ADS1282IPW datasheet(21/52 Pages) TI

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GROUP DELAY AND STEP RESPONSE

Linear Phase Response

0.08

NormalizedInputFrequency(f /f

)

IN DATA

0.003

0.002

0.001

0.00

0.16

0.24

0.32

0.40

TimeIndex(1/f

)

DATA

1.4

1.2

1.0

0.8

0.6

0.4

0.2

-0.2

15 20 25 30 35 40 45 50 55

LinearPhaseFilter

MinimumPhaseFilter

0.35

0.40

0.45

NormalizedInputFrequency(f /f

)

IN DATA

-20

-40

-60

-80

-100

-140

0.65

-120

0.50

0.55

0.60

ADS1282

www.ti.com ........................................................................................................................................ SBAS418E – SEPTEMBER 2007 – REVISED OCTOBER 2008

As shown in Figure 46, the FIR frequency response

provides a flat passband to 0.375 of the data rate

The FIR block is implemented as a multi-stage FIR

(±0.003dB passband ripple). Figure 47 shows the

structure with selectable linear or minimum phase

transition from passband to stop band.

response. The passband, transition band, and stop

band responses of the filters are nearly identical but

differ in the respective phase responses.

Linear phase filters exhibit constant delay time versus

input frequency (that is, constant group delay). Linear

phase filters have the property that the time delay

from any instant of the input signal to the same

instant of the output data is constant and is

independent of the signal nature. This filter behavior

results in essentially zero phase error when analyzing

multi-tone signals. However, the group delay and

settling time of the linear phase filter are somewhat

larger than the minimum phase filter, as shown in

Figure 48.

Figure 46. FIR Passband Magnitude Response

(fDATA = 500Hz)

Figure 48. FIR Step Response

Figure 47. FIR Transition Band Magnitude

Response

Although not shown in Figure 47, the passband

response repeats at multiples of the modulator

frequency (NfMOD – f0 and NfMOD + f0, where N = 1, 2,

etc. and f0 = passband). These image frequencies, if

present in the signal and not externally filtered, fold

back (or alias) into the passband and cause errors. A

low-pass signal filter reduces the effect of aliasing.

Often, the RC low-pass filter provided by the PGA

output resistors and the external capacitor connected

CAPP

and

CAPN

provides

sufficient

signal

attenuation.

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