ADS1258IPHPREP datasheet(22/57 Pages) TI

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H f

+ Hsinc

Averager f

sin

128p

CLK

sin

CLK

sin

128p

Num_Ave

CLK

Num_Ave

sin

128p

CLK

−20

−40

−60

−80

−100

−120

−140

Frequency (kHz)

125

250

375

500

625

Data Rate

Auto−Scan Mode

(23.739kSPS)

Data Rate

Fixed−Channel Mode

(125kSPS)

−20

−40

−60

−80

−100

−120

−140

Frequency (kHz)

125

250

375

500

625

Data Rate

Auto−Scan Mode

(15.123kSPS)

Data Rate

Fixed−Channel Mode

(31.25kSPS)

ADS1258-EP

SBAS445C – MARCH 2009 – REVISED DECEMBER 2009

www.ti.com

Table 3 shows a listing of the averaging and data

Figure 44 shows the response with averaging set to 4

rates for each of the four DRATE[1:0] register

(DRATE[1:0]

10).

4-reading,

post-averaging

settings for the Auto-Scan and Fixed-Channel modes,

produces three equally-spaced notches between

with CHOP, DLY = 0. Note that the data rate scales

each main notch of the sinc5 filter. The frequency

directly with fCLK. For example, reducing fCLK by 2x

response of DRATE[1:0] = 01 and 00 follows a similar

reduces the maximum data rate by 2x.

pattern, but with 15 and 63 equally-spaced notches

between the main sinc5 notches, respectively.

FREQUENCY RESPONSE

The low-pass digital filter sets the overall frequency

response for the ADS1258. The filter response is the

product

the

responses

the

fixed

and

programmable

filter

sections

and

given

Equation 4:

(4)

The digital filter attenuates noise on the modulator

output including noise from within the ADS1258 and

external noise present within the ADS1258 input

signal. Adjusting the filtering by changing the number

Figure 43. Frequency Response, DRATE[1:0] = 11

of averages used in the programmable filter changes

the filter bandwidth. With a higher number of

averages, the bandwidth is reduced and more noise

is attenuated.

The low-pass filter has notches (or zeros) at the data

output rate and multiples thereof. The sinc5 part of

the filter produces wide notches at fCLK/128 and

multiples thereof. At these frequencies, the filter has

zero gain. Figure 43 shows the response with no post

averaging. Note that in Auto-Scan mode, the data

rate is reduced while retaining the same frequency

response as in Fixed-Channel mode.

With programmable averaging, the wide notches

produced by the sinc5 filter remain, but a number of

narrow notches are superimposed in the response.

The

number

the

superimposed

notches

determined

the

number

readings

averaged (minus one).

Figure 44. Frequency Response, DRATE[1:0] = 10

Table 3. Data Rates(1)

DATA RATE AUTO-SCAN

DATA RATE FIXED-CHANNEL

–3dB BANDWIDTH

DRATE[1:0]

Num_Ave(2)

MODE (SPS)(3)

MODE (SPS)

(Hz)

23739

125000

25390

15123

31250

12402

6168

7813

3418

1831

1953

869

(1)

fCLK = 16 MHz, Chop = 0, and Delay = 0.

(2)

Num_Ave is the number of averages performed by the digital filter second stage.

(3)

In Auto-Scan mode, the data rate listed is for a single channel; the effective data rate for multiple channels (on a per-channel basis) is

the value shown in Figure 43 and Figure 44 divided by the number of active channels in a scan loop.

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