TC7109CLW datasheet(15/21 Pages) TELCOM

15 / 21 page

3-105

TELCOM SEMICONDUCTOR, INC.

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.

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

have a full-scale reading when the voltage for the transducer

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 k

Ω 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

temperature and weight measurements with an offset or tare

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

A major factor in the absolute accuracy of the ADC is the

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.

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 k

Ω

precision potentiometer in series with a 24 k

Ω fixed resistor.

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.

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.

TC7109

TC7109A

12-BIT

µP-COMPATIBLE

ANALOG-TO-DIGITAL CONVERTERS

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