ADS1606IPAPR datasheet(23/38 Pages) TI

23 / 38 page

ADS1605

ADS1606

SBAS274H − MARCH 2003 − REVISED MAY 2007

www.ti.com

FREQUENCY RESPONSE

The linear phase FIR digital filter sets the overall fre-

quency response. The decimation rate is set to 8

(2XMODE = low) for all the figures shown in this section.

Figure 19 shows the frequency response from dc to

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

the ADS1605/6 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 2.2MHz

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

passband transition by plotting the response from

2.0MHz to 2.5MHz (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 sig-

nal. 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 ADS1605/6. To prevent this, place an anti-alias filter

in front of the ADS1605/6 that rolls off before 37MHz.

−20

−40

−60

−80

−100

−120

Frequency (MHz)

CL K =40MHz

Figure 19. Frequency Response

0.0025

0.0020

0.0015

0.0010

0.0005

−0.0005

−0.0010

−0.0015

−0.0020

00.5

1.0

Frequency (MHz)

1.5

2.0

2.5

CLK =40MHz

Figure 20. Passband Ripple

−1

−2

−3

−4

−5

−6

−7

2.0

2.1

2.05

2.15

2.25

2.35

2.45

2.2

Frequency (MHz)

2.3

2.4

2.5

CL K =40MHz

Figure 21. Passband Transition

−20

−40

−60

−80

−100

020

Frequency (MHz)

120

100

CL K = 40MHz

Figure 22. Frequency Response Out to 120MHz

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