5V PRECISION DATA ACQUISITION
SUBSYSTEMS
1
2
3
4
5
6
7
8
TC530
TC534
(1) Select Integration Time
Auto Zero Phase (AZ)
Integration time must be picked as a multiple of the
period of the line frequency. For example, tINT times
of 33msec, 66msec and 132msec maximize 60Hz
line rejection.
This phase compensates for errors due to buffer, inte-
gratorandcomparatoroffsetvoltages. Duringthisphase, an
internal feedback loop forces a compensating error voltage
on auto zero capacitor (CAZ). The duration of the AZ phase
is programmable via the serial port (see also Programming
AZ and INT Phase Duration paragraph of this document).
(2) Estimate Crystal Frequency
Crystal frequencies as high as 2MHz are allowed.
Crystal frequency is estimated using:
Input Integrate Phase (INT)
In this phase, a current directly proportional to differen-
2(R)
tINT
FIN
=
tial input voltage is sourced into integrating capacitor CINT
.
The amount of voltage stored on CINT at the end of the INT
phase is directly proportional to the applied differential input
voltage. Input signal polarity (sign bit) is determined at the
end of this phase. Converter resolution and conversion
speed is a function of the duration of the INT phase, which
is programmable by the user via the serial port (see also
Programming AZ and INT Phase Duration paragraph of this
document). The shorter the integration time, the faster the
speed of conversion, but the lower the resolution. Con-
versely, the longer the integration time, the greater the
resolution, but at slower the speed of conversion.
where: R = Desired Converter Resolution
(in counts)
FIN = Input Frequency (in MHz)
INT = Integration Time (in seconds)
(3) Calculate LOAD VALUE
(t )(F )
INT
IN
[LOAD VALUE] = 256 –
10
1024
FIN can be adjusted to a standard value during this step.
The resulting base -10 LOAD VALUE must be converted to
a hexadecimal number, then loaded into the serial port prior
to initiating A/D conversion.
Reference Deintegrate Phase (DINT)
This phase consists of measuring the time for the
integrator output to return (at a rate determined by the
external reference voltage) from its initial voltage to 0V. The
resulting timer data is stored in the output shift register as
converted analog data.
DINT and IZ Phase Timing
The duration of the DINT phase is a function of the
amount of voltage stored on the integrator capacitor during
INT, and the value of VREF. The DINT phase is initiated
immediately following INT and terminated when an integra-
tor output zero-crossing is detected. In general, the maxi-
mum number of counts chosen for DINT is twice that of INT
(with VREF chosen at VIN(max)/2).
Integrator Output Zero Phase (IZ)
This phase guarantees the integrator output is at zero
volts when the AZ phase is entered so that only true system
offset voltages will be compensated for.
All internal converter timing is derived from the fre-
quency source at OSCIN and OSCOUT. This frequency
source must be either an externally provided clock signal, or
an external crystal. If an external clock is used, it must be
connected to the OSCIN pin and the OSCOUT pin must
remain floating. If a crystal is used, it must be connected
between OSCIN and OSCOUT and physically located as
close to the OSCIN and OSCOUT pins as possible. In either
case,theincomingclockfrequencyisdividedbyfourandthe
resulting clock serves as the internal TC530/534 timebase.
System RESET
The TC530/534 must be forced into the AZ state when
power is first applied. A .01µF capacitor connected from
RESET to VCC (or external system reset logic signal) can be
used to momentarily drive RESET high for a minimum of
100msec.
Selecting Component Values for the TC530/534
(1) Calculate Integrating Resistor (RINT
The desired full-scale input voltage and amplifier
outputcurrentcapabilitydeterminethevalueofRINT
)
APPLICATIONS
.
Programming the TC530/534
The buffer and integrator amplifiers each have a full-
scale current of 20µA.
AZ and INT Phase Duration:
These two phases have equal duration determined by
the crystal (or external) frequency and the timer initialization
byte (LOAD VALUE). Timing is selected as follows:
The value of RINT is therefore directly calculated as
follows:
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3-55
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