MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA
33972
21
Figure 11. Analog Ratiometric Conversion
To read a potentiometer sensor, the wiper should be
grounded and brought back to the module ground, as illustrated
in
Figure 11
. With the wiper changing the impedance of the
sensor, the analog voltage on the input will represent the
position of the sensor.
Using the Analog feature to provide 2.0 mA of pullup current
to an analog sensor may induce error due to the accuracy of the
current source. For this reason, a ratiometric conversion must
be considered. Using two current sources (one for the sensor
and one to set the reference voltage to the A/D converter) will
yield a maximum error (owing to the 33972) of 4%.
Higher accuracy may be achieved through module level
calibration. In this example, we use the resistor values from
Figure 11
and assume the current sources are 4% from each
other. The user may use the module end-of-line tester to
calculate the error in the A/D conversion. By placing a 2.0 k
,
0.1% resistor in the end-of-line test equipment and assuming a
perfect 2.0 mA current source from the 33972, a calculated A/D
conversion may be obtained. Using the equation yields the
following:
The ADC value of 213 counts is the value with 0% error
(neglecting the resistor tolerance and AMUX input offset
voltage). Now we can calculate the count value induced by the
mismatch in current sources. From a sample device the
maximum current source was measured at 2.05 mA and
minimum current source was measured at 1.99 mA. This yields
3% error in A/D conversion. The A/D measurement will be as
follows:
This A/D conversion is 3% low in value. The error correction
factor of 1.03 may be used to correct the value:
An error correction factor may then be stored in E
2
memory
and used in the A/D calculation for the specific input. Each input
used as analog measurement will have a dedicated calibrated
error correction factor.
Power MOSFET/LED Driver and Monitor
Because of the flexible programming of the 33972 device, it
may be used to drive small loads like LEDs or MOSFET gates.
It was specifically designed to power up in the Normal mode
with the inputs tri-state. This was done to ensure the LEDs or
MOSFETs connected to the 33972 power up in the off-state.
The Switch Programmable (SP0–SP7) inputs have a source-
and-sink capability, providing effective MOSFET gate control.
To complete the circuit, a pulldown resistor should be used to
keep the gate from floating during the Sleep modes.
Figure 12
,
page 22, shows an application where the SG0 input is used to
monitor the drain-to-source voltage of the external MOSFET.
The 1.5 k
resistor is used to set the drain-to-source trip
voltage. With the 2.0 mA current source enabled, an interrupt
will be generated when the drain-to-source voltage is
approximately 1.0 V.
V
DD
V
PWR
V
DD
SI
SO
INT
SCLK
CS
AMUX
MOSI
SCLK
CS
MISO
INT
AN0
VBAT
SP0
SP1
SP7
SG1
SG0
SG12
SG13
WAKE
VBAT
16
mA
2.0
mA
V
PWR
2.0 mA
2.0 mA
2.39 k
0.1%
4.54 V to 5.02 V
Analog
Ports
V
REF(H)
V
REF(L)
Analog Sensor
or Analog Switch
R
2
I
I
1
R
1
V
PWR
2.0
mA
V
PWR
V
PWR
mA
16
33972
MCU
ADC = I1 x R1
I2 x R2x 225
ADC = 2.0 mA x 2.0 k
2.0 mA x 2.39 k
x 225
ADC = 213 counts
ADC = 1.99 mA x 2.0 k
2.05 mA x 2.39 k
x 225
ADC = 207 counts
ADC = 207 counts x 1.03
ADC = 213 counts
-
F
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
.