LTC1850CFW datasheet(14/28 Pages) LINER

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LTC1850/LTC1851

18501f

APPLICATIO S I FOR ATIO

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

to the ADC input, nonlinearities in the ADC transfer func-

tion can create distortion products at the sum and differ-

ence 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

Log

Amplitude at fa

()=

()

Peak Harmonic or Spurious Noise

The peak harmonic or spurious noise is the largest spec-

tral 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 LTC1851 (11

effective bits) or 56dB for the LTC1850 (9 effective bits).

The LTC1850/LTC1851 have been designed to optimize

input bandwidth, allowing the ADC to undersample input

signals with frequencies above the converter’s Nyquist

frequency. The noise floor stays very low at high frequen-

cies; 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 common 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 A1 A0 CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM

–

MUX ADDRESS

DIFFERENTIAL CHANNEL SELECTION

DIFF A2 A1 A0 CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM

–

*Not used in differential mode. Connect to GND.

In addition to selecting the MUX channel, the LTC1850/

LTC1851 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 summa-

rizes the possible input spans.

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