Datasheet

PHASE RESPONSE
SETTLING TIME
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
NormalizedInputFrequency(f /f
IN TDA A
)
Amplitude(dB)
0.45 0.47 0.49 0.51 0.53 0.55
20
0
-20
-40
-60
-80
-100
-120
-140
-160
InputFrequency(f /f
IN DATA
)
Gain(dB)
0 16 32 48 64
100
0
Settling(%)
Conversions(1/f
DATA
)
0 2010 4030 6050 8070
FullySettledData
at76Conversions
InitialValue
FinalValue
ANALOG INPUTS (AINP, AINN)
ADS1174
ADS1178
www.ti.com ........................................................................................................................................ SBAS373B – OCTOBER 2007 – REVISED SEPTEMBER 2008
The ADS1174/78 incorporates a multiple stage, linear
phase digital filter. Linear phase filters exhibit
constant delay time versus input frequency (constant
group delay), which means the time delay from any
instant of the input signal to the same instant of the
output data is constant, and is independent of input
signal frequency. This behavior results in essentially
zero phase errors when analyzing multi-tone signals.
As with frequency and phase response, the digital
filter also determines settling time. Figure 6 shows
the output settling behavior after a step change on
the analog inputs normalized to conversion periods.
Figure 4. Transition Band Response
The X-axis is given in units of conversion. Note that
after the step change on the input occurs, the output
data change very little prior to 30 conversion periods.
The output data are fully settled after 76 conversion
periods.
Figure 5. Frequency Response Out to f
MOD
These image frequencies, if present in the signal and
Figure 6. Step Response
not externally filtered, fold back (or alias) into the
passband, causing errors. Table 2 lists the degree of
image rejection versus external antialiasing filter
order. The stop band of the ADS1174/78 provides
100dB attenuation of frequencies that begin just
The ADS1174/78 measures each differential input
beyond the passband and continue out to f
MOD
.
signal V
IN
= (AINP – AINN) against the common
Placing an antialiasing, low-pass filter in front of the
differential reference V
REF
= (VREFP – VREFN). The
ADS1174/78 inputs is recommended to limit possible
most positive measurable differential input is +V
REF
,
high-amplitude, out-of-band signals and noise.
which produces the most positive digital output code
of 7FFFh. Likewise, the most negative measurable
Table 2. Antialiasing Filter Order Image Rejection
differential input is – V
REF
, which produces the most
negative digital output code of 8000h.
ANTIALIASING FILTER IMAGE REJECTION (dB)
ORDER (f
– 3dB
at f
DATA
)
For optimal performance, drive the ADS1174/78
1 39
inputs differentially. For single-ended input
2 75
applications, one of the inputs (AINP or AINN) can be
3 111
driven while the other input is fixed (typically, to
AGND or +2.5V); fixing the input to +2.5V permits
bipolar operation, thereby using the full range of the
converter.
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