Datasheet

Full-scalerange
RMSnoise
ln(2)
ENOB=
ln
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
V
IN
= (AINP – AINN), against the differential
under the conditions shown.
reference, V
REF
= (VREFP – VREFN). The most
positive measurable differential input is V
REF
, 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 –V
REF
, 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 is 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 2V
REF
.
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 (mV
RMS
) 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 94 44.02 17.06 14.33
(1) V
REF
= 3V, f
CLK
= 32MHz.
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