REV. F
AD7712
–11–
Tables I and II show the output rms noise for some typical
notch and –3 dB frequencies. The numbers given are for the
bipolar input ranges with a V
REF
of 2.5 V. These numbers are
typical and are generated with an analog input voltage of 0 V.
The output noise from the part comes from two sources. First,
there is the electrical noise in the semiconductor devices used in
the implementation of the modulator (device noise). Second,
when the analog input signal is converted into the digital do-
main, quantization noise is added. The device noise is at a low
level and is largely independent of frequency. The quantization
noise starts at an even lower level but rises rapidly with increasing
frequency to become the dominant noise source. Consequently,
lower filter notch settings (below 60 Hz approximately) tend to
be device noise dominated while higher notch settings are domi-
nated by quantization noise. Changing the filter notch and cutoff
frequency in the quantization noise dominated region results in a
more dramatic improvement in noise performance than it does
in the device noise dominated region as shown in Table I.
Furthermore, quantization noise is added after the PGA, so
effective resolution is independent of gain for the higher filter
Table I. Output Noise vs. Gain and First Notch Frequency
First Notch of
Filter and O/P –3 dB Gain of Gain of Gain of Gain of Gain of Gain of Gain of Gain of
Data Rate
1
Frequency 1248163264128
10 Hz
2
2.62 Hz 1.0 0.78 0.48 0.33 0.25 0.25 0.25 0.25
25 Hz
2
6.55 Hz 1.8 1.1 0.63 0.5 0.44 0.41 0.38 0.38
30 Hz
2
7.86 Hz 2.5 1.31 0.84 0.57 0.46 0.43 0.4 0.4
50 Hz
2
13.1 Hz 4.33 2.06 1.2 0.64 0.54 0.46 0.46 0.46
60 Hz
2
15.72 Hz 5.28 2.36 1.33 0.87 0.63 0.62 0.6 0.56
100 Hz
3
26.2 Hz 13 6.4 3.7 1.8 1.1 0.9 0.65 0.65
250 Hz
3
65.5 Hz 130 75 25 12 7.5 4 2.7 1.7
500 Hz
3
131 Hz 0.6 ⫻ 10
3
0.26 ⫻ 10
3
140 70 35 25 15 8
1 kHz
3
262 Hz 3.1 ⫻ 10
3
1.6 ⫻ 10
3
0.7 ⫻ 10
3
0.29 ⫻ 10
3
180 120 70 40
NOTES
1
The default condition (after the internal power-on reset) for the first notch of filter is 60 Hz.
2
For these filter notch frequencies, the output rms noise is primarily dominated by device noise, and, as a result, is independent of the value of the reference voltage.
Therefore, increasing the reference voltage will give an increase in the effective resolution of the device (i.e., the ratio of the rms noise to the input full scale is
increased since the output rms noise remains constant as the input full scale increases).
3
For these filter notch frequencies, the output rms noise is dominated by quantization noise, and, as a result, is proportional to the value of the reference voltage.
Table II. Effective Resolution vs. Gain and First Notch Frequency
First Notch of
Filter and O/P –3 dB Gain of Gain of Gain of Gain of Gain of Gain of Gain of Gain of
Data Rate Frequency 1248163264128
10 Hz 2.62 Hz 22.5 21.5 21.5 21 20.5 19.5 18.5 17.5
25 Hz 6.55 Hz 21.5 21 21 20 19.5 18.5 17.5 16.5
30 Hz 7.86 Hz 21 21 20.5 20 19.5 18.5 17.5 16.5
50 Hz 13.1 Hz 20 20 20 20 19 18.5 17.5 16.5
60 Hz 15.72 Hz 20 20 20 19.5 19 18 17 16
100 Hz 26.2 Hz 18.5 18.5 18.5 18.5 18 17.5 17 16
250 Hz 65.5 Hz 15 15.5 15.5 15.5 15.5 15.5 15 14.5
500 Hz 131 Hz 13 13 13 13 13 12.5 12.5 12.5
1 kHz 262 Hz 10.5 10.5 11 11 11 10.5 10 10
*Effective resolution is defined as the magnitude of the output rms noise with respect to the input full scale (i.e., 2 ⫻ V
REF
/GAIN). The above table applies for
a V
REF
of 2.5 V and resolution numbers are rounded to the nearest 0.5 LSB.
Typical Output RMS Noise (V)
Effective Resolution* (Bits)
notch frequencies. Meanwhile, device noise is added in the PGA
and, therefore, effective resolution suffers a little at high gains
for lower notch frequencies.
At the lower filter notch settings (below 60 Hz), the no missing
codes performance of the device is at the 24-bit level. At the
higher settings, more codes will be missed until at the 1 kHz
notch setting; no missing codes performance is guaranteed only
to the 12-bit level. However, since the effective resolution of the
part is 10.5 bits for this filter notch setting, this no missing codes
performance should be more than adequate for all applications.
The effective resolution of the device is defined as the ratio of
the output rms noise to the input full scale. This does not
remain constant with increasing gain or with increasing band-
width. Table II is the same as Table I except that the output is
expressed in terms of effective resolution (the magnitude of the
rms noise with respect to 2 ⴛ V
REF
/GAIN, i.e., the input full
scale). It is possible to do post filtering on the device to improve
the output data rate for a given –3 dB frequency and to further
reduce the output noise (see the Digital Filtering section).
