TC850
DS21479D-page 10 2001-2012 Microchip Technology Inc.
4.2 Zero Integrator Phase
During the zero integrator phase, the differential input
signal is disconnected from the circuit by opening inter-
nal analog gates. The internal nodes are shorted to
analog common (ground) to establish a zero input con-
dition. At the same time, a feedback loop is closed
around the input buffer, integrator and comparator. The
feedback loop ensures the integrator output is near 0V
before the signal integrate phase begins.
During this phase, a chopper-stabilization technique is
used to cancel offset errors in the input buffer, integra-
tor and comparator. Error voltages are stored on the
C
BUFF
, C
INT
and COMP capacitors. The zero integrate
phase requires 246 clock cycles.
4.3 Signal Integrate Phase
The zero integrator loop is opened and the internal dif-
ferential inputs are connected to IN
+ and IN-. The differ-
ential input signal is integrated for a fixed time period.
The TC850 signal integrate period is 256 clock periods,
or counts. The crystal oscillator frequency is before
clocking the internal counters.
The integration time period is:
EQUATION 4-1:
4.4 Reference Integrate Phase
During reference integrate phase, the charge stored on
the integrator capacitor is discharged. The time
required to discharge the capacitor is proportional to
the analog input voltage.
The reference integrate phase is divided into three
subphases:
1. Fast
2. Slow
3. Overrange de-integrate
During fast de-integrate, V
IN
- is internally connected to
analog common and V
IN
+ is connected across the pre-
viously-charged reference capacitor (C
REF1
). The inte-
grator capacitor is rapidly discharged for a maximum of
512 internal clock pulses, yielding 9 bits of resolution.
During the slow de-integrate phase, the internal V
IN
+
node is now connected to the C
REF2
capacitor and the
residual charge on the integrator capacitor is further
discharged a maximum of 64 clock pulses. At this point,
the analog input voltage has been converted with 15
bits of resolution.
If the analog input is greater than full scale, the TC850
performs up to three overrange de-integrate sub-
phases. Each subphase occupies a maximum of 64
clock pulses. The overrange feature permits analog
inputs up to 192 LSBs greater than full scale to be
correctly converted. This feature permits the user to
digitally null up to 192 counts of input offset, while
retaining full 15-bit resolution.
In addition to 512 counts of fast, 64 counts of slow and
192 counts of overrange de-integrate, the reference
integrate phase uses 10 clock pulses to permit internal
nodes to settle. Therefore, the reference integrate
cycle occupies 778 clock pulses.
