TC7126IPL Ver la hoja de datos (PDF) - Microchip Technology
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TC7126IPL Datasheet PDF : 26 Pages
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TC7126/A
4.0 ANALOG SECTION
In addition to the basic integrate and de-integrate dual
slope cycles discussed above, the TC7126A design
incorporates an auto-zero cycle. This cycle removes
buffer amplifier, integrator and comparator offset volt-
age error terms from the conversion. A true digital zero
reading results without external adjusting potentiome-
ters. A complete conversion consists of three phases:
1. Auto-Zero phase
2. Signal Integrate phase
3. Reference Integrate phase
4.1 Auto-Zero Phase
During the auto-zero phase, the differential input signal
is disconnected from the circuit by opening internal
analog gates. The internal nodes are shorted to analog
common (ground) to establish a zero input condition.
Additional analog gates close a feedback loop around
the integrator and comparator. This loop permits com-
parator offset voltage error compensation. The voltage
level established on CAZ compensates for device offset
voltages. The auto-zero phase residual is typically
10μV to 15μV. The auto-zero cycle length is 1000 to
3000 clock periods.
4.2 Signal Integrate Phase
The auto-zero loop is entered and the internal differen-
tial inputs connect to VIN+ and VIN-. The differential
input signal is integrated for a fixed time period. The
TC7126/A signal integration period is 1000 clock
periods or counts. The externally set clock frequency is
divided by four before clocking the internal counters.
The integration time period is:
EQUATION 4-1:
TSI =
4
FOSC
x 1000
Where: FOSC = external clock frequency.
The differential input voltage must be within the device
Common mode range when the converter and mea-
sured system share the same power supply common
(ground). If the converter and measured system do not
share the same power supply common, VIN- should be
tied to analog common.
Polarity is determined at the end of signal integrate
phase. The sign bit is a true polarity indication, in that
signals less than 1LSB are correctly determined. This
allows precision null detection limited only by device
noise and auto-zero residual offsets.
4.3 Reference Integrate Phase
The third phase is reference integrate or de-integrate.
VIN- is internally connected to analog common and
VIN+ is connected across the previously charged refer-
ence capacitor. Circuitry within the chip ensures that
the capacitor will be connected with the correct polarity
to cause the integrator output to return to zero. The
time required for the output to return to zero is propor-
tional to the input signal and is between 0 and 2000
counts. The digital reading displayed is:
EQUATION 4-2:
1000
VIN
VREF
DS21458C-page 10
© 2006 Microchip Technology Inc.
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