ADS1178IPAPR datasheet(11/39 Pages) TI1

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PHASE RESPONSE

SETTLING TIME

-1

-2

-3

-4

-5

-6

-7

-8

-9

-10

NormalizedInputFrequency(f /f

DA A)

0.45

0.47

0.49

0.51

0.53

0.55

-20

-40

-60

-80

-100

-120

-140

-160

InputFrequency(f /f

IN DATA)

100

Conversions(1/f

DATA)

FullySettledData

at76Conversions

InitialValue

FinalValue

ANALOG INPUTS (AINP, AINN)

ADS1174

ADS1178

www.ti.com ........................................................................................................................................ SBAS373B – OCTOBER 2007 – REVISED SEPTEMBER 2008

The ADS1174/78 incorporates a multiple stage, linear

phase

digital

filter.

Linear

phase

filters

exhibit

constant delay time versus input frequency (constant

group delay), which means the time delay from any

instant of the input signal to the same instant of the

output data is constant, and is independent of input

signal frequency. This behavior results in essentially

zero phase errors when analyzing multi-tone signals.

As with frequency and phase response, the digital

filter also determines settling time. Figure 6 shows

the output settling behavior after a step change on

the analog inputs normalized to conversion periods.

Figure 4. Transition Band Response

The X-axis is given in units of conversion. Note that

after the step change on the input occurs, the output

data change very little prior to 30 conversion periods.

The output data are fully settled after 76 conversion

periods.

Figure 5. Frequency Response Out to fMOD

These image frequencies, if present in the signal and

Figure 6. Step Response

not externally filtered, fold back (or alias) into the

passband, causing errors. Table 2 lists the degree of

image rejection versus external antialiasing filter

order. The stop band of the ADS1174/78 provides

100dB attenuation of frequencies that begin just

The ADS1174/78 measures each differential input

beyond the passband and continue out to fMOD.

signal VIN = (AINP – AINN) against the common

Placing an antialiasing, low-pass filter in front of the

differential reference VREF = (VREFP – VREFN). The

ADS1174/78 inputs is recommended to limit possible

most positive measurable differential input is +VREF,

high-amplitude, out-of-band signals and noise.

which produces the most positive digital output code

of 7FFFh. Likewise, the most negative measurable

Table 2. Antialiasing Filter Order Image Rejection

differential input is –VREF, which produces the most

negative digital output code of 8000h.

ANTIALIASING FILTER

IMAGE REJECTION (dB)

ORDER

(f–3dB at fDATA)

For optimal performance, drive the ADS1174/78

inputs

differentially.

For

single-ended

input

applications, one of the inputs (AINP or AINN) can be

111

driven while the other input is fixed (typically, to

AGND or +2.5V); fixing the input to +2.5V permits

bipolar operation, thereby using the full range of the

converter.

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