12-BIT µP-COMPATIBLE
ANALOG-TO-DIGITAL CONVERTERS
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TC7109
TC7109A
A reference output (pin 29) is provided which may be
used with a resistive divider to generate a suitable reference
voltage (20 mA may be sunk without significant variation in
output voltage). A pull-up bias device is provided which
sources about 10 µA. The output voltage is nominally 2.8V
below V+. When using the on-board reference, REF OUT
(pin 29) should be connected to REF– (pin 39), and REF+
should be connected to the wiper of a precision potentiom-
eter between REF OUT and V+. The test circuit shows the
circuit for a 204.8 mV reference, generated by a 2 kΩ
precision potentiometer in series with a 24 kΩ fixed resistor.
The inner foil of CAZ should be connected to pin 31 and
the outer foil to the RC summing junction. The inner foil of
CINT should be connected to the RC summing junction and
the outer foil to pin 32 for best rejection of stray pickups. For
low leakage at temperatures above +85°C, use Teflon
capacitors.
Reference Capacitor
A 1 µF capacitor is recommended for most circuits.
However, where a large common-mode voltage exists, a
larger value is required to prevent roll-over error (e.g., the
reference low is not analog common), and a 409.6 mV scale
is used. The roll-over error will be held to 0.5 count with a 10
µF capacitor. For temperatures above +80°C use Teflon or
equivalent capacitors for their low leakage characteristics.
Interfacing
Direct Mode
Combinations of chip-enable and byte-enable control
signals which may be used when interfacing the TC7109A
to parallel data lines are shown in Figure 12. The CE/LOAD
input may be tied low, allowing either byte to be controlled
by its own enable (Figure 12A). Figure 12B shows the
HBEN and LBEN as flag inputs, and CE/LOAD as a master
enable, which could be the READ strobe available from
most microprocessors. Figure 12C shows a configuration
where the two byte enables are connected together. The
CE/LOAD is a chip enable, and the HBEN and LBEN may
be used as a second chip enable, or connected to ground.
The 14 data outputs will be enabled at the same time. In
the direct MODE, SEND should be tied to V+.
Figure 13 shows interfacing several TC7109A's to a
bus, ganging the HBEN and LBEN signals to several con-
verters together, and using the CE/LOAD input to select
the desired converter.
Figures 14–19 give practical circuits utilizing the paral-
lel three-state output capabilities of the TC7109A. Figure
14 shows parallel interface to the Intel MCS-48, -80 and -
85 systems via an 8255 PPI, where the TC7109A data
outputs are active at all times. The 8155 I/O ports may be
used in an identical manner. This interface can be used in a
read-after-update sequence, as shown in Figure 15. The
data is accessed by the high-to-low transition of the STA-
TUS driving an interrupt to the microprocessor.
Reference Voltage
To generate full-scale output of 4096 counts, the analog
input required is VIN = 2 VREF. For 409.6 mV full scale, use
a reference of 204.8 mV. In many applications, where the
ADC is connected to a transducer, a scale factor will exist
between the input voltage and the digital reading. For
instance, in a measuring system, the designer might like to
haveafull-scalereadingwhenthevoltageforthetransducer
is 700 mV. Instead of dividing the input down to 409.6 mV,
the designer should use the input voltage directly and select
VREF = 350 mV. Suitable values for integrating resistor and
capacitor would be 34 kΩ and 0.15 µF. This makes the
system slightly quieter and also avoids a divider network on
the input. Another advantage of this system occurs when
temperatureandweightmeasurementswithanoffsetortare
are desired for non-zero input. The offset may be introduced
by connecting the voltage output of the transducer between
common and analog high, and the offset voltage between
common and analog low, observing polarities carefully. In
processor-based systems using the TC7109A, it may be
more desirable to use software and perform this type of
scaling or tare subtraction digitally.
Reference Sources
AmajorfactorintheabsoluteaccuracyoftheADCisthe
stability of the reference voltage. The 12-bit resolution of the
TC7109A is one part in 4096, or 244 ppm. Thus, for the on-
board reference temperature coefficient of 70 ppm/°C, a
temperature difference of 3°C will introduce a one-bit abso-
lute error. Where the ambient temperature is not controlled,
or where high-accuracy absolute measurements are being
made, it is recommended that an external high-quality
reference be used.
The RUN/HOLD input is also used to initiate conver-
sions under software control. Figure 16 gives an interface
to Motorola MC6800 or MOS Technology MCS650X sys-
tem.
An interrupt is generated through the Control Register
B, CB1 line from the high-to-low transition of the STATUS
output. The RUN/HOLD pin is controlled by CB2 through
Control Register B, allowing software control of conver-
sions.
TELCOM SEMICONDUCTOR, INC.
3-105
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
