CS5530
18
DS742F3
the magnitude of the reference voltage to achieve
optimal performance. Figures 8 and 9 model the ef-
fects on the reference’s input impedance and input
current for each VRS setting. As the models show,
the reference includes a coarse/fine charge buffer
which reduces the dynamic current demand of the
external reference.
The reference’s input buffer is designed to accom-
modate rail-to-rail (common-mode plus signal) in-
put voltages. The differential voltage between the
VREF+ and VREF- can be any voltage from 1.0 V
up to the analog supply (depending on how VRS is
configured), however, the VREF+ cannot go above
VA+ and the VREF- pin can not go below VA-.
Note that the power supplies to the chip should be
established before the reference voltage.
2.3.5 Output Latch Pins
The A1-A0 pins of the ADC mimic the D24-D23
bits of the configuration register. A1-A0 can be
used to control external multiplexers and other log-
ic functions outside the converter. The A1-A0 out-
puts can sink or source at least 1 mA, but it is
recommended to limit drive currents to less than
20
μ
A to reduce self-heating of the chip. These out-
puts are powered from VA+ and VA-. Their output
voltage will be limited to the VA+ voltage for a
logic 1 and VA- for a logic 0. Note that if the latch
bits are used to modify the analog input signal the
user should delay performing a conversion until he
knows the effects of the A0/A1 bits are fully set-
tled.
2.3.6 Filter Rate Select
The Filter Rate Select bit (FRS) modifies the output
word rates of the converter to allow either 50 Hz or
60 Hz rejection when operating from a 4.9152
MHz crystal. If FRS is cleared to logic 0, the word
rates and corresponding filter characteristics can be
selected using the Configuration Register. Rates
can be 7.5, 15, 30, 60, 120, 240, 480, 960, 1920, or
3840 Sps when using a 4.9152 MHz clock. If FRS
is set to logic 1, the word rates and corresponding
filter characteristics scale by a factor of 5/6, mak-
ing the selectable word rates 6.25, 12.5, 25, 50,
100, 200, 400, 800, 1600, and 3200 Sps when using
a 4.9152 MHz clock. When using other clock fre-
quencies, these selectable word rates will scale lin-
early with the clock frequency that is used.
2.3.7 Word Rate Select
The Word Rate Select bits (WR3-WR0) allow slec-
tion of the output word rate of the converter as de-
picted in the Configuration Register Descriptions.
The word rate chosen by the WR3-WR0 bits is
modified by the setting of the FRS bit as presented
in the previous paragraph.
2.3.8 Unipolar/Bipolar Select
The UP/BP Select bit sets the converter to measure
either a unipolar or bipolar input span.
2.3.9 Open Circuit Detect
When the OCD bit is set it activates a current
source as a means to test for open thermocouples.
VREF
C = 14pF
f =
2
φ
Fine
1
V
≤
8 mV
i = fV
C
os
os
n
φ
Coarse
MCLK
16
VRS = 1; 1 V
≤
V
≤
2.5 V
REF
Figure 8. Input Reference Model when VRS = 1
VREF
C = 7 pF
f =
2
φ
Fine
1
V
≤
16 mV
i = fV
C
os
os
n
φ
Coarse
MCLK
16
VRS = 0; 2.5 V < V
≤
VA+
REF
Figure 9. Input Reference Model when VRS = 0
