AN1740 Freescale Semiconductor / Motorola, AN1740 Datasheet - Page 71

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

Recommended
Ramp Capacitors
Internal Analog
V
Current Spikes
AN1740
MOTOROLA
SS
Offset
The desired capacitor for the ramp capacitor is any of the “poly” film
types which have both low leakage and low dielectric absorption
(sometimes referred to as memory behavior). Capacitor parameters
other than capacitance and working voltage are equivalent series
resistance (ESR), equivalent series inductance (ESL), leakage, and
dielectric absorption (expressed as RDA or CDA).
Dielectric absorption may leave some charge in the capacitor after a
discharge. Such remaining charge will affect subsequent conversion
ramps. Leakage acts as a parallel resistor which can make the charging
ramp non-linear.
Polystyrene capacitors are usually the best choice. Low-cost monolithic
ceramics are good for bypass use, but have high dielectric absorption
which makes them less desirable for an integration or storage
application. Tantalum or aluminum electrolytics have high dielectric
absorption and too much leakage as well. Integration or storage
capacitors should have a dielectric absorption of less than 0.01%.
The JJ/JP series of MCUs has only a single V
both the analog and digital currents. Also, any external load which sinks
current into the device will pass this current through the V
internal analog and digital V
through separate bonding wires. But the current of both must flow
through the leadframe and out of the V
combined current can result in a small IR voltage drop which adds a
positive offset to the internal analog V
resistance of the V
analog V
current that flows through the V
Similar to the voltage offset just described, current spikes from high-
speed digital switching circuits will generate a voltage drop across the
package pin shared with the analog subsystem. The voltage drop due to
the resistance of a package lead is minor compared to the inductance of
the lead, which is particularly a problem with fast switching currents.
Freescale Semiconductor, Inc.
For More Information On This Product,
SS
will rise about one millivolt for every 10 milliamperes of
Go to: www.freescale.com
SS
pin is about 0.1 ohm, which means that internal
SS
lines are connected to the V
SS
pin.
SS
SS
, called V
pin to the user’s PWB. This
SS
pin through which flows
AOFF
General A/D Techniques
in
Figure
SS
Application Note
SS
pin. The
pin
1. The
71