
SECTION 10: A/D CONVERTER
MOTOROLA
Page 65
SECTION 10
A/D CONVERTER
The MC68HC705V7 includes a 16 channel, 8-bit, multiplexed input, successive
approximation A/D converter. When the device is packaged (in a 56-pin SDIP), pin number
restrictions require that 8 channels, AN8 through AN15, are not bonded out.
10.1
ANALOG SECTION
10.1.1
The A/D is ratiometric, with two dedicated pins supplying the reference voltages (V
REFH
and V
REFL
). An input voltage equal to V
REFH
converts to $FF (full scale) and an input
voltage equal to V
REFL
converts to $00. An input voltage greater than V
REFH
will convert
to $FF with no overflow indication. For ratiometric conversions, the source of each analog
input should use V
REFH
as the supply voltage and be referenced to V
REFL
.
RATIOMETRIC CONVERSION
10.1.2
V
REFH
AND V
REFL
The reference supply for the converter uses two dedicated pins rather than being driven by
the system power supply lines because the voltage drops in the bonding wires of those
heavily loaded pins would degrade the accuracy of the A/D conversion. V
REFH
and V
REFL
can be any voltage between V
SSA1
and V
CCA
, as long as V
REFH
> V
REFL
; however, the
accuracy of conversions is tested and guaranteed only for V
REFL
= V
SSA1
and V
REFH
= V
CCA
.
10.1.3
ACCURACY AND PRECISION
The 8-bit conversions shall be accurate to within
±
1
1
/
2
LSB including quantization.
10.2
The A/D reference inputs are applied to a precision internal digital-to-analog converter.
Control logic drives this D/A and the analog output is successively compared to the
selected analog input which was sampled at the beginning of the conversion time. The
conversion process is monotonic and has no missing codes.
CONVERSION PROCESS
10.3
DIGITAL SECTION
10.3.1
Each channel of conversion takes 32 clock cycles, which must be at a frequency equal to
or greater than 1 MHz.
CONVERSION TIMES
F
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
.