Rainbow Electronics MAX111 - Manual (Page 18)

MAX110/MAX111
Low-Cost, 2-Channel, ±14-Bit Serial ADCs
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Selecting the Oversampling
Clock Frequency
Choose the oversampling frequency, f OSC , carefully to
achieve the best relative-accuracy performance from the
MAX110/MAX111 (see
Typical Operating Characteristics ).
Clock Divider-Ratio Control Bits
Bits 7 and 8 (DV2 and DV4) program the clock-
frequency divider network. The divider network sets the
frequency ratio between f
XCLK (the frequency of the
external TTL/CMOS clock or internal RC oscillator) and
f
OSC (the oversampling frequency used by the ADC).
An oversampling clock frequency between 450kHz and
700kHz is optimum for the converter.
Best perfor-
mance over the extended temperature range is
obtained by choosing 1MHz or 1.024MHz with the
divide-by-2 option (DV2 = 1) (see the section
Effect
of Dither on INL ) . To determine the converter’s accura-
cy at other clock frequencies, see the Typical
Operating Characteristics and Table 5.
Effect of Dither on Relative Accuracy
First-order sigma-delta converters require dither for
randomizing any systematic tone being generated in
the modulator. The frequency of the dither source plays
an important role in linearizing the modulator. The ratio
of the dither generator’s frequency to that of the modu-
lator’s oversampling clock can be changed by setting
the DV2/DV4 bits. The XCLK clock is directly used by
the dither generator while the DV2/DV4 bits reduce the
oversampling clock by a ratio of 2 or 4. Over the com-
mercial temperature range, any ratio (i.e., 1, 2, or 4)
between the dither frequency and the oversampling
clock frequency can be used for best performance.
Over the extended and military temperature ranges, the
ratio of 2 or 4 gives the best performance. See the
Typical Operating Characteristics to observe the effect
of the clock divider on the converter’s linearity.
50Hz/60Hz Line Frequency Rejection
High rejection of 50Hz or 60Hz is obtained by using an
oversampling clock frequency and a clock-cycles/con-
version setting so the conversion time equals an inte-
gral number of line cycles, as in the following equation:
f OSC = f LINE x m / n
where f OSC is the oversampling clock frequency, f LINE
= 50Hz or 60Hz, m is the number of clock cycles per
conversion (see Table 4), and n is the number of line
cycles averaged every conversion.
This noise rejection is inherent in integrating and
sigma-delta ADCs, and follows a SIN(X) / X function
(Figure 9). Notches in this function represent extremely
high rejection, and correspond to frequencies with an
integral number of cycles in the MAX110/MAX111’s
selected conversion time.
The shortest conversion time resulting in maximum
simultaneous rejection of both 60Hz and 50Hz line fre-
quencies is 100ms. When using the MAX111, use a
200ms conversion time for maximum 60Hz and 50Hz
rejection
and optimum performance. For either device,
select the appropriate oversampling clock frequency
and either an 81,240 or 102,400 clock cycles per con-
version (CCPC) ratio. Table 6 suggests the possible
configurations.
0
-10
-20
-30
-40
-50
-60
0.1
1
CONVERSION TIME
LINE CYCLE PERIOD
SIGNAL FREQUENCY IN Hz
FOR 100ms CONVERSION
TIME (see Table 6)
1
10 20 30 40 50 60 70 80 90100
2 3 4 5 6 7 8 9 10
GAIN (dB)
Figure 9. MAX110/MAX111 Noise Rejection Follows SIN(X) / X Function