ADS1675 datasheet(16/40 Pages) TI2

16 / 40 page

Full-scalerange

RMSnoise

ln(2)

ENOB=

ADS1675

SBAS416D – DECEMBER 2008 – REVISED AUGUST 2010

www.ti.com

NOISE PERFORMANCE

Noise-free bits specifies noise, again from a dc

perspective using Equation 1, with peak-to-peak

The ADS1675 offers outstanding noise performance

noise substituted for RMS noise.

that can be optimized by adjusting the data rate. As

the averaging is increased (thus reducing the data

ANALOG INPUTS (AINP, AINN)

rate), the noise drops correspondingly. Table 1 shows

the noise as a function of data rate for both the

The

ADS1675

measures

the

differential

signal,

Low-Latency and the Wide-Bandwidth filter paths

VIN = (AINP – AINN), against the differential

under the conditions shown.

reference, VREF = (VREFP – VREFN). The most

positive measurable differential input is VREF, which

Table 1 lists some of the more common methods of

produces the most positive digital output code of

specifying noise. The dynamic range is the ratio of

7FFFFFh. Likewise, the most negative measurable

the root-mean-square (RMS) value of a full-scale sine

differential input is –VREF, which produces the most

wave to the RMS noise with the inputs shorted

negative digital output code of 800000h.

together. This value is expressed in decibels relative

to full-scale (dBFS). The input-referred noise is the

Analog inputs must be driven with a differential signal

RMS value of the noise with the inputs shorted,

to achieve optimum performance. The recommended

referred to the input of the ADS1675. The effective

common-mode

voltage

2.5V.

The

ADS1675

number of bits (ENOB) is calculated from a dc

samples the analog inputs at very high speeds. It is

perspective using the formula in Equation 1, where

critical that a suitable driver be used. See the

full-scale range equals 2VREF.

Application Information section for recommended

circuit designs.

(1)

Table 1. Noise Performance(1)

INPUT-

DATA RATE

DYNAMIC

REFERRED

NOISE-FREE

FILTER PATH

DATA RATE[2:0]

(kSPS)

RANGE (dB)

NOISE (mVRMS)

ENOB

BITS

000

125

111

6.30

19.86

17.14

001

250

109

7.47

19.61

16.89

Low-speed

Low-Latency

modes

010

500

107

9.51

19.27

16.54

(Fast Response

Mode

011

1000

105

11.72

18.97

16.24

configuration)

100

2000

104

13.72

18.74

16.02

High-speed

modes

101

4000

103

14.23

18.69

15.96

000

125

111

6.17

19.89

17.17

001

250

109

7.44

19.62

16.90

Low-speed

modes

010

500

107

9.66

19.25

16.52

Wide-Bandwidth

011

1000

104

12.99

18.82

16.09

100

2000

101

18.64

18.30

15.57

High-speed

modes

101

4000

44.02

17.06

14.33

(1)

VREF = 3V, fCLK = 32MHz.

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