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

require very precise amplifiers. The expression for the differ-

ential output voltage of the preamp is:

where I

V

OS3

−V

This is the principle of this auto-zeroing

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

SAMPLE PROGRAM FOR

Figure 17 INTERFACING MULTIPLE A/D’s IN AN MC6800 SYSTEM

DS005671-A3

SAMPLE PROGRAM FOR

Figure 17 INTERFACING MULTIPLE A/D’s IN AN MC6800 SYSTEM

DS005671-A4

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