LTC1853_15 datasheet(11/24 Pages) LINER

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LTC1852/LTC1853

18523fa

If two pure sine waves of frequencies fa and fb are applied

to the ADC input, nonlinearities in the ADC transfer function

can create distortion products at the sum and difference

frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc.

For example, the 2nd order IMD terms include (fa ± fb).

If the two input sine waves are equal in magnitude, the

value (in decibels) of the 2nd order IMD products can be

expressed by the following formula:

IMD fa ± fb

()=20Log

Amplitude at fa ± fb

()

Amplitude at fa

Peak Harmonic or Spurious Noise

The peak harmonic or spurious noise is the largest spectral

component excluding the input signal and DC. This value

is expressed in decibels relative to the RMS value of a

full-scale input signal.

Full-Power and Full-Linear Bandwidth

The full-power bandwidth is that input frequency at which

the amplitude of the reconstructed fundamental is reduced

by 3dB for a full-scale input signal.

The full-linear bandwidth is the input frequency at which

the S/(N + D) has dropped to 68dB for the LTC1853 (11

effective bits) or 56dB for the LTC1852 (9 effective bits).

The LTC1852/LTC1853 have been designed to optimize

input bandwidth, allowing the ADC to undersample input

signals with frequencies above the converter’s Nyquist fre-

quency. The noise ﬂoor stays very low at high frequencies;

S/(N + D) becomes dominated by distortion at frequencies

far beyond Nyquist.

ANALOG INPUT MULTIPLEXER

The analog input multiplexer is controlled using the

single-ended/differential pin (DIFF), three MUX address

pins (A2, A1, A0), the unipolar/bipolar pin (UNI/BIP) and

the gain select pin (PGA). The single-ended/differential

pin (DIFF) allows the user to conﬁgure the MUX as eight

single-ended channels relative to the analog input com-

mon pin (COM) when DIFF is low or as four differential

pairs (CH0 and CH1, CH2 and CH3, CH4 and CH5, CH6

and CH7) when DIFF is high. The channels (and polarity in

the differential case) are selected using the MUX address

inputs as shown in Table 1. Unused inputs (including

the COM in the differential case) should be grounded to

prevent noise coupling.

Table 1. Multiplexer Address Table

MUX ADDRESS

SINGLE-ENDED CHANNEL SELECTION

DIFF A2

A0 CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM

–

MUX ADDRESS

DIFFERENTIAL CHANNEL SELECTION

DIFF A2

A0 CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM

–*

–

–*

–

–*

–

–*

–

*Not used in differential mode. Connect to AGND.

In addition to selecting the MUX channel, the LTC1852/

LTC1853 also allows the user to select between two gains

and unipolar or bipolar inputs for a total of four input spans.

PGA high selects a gain of 1 (the input span is equal to the

voltage on REFCOMP). PGA low selects a gain of 2 where

the input span is equal to half of the voltage on REFCOMP.

UNI/BIP low selects a unipolar input span, UNI/BIP high

selects a bipolar input span. Table 2 summarizes the pos-

sible input spans.

Table 2. Input Span Table

INPUT SPAN

UNI/BIP

PGA

REFCOMP = 4.096V

0 – REFCOMP/2

0 – 2.048V

0 – REFCOMP

0 – 4.096V

± REFCOMP/4

±1.024V

±REFCOMP/2

±2.048V

APPLICATIONS INFORMATION

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