ADS1625IPAPR datasheet(23/37 Pages) TI

23 / 37 page

ADS1625

ADS1626

SBAS280E − JUNE 2003 − REVISED MAY 2007

www.ti.com

FREQUENCY RESPONSE

The linear phase FIR digital filter sets the overall frequency

response. Figure 19 shows the frequency response from dc

to 20MHz for fCLK = 40MHz. The frequency response of the

ADS1625/6 filter scales directly with CLK frequency. For

example, if the CLK frequency is decreased by half (to

20MHz), the values on the X-axis in Figure 19 would need to

be scaled by half, with the span becoming dc to 10MHz.

Figure 20 shows the passband ripple from dc to 550kHz

(fCLK = 40MHz). Figure 21 shows a closer view of the

passband transition by plotting the response from 500kHz

to 640kHz (fCLK = 40MHz).

The overall frequency response repeats at multiples of the

CLK frequency. To help illustrate this, Figure 22 shows the

response out to 120MHz (fCLK = 40MHz). Notice how the

passband response repeats at 40MHz, 80MHz and

120MHz; it is important to consider this when there is

high-frequency noise present with the signal. The

modulator bandwidth extends to 100MHz. High-frequency

noise around 40MHz and 80MHz will not be attenuated by

either the modulator or the digital filter. This noise will alias

back in-band and reduce the overall SNR performance

unless it is filtered out prior to the ADS1625/6. To prevent

this, place an anti-alias filter in front of the ADS1625/6 that

rolls off before 39MHz.

−20

−40

−60

−80

−100

−120

−140

Frequency (MHz)

CLK =40MHz

Figure 19. Frequency Response.

0.0025

0.0020

0.0015

0.0010

0.0005

−0.0005

−0.0010

−0.0015

−0.0020

−0.0025

100

200

Frequency (kHz)

300

400

500

600

CLK = 40MHz

Figure 20. Passband Ripple

−1

−2

−3

−4

−5

−6

500

520

560

600

620

540

Frequency (kHz)

580

640

CLK =40MHz

Figure 21. Passband Transition

−20

−40

−60

−80

−100

−120

−140

020

Frequency (MHz)

120

100

CL K =40MHz

Figure 22. Frequency Response Out to 120MHz

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