LTC2445 datasheet(26/28 Pages) LINER

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LTC2444/LTC2445/

LTC2448/LTC2449

sn2444589 2444589fs

Example:

If an amplifier (e.g. LT1219) driving the input of an

LTC2444/LTC2445/LTC2448/LTC2449 has wideband

noise of 33nV/√Hz, band-limited to 1.8MHz, the total

noise entering the ADC input is:

33nV/√Hz • √1.8MHz = 44.3µV.

When the ADC digitizes the input, its digital filter filters out

the wideband noise from the input signal. The noise

reduction depends on the oversample ratio which defines

the effective bandwidth of the digital filter.

At an oversample of 256, the noise bandwidth of the ADC

is 787Hz which reduces the total amplifier noise to:

33nV/√Hz • √787Hz = 0.93µV.

The total noise is the RMS sum of this noise with the 2µV

noise of the ADC at OSR=256.

√(0.93µV)2 + (2uV)2 = 2.2µV.

Increasing the oversample ratio to 32768 reduces the

noise bandwidth of the ADC to 6.2Hz which reduces the

total amplifier noise to:

33nV/√Hz • √6.2Hz = 82nV.

The total noise is the RMS sum of this noise with the 200nV

noise of the ADC at OSR = 32768.

√(82nV)2 + (200nV)2 = 216nV.

In this way, the digital filter with its variable oversampling

ratio can greatly reduce the effects of external noise

sources.

APPLICATIO S I FOR ATIO

Automatic Offset Calibration of External

Buffers/Amplifiers

The LTC2445/LTC2449 enable an external amplifier to be

inserted between the multiplexer output and the ADC

input. This enables one external buffer/amplifier circuit to

be shared between all 17 analog inputs (16 single-ended

or 8 differential). The LTC2445/LTC2449 perform an inter-

nal offset calibration every conversion cycle in order to

remove the offset and drift of the ADC. This calibration is

performed through a combination of front end switching

and digital processing. Since the external amplifier is

placed between the multiplexer and the ADC, it is inside the

correction loop. This results in automatic offset correction

and offset drift removal of the external amplifier.

The LT1368 is an excellent amplifier for this function. It

has rail-to-rail inputs and outputs, and it operates on a

single 5V supply. Its open-loop gain is 1M and its input

bias current is 10nA. It also requires at least a 0.1µF load

capacitor for compensation. It is this feature that sets it

apart from other amplifiers—the load capacitor attenu-

ates sampling glitches from the LTC2445/LTC2449 ADCIN

terminals, allowing it to achieve full performance of the

ADC with high impedance at the multiplexer inputs.

Another benefit of the LT1368 is that it can be powered

from supplies equal to or greater than that of the ADC. This

can allow the inputs to span the entire absolute maximum

of GND – 0.3V to VCC + 0.3V. Using a positive supply of

7.5V to 10V and a negative supply of –2.5 to –5V gives the

amplifier plenty of headroom over the LTC2445/LTC2449

input range.

Figure 16. Simple External Clock Source

24448 F17

1,4,5,6,31,32,33

REFERENCE

VOLTAGE

0.1V TO VCC

ANALOG INPUT

–0.5VREF TO

0.5VREF

3-WIRE

SPI INTERFACE

4.5V TO 5.5V

VCC

BUSY

REF+

SCK

CH0

CH1

SDO

GND

EXT

LTC2448

REF–

1µF

0.1µF

LTC1799

OUT

DIV

SET

GND

V+

RSET

•

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