ADC0801 National Semiconductor, ADC0801 Datasheet - Page 33

• Manufacturer Part Number: ADC0801
• Description: 8 BIT UP COMPATIBLE A/D CONVERTERS
• Manufacturer: National Semiconductor

Functional Description
Note 25: In order for the microprocessor to service subroutines and interrupts, the stack pointer must be dimensioned in the user’s program.
For amplification of DC input signals, a major system error is
the input offset voltage of the amplifiers used for the preamp.
Figure 18 is a gain of 100 differential preamp whose offset
voltage errors will be cancelled by a zeroing subroutine
which is performed by the INS8080A microprocessor sys-
tem. The total allowable input offset voltage error for this
preamp is only 50 µV for
require very precise amplifiers. The expression for the differ-
ential output voltage of the preamp is:
SAMPLE PROGRAM FOR Figure 17 INTERFACING MULTIPLE A/D’s IN AN MC6800 SYSTEM
SAMPLE PROGRAM FOR Figure 17 INTERFACING MULTIPLE A/D’s IN AN MC6800 SYSTEM
1
⁄
4
LSB error. This would obviously
(Continued)
33
where I
error terms can be cancelled by making
V
scheme.
The INS8080A uses the 3 I/O ports of an INS8255 Program-
able Peripheral Interface (PPI) to control the auto zeroing
and input data from the ADC0801 as shown in Figure 19 .
The PPI is programmed for basic I/O operation (mode 0) with
Port A being an input port and Ports B and C being output
ports. Two bits of Port C are used to alternately open or close
the 2 switches at the input of the preamp. Switch SW1 is
closed to force the preamp’s differential input to be zero dur-
ing the zeroing subroutine and then opened and SW2 is then
closed for conversion of the actual differential input signal.
Using 2 switches in this manner eliminates concern for the
ON resistance of the switches as they must conduct only the
input bias current of the input amplifiers.
Output Port B is used as a successive approximation regis-
ter 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
quired to make the differential output voltage equal to zero.
This is accomplished by ensuring that the voltage at the out-
put of A1 is approximately 2.5V so that a logic “1” (5V) on
OS3
− V
X
is the current through resistor R
OS2
. This is the principle of this auto-zeroing
x
increases or decreases as re-
X
. All of the offset
±
I
DS005671-A3
X
DS005671-A4
R
www.national.com
X
= V
OS1
+