AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 25

• Manufacturer Part Number: AN1740
• Description: Applications Using the Analog Subsystem on MC68HC05JJ/JP Series Microcontrollers
• Manufacturer: Freescale Semiconductor / Motorola

Current Source
AN1740
MOTOROLA
The source current can vary as much as 10% due to the combination of
temperature ranging from –40 C to +85 C, supply voltages within
in the source current, then one or more of the various effects needs to
be constrained to a tighter range.
The typical effects are:
This source supplies a constant current out of PB0 at voltages from V
to within 0.5 volt of V
V
100- A value. The current source continues to work above the common
mode range, but starts to degrade below its nominal value as shown in
Figure
The internal current source circuitry contains a similar current mirror with
bias currents totaling about 120 A. To prevent the current source from
lagging when the PB0 pin is being switched between sourcing or
discharging current, it is either directed out of the PB0 pin or redirected
to V
of single slope A/D conversions. Therefore, the current source draws
about 220 A whenever the ISEN bit is set, even if the PB0 pin is not
sourcing current and the discharge device is active. As the voltage on
PB0 changes, non-linearity creates up to about 1% variation in the
nominal 100- A source current.
15%, and between different units. If the user wants a closer tolerance
SS
1. A 10% change in the supply voltage typically creates about a 1.5%
2. A temperature drop from 25 C to –40 C typically creates about a
3. A temperature increase from 25 C to 85 C typically creates about
4. As the voltage on PB0 changes, non-linearity creates up to about
5. Variations between different devices can typically have about 1%
Freescale Semiconductor, Inc.
SS
to V
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change in the source current.
1% change for each 10 C change in temperature.
a 0.01% change for each 10 C change in temperature.
1% variation in the nominal 100 A source current.
variation in the source current.
. Otherwise, the lag in the current source would affect the timing
10.
DD
– 1.5 volts, this current is constant within 1% of the nominal
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DD
. Over the common mode range of
Current Source/Discharge
Application Note
SS
25