ADC0801_15 Datasheet, PDF (34/56 Pages) Texas Instruments – Analog-to-Digital Converters

ADC0801, ADC0802, ADC0803, ADC0804, ADC0805
SNOSBI1C – NOVEMBER 2009 – REVISED JUNE 2015
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Output Port B is used as a successive approximation register by the 8080 and the binary scaled resistors in
series with each output bit create a D/A converter. During the zeroing subroutine, the voltage at V_x increases or
decreases as required to make the differential output voltage equal to zero. This is accomplished by ensuring
that the voltage at the output of A1 is approximately 2.5V so that a logic "1" (5V) on any output of Port B will
source current into node V_X thus raising the voltage at V_X and making the output differential more negative.
Conversely, a logic "0" (0V) will pull current out of node V_X and decrease the voltage, causing the differential
output to become more positive. For the resistor values shown, V_X can move ±12 mV with a resolution of 50 µV,
which will null the offset error term to /⁄4 LSB of full-scale for the ADC0801. It is important that the voltage levels
that drive the auto-zero resistors be constant. Also, for symmetry, a logic swing of 0V to 5V is convenient. To
achieve this, a CMOS buffer is used for the logic output signals of Port B and this CMOS package is powered
with a stable 5V source. Buffer amplifier A1 is necessary so that it can source or sink the D/A output current.
Figure 44. Microprocessor Interface Circuitry for Differential Preamp
A flow chart for the zeroing subroutine is shown in Figure 45. It must be noted that the ADC0801 series will
output an all zero code when it converts a negative input [V_IN(−) ≥ V_IN(+)]. Also, a logic inversion exists as all of
the I/O ports are buffered with inverting gates.
Basically, if the data read is zero, the differential output voltage is negative, so a bit in Port B is cleared to pull VX
more negative which will make the output more positive for the next conversion. If the data read is not zero, the
output voltage is positive so a bit in Port B is set to make VX more positive and the output more negative. This
continues for 8 approximations and the differential output eventually converges to within 5 mV of zero.
The actual program is given in Figure 46. All addresses used are compatible with the BLC 80/10 microcomputer
system. In particular:
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