TC7126 View Datasheet(PDF) - TelCom Semiconductor Inc => Microchip

Part Name

Description

Manufacturer

TC7126

3-1/2 DIGIT ANALOG-TO-DIGITAL CONVERTERS

TelCom Semiconductor Inc => Microchip 

TC7126

TC7126A

3-1/2 DIGIT

ANALOG-TO-DIGITAL CONVERTERS

When a segment driver is in-phase with the backplane

signal, the segment is OFF. An out-of-phase segment drive

signal causes the segment to be ON, or visible. This AC drive

configuration results in negligible DC voltage across each

LCD segment, ensuring long LCD life. The polarity segment

driver is ON for negative analog inputs. If VIN+ and VIN– are

reversed, this indicator would reverse.

On the TC7126A, when the TEST pin is pulled to V+, all

segments are turned ON. The display reads –1888. During

this mode, LCD segments have a constant DC voltage

impressed. DO NOT LEAVE THE DISPLAY IN THIS MODE

FOR MORE THAN SEVERAL MINUTES; LCDS MAY BE

DESTROYED IF OPERATED WITH DC LEVELS FOR

EXTENDED PERIODS.

The display font and segment drive assignment are

shown in Figure 4.

System Timing

The oscillator frequency is Ϭ4 prior to clocking the

internal decade counters. The three-phase measurement

cycle takes a total of 4000 counts (16,000 clock pulses).

The 4000-count cycle is independent of input signal magni-

tude.

Each phase of the measurement cycle has the following

length:

(1) Auto-zero phase: 1000 to 3000 counts

(4000 to 12,000 clock pulses)

For signals less than full scale, the auto-zero phase

is assigned the unused reference integrate time

period.

(2) Signal integrate: 1000 counts

(4000 clock pulses)

This time period is fixed. The integration period is:

1

tSI = 4000 fOSC ,

where fOSC is the externally-set clock frequency.

(3) Reference integrate: 0 to 2000 counts

(0 to 8000 clock pulses)

DISPLAY FONT

1000's 100's

10's

1's

3-224

Figure 4. Display Font and Segment Assignment

The TC7126A is a drop-in replacement for the TC7126

and ICL7126 that offers a greatly improved internal refer-

ence temperature coefficient. No external component value

changes are required to upgrade existing designs.

COMPONENT VALUE SELECTION

Auto-Zero Capacitor (CAZ)

The CAZ size has some influence on system noise. A

0.33 µF capacitor is recommended for 200 mV full-scale

applications where 1 LSB is 100 µV. A 0.033 µF capacitor is

adequate for 2V full-scale applications. A Mylar-type dielec-

tric capacitor is adequate.

Reference Voltage Capacitor (CREF)

The reference voltage, used to ramp the integrator

output voltage back to zero during the reference integrate

phase, is stored on CREF. A 0.1 µF capacitor is acceptable

when VREF– is tied to analog common. If a large common-

mode voltage exists (VREF– ≠ analog common) and the

application requires a 200 mV full scale, increase CREF to

1 µF. Roll-over error will be held to less than 0.5 count. A

Mylar-type dielectric capacitor is adequate.

Integrating Capacitor (CINT)

CINT should be selected to maximize integrator output

voltage swing without causing output saturation. Due to

the TC7126A's superior analog common temperature co-

efficient specification, analog common will normally sup-

ply the differential voltage reference. For this case, a ±2V

full-scale integrator output swing is satisfactory. For 3

readings per second (fOSC = 48 kHz), a 0.047 µF value is

suggested. For 1 reading per second, 0.15 µF is recom-

mended. If a different oscillator frequency is used, CINT

must be changed in inverse proportion to maintain the

nominal ±2V integrator swing.

An exact expression for CINT is:

( ) ( ) (4000)

CINT =

1

fOSC

VFS

RINT ,

VINT

where: fOSC = Clock frequency at pin 38

VFS = Full-scale input voltage

RINT = Integrating resistor

VINT = Desired full-scale integrator output swing.

At 3 readings per second, a 750Ω resistor should be

placed in series with CINT. This increases accuracy by

compensating for comparator delay. CINT must have low

dielectric absorption to minimize roll-over error. A polypro-

pylene capacitor is recommended.

TELCOM SEMICONDUCTOR, INC.

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