
THEORY OF OPERATION
The timing control circuitry governs the counters that
measure the integration time in both directions.
The ADC100's on-board microprocessor is used to
calculate the results of the integration equation above.
It is also used to perform error corrections and to
control the built-in-auto-zero function. Note that the
mP automatically performs an auto-zero function at
start-up, but it is recommended, to achieve maximum
accuracy, that an auto-zero be performed again after
the ADC100 is fully warmed up.
When the μP detects a convert signal, it lowers the
status lines to indicate that the ADC is involved in a
conversion. When it detects a change in slope
direction, the μP will collect the counts for the
integration time. When sufficient counts have been
collected, the μP performs the calculations described
above.
When the calculations are complete, the μmP places
the most significant byte in the output buffer and
raises the S
flag. When another pulse is placed on
the convert line, the middle byte is placed on the
output, the S
flag is lowered and the S
flag raised.
When the last pulse is placed in the convert line, the
least significant byte is placed in the output buffer and
both status flags are high indicating that the ADC100
is ready for another conversion.
Status line summary:
FIGURE 1. BLOCK DIAGRAM
Conversion in progress.
Conversion complete. MSB in output.
Middle byte in output register.
LSB in output. Ready for next conversion.
0 0
0 1
1 0
1 1
S
1
S
0
In the ADC100 block diagram (see Figure 1), V
and V
low
are the inputs. Both are buffered and fed
into a differential, voltage controlled, single output
current source. This current is added to the
reference current at the input of the op amp
integrator. The output of the integrator is fed into
a Schmitt trigger, which in turn, is fed into the
ADC's timing control circuitry. When the
integrator output actuates the Schmitt trigger, the
timing circuit changes the direction of the
reference current source and the integrator
begins integrating in the opposite direction. This
continues until the Schmitt trigger is actuated
again by the integrator and reverses the direction
of the reference current.
The equation for integration times are:
T
p
=
V X C
I
ref
+ I
inp
T
m
=
V X C
-I
ref
+ I
inp
Resolving these equations produces:
I
inp
= I
ref
T
p
- T
m
T
p
+ T
m
T
p =
Time Positive
T
m =
Time Negative
V = Voltage
C= Integration Capacitor Value
I
ref =
Reference Current
I
inp =
Input Current
ADC100DS REV. E MAR 00
Auto
Zero
Switch
Schmitt
Trigger
Bidirectional
Reference
Current Source
Current
Directional
Switch
Timing
Control
and
Counter
Microprocessor
Output
Buffer
Clock
Differential
Voltage Controlled
Current Source
V
hi
V
low
+15V
-15V
Auto
Zero
Convert
Status
Lines
Output Enable
Data
Output