Cirrus Logic CS5460A - Manual (Page 14)

CS5460A
14 DS487F5
full-scale. Note that the 24-bit signed output words
are expressed in two’s complement format. The
24-bit data words in the CS5460A output registers
represent values between 0 and 1 (for unsigned
output registers) or between -1 and +1 (for signed
output registers). A register value of 1 represents
the maximum possible value. Note that a value of
1.0 is never actually obtained in the registers of the
CS5460A. As an illustration, in any of the signed
output registers, the maximum register value is
[(2^23 - 1) / (2^23)] = 0.999999880791. After each
A/D conversion, the CRDY bit will be asserted in
the Status Register, and the INT pin will also be-
come active if the CRDY bit is unmasked (in the
Mask Register). The assertion of the CRDY bit in-
dicates that new instantaneous 24-bit voltage and
current samples have been collected, and these
two samples have also been multiplied together to
provide a corresponding instantaneous 24-bit pow-
er sample.
Table 1 conveys the typical relationship between
the differential input voltage (across the “+” and “-”
input pins of the voltage channel input) and the cor-
responding output code in the Instantaneous Volt-
age Register. Note that this table is applicable for
the current channel if the current channel’s PGA
gain is set for the “10x” gain mode.
The V
RMS
, I
RMS
, and energy calculations are up-
dated every N conversions (which is known as 1
“computation cycle”), where N is the value in the
Cycle Count Register. At the end of each computa-
tion cycle, the DRDY bit in the Mask Register will
be set, and the INT pin will become active if the
DRDY bit is unmasked.
DRDY is set only after each computation cycle has
completed, whereas the CRDY bit is asserted after
each individual A/D conversion. Bits asserted by
the CS5460A must be cleared before being assert-
ed again. If the Cycle Count Register value (N) is
set to 1, all output calculations are instantaneous,
and DRDY will indicate when instantaneous calcu-
lations are finished, just like the CRDY bit. For the
RMS results to be valid, the Cycle-Count Register
must be set to a value greater than 10.
The computation cycle frequency is derived from
the master clock, and has a value of
(MCLK/K)/(1024*N). Under default conditions, with
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
Frequency (Hertz)
G
a
in (
d
B)
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Figure 4. Voltage Input Filter Characteristics
Input Voltage (DC)
Output Code
(hexidecimal)
Output Code
(decimal)
+250 mV 7FFFFF 8388607
14.9 nV to 44.7 nV 000001 1
-14.9 nV to 14.9 nV 000000 0
-44.7 nV to -14.9 nV FFFFFF -1
-250 mV 800000 -8388608
Table 1. Differential Input Voltage vs. Output Code
-2.5
-2
-1.5
-1
-0.5
0
0.5
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Ga
in
(
d
B
)
Frequency (Hertz)
Figure 5. Current Input Filter Characteristics