions should be prevented either with proper supply

sequencing or by use of external circuitry to clamp or

current limit the input source. As shown in Figure 20, a

Ω resistor is enough to stand off ±15V (15mA for one

only 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.

This means four channels can handle 7mA of input current

each. Reducing the ACLK and SCLK frequencies from the

maximum of 2MHz and 1MHz, respectively (see Typical

Peformance Characteristics curves Maximum ACLK Fre-

quency vs Source Resistance and Sample and Hold Acqui-

sition Time vs Source Resistance) allows the use of 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 LTC1289

MUX inputs.

How the various power supplies to the LTC1289 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 22) 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.

3.3V

LTC1289 AIF20

DGND

V–

AGND

VCC

LTC1289

CH0

VIN

–3.3V

0.1

µF

Figure 20. Overvoltage Protection for MUX

3.3V

LTC1289 AIF21

REF+

VCC

LTC1289

µF

1N4148

VREF

Figure 21.

3.3V

LTC1289 AIF22

DGND

V –

AGND

VCC

LTC1289

–3.3V

0.1

µF

1N5817

Figure 22. Power Supply Reversal

LTC1289BCSW Datenblatt(PDF) 22 Page - Linear Technology

LTC1289BCSW Datenblatt(HTML) 22 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datenblatt Download

Link URL