MAX11040 datasheet(29/31 Pages) MAXIM

29 / 31 page

24-Bit, 4-Channel, Simultaneous-Sampling,

Cascadable, Sigma-Delta ADC

______________________________________________________________________________________

Source Impedance and

Input Sampling Network

The source impedance that drives the analog inputs

affects the sampling period.

Low-Impedance Sources

Minimize the source impedance to ensure the input

capacitor fully charges during the sampling phase.

The required source resistance is defined by the

equation below:

where K = 1.5 and RINT = 2600

Ω.

For example, the required source resistance to achieve

0.1% accuracy is:

High-Impedance Sources

the

source

impedance

greater

than

RSOURCE_MAX, as defined in the Low-Impedance

Sources section, place a 0.1µF bypass capacitor

between AIN_+ and AIN_- to provide transient charge.

The average switched-capacitor load with a proper

bypass capacitor and XIN clock frequency =

24.576MHz is equivalent to a 130k

Ω resistor connect-

ed between AIN_+ and AIN_-. This resistance is inde-

pendent of the value of the 0.1µF bypass capacitor. If

another XIN clock frequency is chosen, this resistance

is directly proportional to the XIN clock period.

Although the addition of a bypass capacitor helps charge

the MAX11040 input capacitor, some gain error due to

resistive drop across the source resistance still remains.

Calculate this gain error using the following equation:

Analog Filtering

The analog filtering requirements in front of the

MAX11040 are considerably reduced compared to a

conventional converter with no on-chip filtering. The

internal digital filter has significant rejection of signals

higher than the Nyquist frequency of the output data

rate that would alias back into the sampled signal.

The internal digital filter does not provide rejection

close to the harmonics of the 3.072MHz modulator fre-

quency. For example, assuming an output data rate of

16ksps if the XIN clock is set to 24.576MHz, then the

band between 3.0686MHz and 3.0750MHz is not

explicitly filtered. Since this unfiltered band is very

small compared to its actual frequency, very little

broadband noise enters through this mechanism. If

focused narrowband noise in this band is present, a

simple analog filter can create significant attenuation at

this frequency because the ratio of passband to stop-

band frequency is large.

In addition, because the device’s common-mode rejec-

tion extends out to several 100kHz, the common-mode

noise susceptibility in this frequency range is substan-

tially reduced.

Providing additional filtering in some applications

ensures that differential noise signals outside the fre-

quency band of interest do not saturate the analog

modulator.

The modulator saturates if the input voltage exceeds its

full scale (±2.2V). The digital filter does not prevent a

large signal in the filter stopband from saturating the

modulator. If signals outside the band of interest cause

violation of this full scale while accurate conversion of

passband signals is desired, then additional analog fil-

tering is required to prevent saturation.

Compensating for the Rolloff of

the Digital Filter in Typical FFT Analysis

To calculate FIR_GAIN(fAIN_):

1) Decide the number of evenly spaced frequencies

between DC and the Nyquist frequency of the output

data rate at which correction factors are desired,

which is usually the same as the FFT result.

2) Create an array with a length that is 2x the number of

the desired frequencies. (Again, the result is likely to

correlate with the time domain array that is loaded

into an FFT algorithm.)

3) Fill this array with the filter coefficients provided in

the

Digital Filter section. Fill the rest of the array with

zeros.

4) Take an FFT of this array. The result represents the

response of the MAX11040 built-in FIR filter.

Gain

SOURCE

LOAD

SOURCE

+ 130

xpF x

SOURCE MAX

⎛

⎝⎜

⎞

⎠⎟

()

−

120

2600

120

1 5

1000

2600

120

6 91

2600

294

ΩΩ

Kx C

x In

Error

xpF x In

Error

SOURCE MAX

SAMP

INT

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

−

120

2600

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